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Infectious Disease and Microbiology – Diverticulitis
Diverticulitis is an inflammatory condition of a diverticulum, which is a pouch formed by herniation of the mucosal layer through a weak point in the muscular wall of the gastrointestinal tract. While diverticula can occur throughout the GI tract, they are most commonly found in the colon, particularly where the vasa recta penetrate the bowel wall. The presence of diverticula without inflammation is referred to as diverticulosis or diverticular disease, whereas diverticulitis specifically indicates inflammation and possible infection.
The condition is more prevalent in Western populations and increases significantly with age, affecting fewer than 5% of individuals under 40 years old but up to 65–80% of those over 70. It accounts for more than 130,000 hospital admissions annually in the United States. Both males and females are affected equally. In Western countries, diverticulitis typically involves the descending and sigmoid colon, whereas right-sided disease is more common in Asian populations.
Risk factors include a low-fiber diet, which leads to decreased stool bulk and increased intraluminal pressure, obesity, sedentary lifestyle, and immunosuppression (e.g., organ transplant recipients). The underlying mechanism involves increased pressure within the colon at weak points, leading to formation of diverticula. Subsequent pressure and irritation can result in inflammation, microperforations, or infection by polymicrobial flora, particularly anaerobes and gram-negative bacteria. Complicated diverticulitis may involve abscess formation, peritonitis, or fistula development and is classified using Hinchey’s staging system.
Clinically, diverticulitis presents with variable severity. Common symptoms include low-grade fever, abdominal pain that often starts in the epigastric region and localizes to the left lower quadrant, and changes in bowel habits such as constipation or diarrhea. In severe cases, perforation may lead to peritoneal signs like guarding and rebound tenderness. Patients with fistula formation may report pneumaturia, fecaluria, or recurrent urinary tract infections.
On physical examination, tenderness is usually localized to the left lower quadrant, sometimes accompanied by signs of peritoneal irritation. A tender mass may be palpable on rectal examination if inflammation is adjacent to the rectum. Mild rectal bleeding may occur but is rarely significant.
Diagnosis is supported by laboratory findings such as leukocytosis and confirmed primarily through imaging. CT scanning is the most reliable diagnostic modality, with high sensitivity and specificity, demonstrating findings such as bowel wall thickening, pericolic fat inflammation, diverticula, and abscess formation. Ultrasound may be used but is more operator-dependent. Endoscopy is generally avoided during the acute phase due to risk of perforation.
Management includes antibiotic therapy targeting anaerobic and gram-negative organisms. Common regimens include ciprofloxacin plus metronidazole, trimethoprim-sulfamethoxazole plus metronidazole, or amoxicillin-clavulanate for 7–10 days. Mild cases can often be managed outpatient, while severe cases or those unable to tolerate oral intake require hospitalization and intravenous therapy. Surgical intervention may be necessary for complications such as abscesses, perforation, or peritonitis. Percutaneous drainage may be used for larger abscesses, and elective surgery may be considered after recovery to prevent recurrence.
After resolution of the acute episode, patients are advised to adopt a high-fiber diet to reduce recurrence risk. Colonoscopy is recommended to exclude underlying malignancy. Although many patients recover fully, recurrence is common, and a subset will require surgical management.
Prognosis depends on disease severity, with mortality rates increasing in advanced stages of disease. Potential complications include perforation, abscess formation, fistula development, bowel obstruction, sepsis, and rarely pylephlebitis or hepatic abscess.
Diverticulitis is an inflammatory condition of a diverticulum, which is a pouch formed by herniation of the mucosal layer through a weak point in the muscular wall of the gastrointestinal tract. While diverticula can occur throughout the GI tract, they are most commonly found in the colon, particularly where the vasa recta penetrate the bowel wall. The presence of diverticula without inflammation is referred to as diverticulosis or diverticular disease, whereas diverticulitis specifically indicates inflammation and possible infection.
The condition is more prevalent in Western populations and increases significantly with age, affecting fewer than 5% of individuals under 40 years old but up to 65–80% of those over 70. It accounts for more than 130,000 hospital admissions annually in the United States. Both males and females are affected equally. In Western countries, diverticulitis typically involves the descending and sigmoid colon, whereas right-sided disease is more common in Asian populations.
Risk factors include a low-fiber diet, which leads to decreased stool bulk and increased intraluminal pressure, obesity, sedentary lifestyle, and immunosuppression (e.g., organ transplant recipients). The underlying mechanism involves increased pressure within the colon at weak points, leading to formation of diverticula. Subsequent pressure and irritation can result in inflammation, microperforations, or infection by polymicrobial flora, particularly anaerobes and gram-negative bacteria. Complicated diverticulitis may involve abscess formation, peritonitis, or fistula development and is classified using Hinchey’s staging system.
Clinically, diverticulitis presents with variable severity. Common symptoms include low-grade fever, abdominal pain that often starts in the epigastric region and localizes to the left lower quadrant, and changes in bowel habits such as constipation or diarrhea. In severe cases, perforation may lead to peritoneal signs like guarding and rebound tenderness. Patients with fistula formation may report pneumaturia, fecaluria, or recurrent urinary tract infections.
On physical examination, tenderness is usually localized to the left lower quadrant, sometimes accompanied by signs of peritoneal irritation. A tender mass may be palpable on rectal examination if inflammation is adjacent to the rectum. Mild rectal bleeding may occur but is rarely significant.
Diagnosis is supported by laboratory findings such as leukocytosis and confirmed primarily through imaging. CT scanning is the most reliable diagnostic modality, with high sensitivity and specificity, demonstrating findings such as bowel wall thickening, pericolic fat inflammation, diverticula, and abscess formation. Ultrasound may be used but is more operator-dependent. Endoscopy is generally avoided during the acute phase due to risk of perforation.
Management includes antibiotic therapy targeting anaerobic and gram-negative organisms. Common regimens include ciprofloxacin plus metronidazole, trimethoprim-sulfamethoxazole plus metronidazole, or amoxicillin-clavulanate for 7–10 days. Mild cases can often be managed outpatient, while severe cases or those unable to tolerate oral intake require hospitalization and intravenous therapy. Surgical intervention may be necessary for complications such as abscesses, perforation, or peritonitis. Percutaneous drainage may be used for larger abscesses, and elective surgery may be considered after recovery to prevent recurrence.
After resolution of the acute episode, patients are advised to adopt a high-fiber diet to reduce recurrence risk. Colonoscopy is recommended to exclude underlying malignancy. Although many patients recover fully, recurrence is common, and a subset will require surgical management.
Prognosis depends on disease severity, with mortality rates increasing in advanced stages of disease. Potential complications include perforation, abscess formation, fistula development, bowel obstruction, sepsis, and rarely pylephlebitis or hepatic abscess.
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Febrile seizure is a seizure occurring in children between 6 months and 5 years of age associated with fever, without evidence of intracranial infection or another primary central nervous system cause. The average age of onset is around 18–22 months. It is the most common pediatric convulsive disorder, affecting approximately 2–4% of children, and typically occurs in otherwise healthy children during a systemic illness, most often viral. Febrile seizures are classified into simple and complex types. Simple febrile seizures are generalized, brief (less than 10–15 minutes), self-limited, and occur only once within a 24-hour period. Complex febrile seizures are longer than 15 minutes, may have focal features, or recur within 24 hours. Risk factors include a family history of febrile seizures, delayed neurologic development, and male sex.
The condition is usually triggered by common childhood infections, particularly viral illnesses. Frequent causes include upper respiratory infections, otitis media, gastroenteritis, and Roseola. The seizure often occurs early in the illness, sometimes coinciding with the initial rapid rise in temperature rather than the peak fever.
Clinically, children present with fever and a seizure, most commonly a generalized tonic–clonic event. The seizure may involve an initial phase of muscle rigidity followed by rhythmic jerking movements and may be associated with apnea or urinary incontinence. Most episodes are brief and resolve spontaneously within a few minutes. Other possible manifestations include staring spells, limpness, or isolated jerking movements. After the seizure, a short postictal phase with drowsiness or confusion is common. A thorough history should include the duration and characteristics of the seizure, symptoms of infection, recent immunizations, medication exposure, trauma, developmental history, and family history of seizures. Physical examination should focus on identifying the source of fever and excluding serious conditions such as meningitis, looking for signs like nuchal rigidity, bulging fontanelle, or persistent altered mental status.
Evaluation is generally minimal for simple febrile seizures. Routine laboratory testing is not required unless there is concern for a serious bacterial infection, in which case tests such as complete blood count, urinalysis, and cultures may be performed. Lumbar puncture is not routinely indicated but should be considered in certain situations, such as in children aged 12–18 months with concerning symptoms (e.g., irritability, lethargy, poor feeding), incomplete immunization status, or signs suggestive of central nervous system infection. It is also indicated in older children if there are clear signs of meningitis or persistent altered mental status. Neuroimaging and EEG are not routinely required and are reserved for atypical presentations, such as focal seizures or underlying neurologic abnormalities.
Management is primarily supportive because most febrile seizures are self-limited. Initial priorities include maintaining airway, breathing, and circulation, and protecting the child from injury during the seizure. Oxygen and supportive care should be provided as needed. Pharmacologic treatment is rarely required, but benzodiazepines such as lorazepam, diazepam, or midazolam may be used for prolonged seizures or if the child is compromised. Rectal diazepam or intranasal midazolam can be effective in emergency settings. If seizures persist despite benzodiazepines, second-line agents such as phenytoin, fosphenytoin, or phenobarbital may be used. Antipyretics such as acetaminophen or ibuprofen are recommended to improve comfort, although they do not prevent recurrence of seizures. If a bacterial infection is identified, appropriate antibiotic therapy should be initiated.
The prognosis is generally excellent. About one-third of children will experience recurrence, especially those with early onset, a family history of seizures, or lower fever at the time of the initial episode. The risk of developing epilepsy later in life is only slightly increased compared to the general population, particularly in children with simple febrile seizures and normal neurologic development. Most children can be safely discharged if they return to baseline, have a normal neurologic examination, and the source of fever is identified and manageable. Parental reassurance and education are essential, as febrile seizures are typically benign. Aggressive fever control does not prevent recurrence, and long-term anticonvulsant prophylaxis is generally not recommended.
The condition is usually triggered by common childhood infections, particularly viral illnesses. Frequent causes include upper respiratory infections, otitis media, gastroenteritis, and Roseola. The seizure often occurs early in the illness, sometimes coinciding with the initial rapid rise in temperature rather than the peak fever.
Clinically, children present with fever and a seizure, most commonly a generalized tonic–clonic event. The seizure may involve an initial phase of muscle rigidity followed by rhythmic jerking movements and may be associated with apnea or urinary incontinence. Most episodes are brief and resolve spontaneously within a few minutes. Other possible manifestations include staring spells, limpness, or isolated jerking movements. After the seizure, a short postictal phase with drowsiness or confusion is common. A thorough history should include the duration and characteristics of the seizure, symptoms of infection, recent immunizations, medication exposure, trauma, developmental history, and family history of seizures. Physical examination should focus on identifying the source of fever and excluding serious conditions such as meningitis, looking for signs like nuchal rigidity, bulging fontanelle, or persistent altered mental status.
Evaluation is generally minimal for simple febrile seizures. Routine laboratory testing is not required unless there is concern for a serious bacterial infection, in which case tests such as complete blood count, urinalysis, and cultures may be performed. Lumbar puncture is not routinely indicated but should be considered in certain situations, such as in children aged 12–18 months with concerning symptoms (e.g., irritability, lethargy, poor feeding), incomplete immunization status, or signs suggestive of central nervous system infection. It is also indicated in older children if there are clear signs of meningitis or persistent altered mental status. Neuroimaging and EEG are not routinely required and are reserved for atypical presentations, such as focal seizures or underlying neurologic abnormalities.
Management is primarily supportive because most febrile seizures are self-limited. Initial priorities include maintaining airway, breathing, and circulation, and protecting the child from injury during the seizure. Oxygen and supportive care should be provided as needed. Pharmacologic treatment is rarely required, but benzodiazepines such as lorazepam, diazepam, or midazolam may be used for prolonged seizures or if the child is compromised. Rectal diazepam or intranasal midazolam can be effective in emergency settings. If seizures persist despite benzodiazepines, second-line agents such as phenytoin, fosphenytoin, or phenobarbital may be used. Antipyretics such as acetaminophen or ibuprofen are recommended to improve comfort, although they do not prevent recurrence of seizures. If a bacterial infection is identified, appropriate antibiotic therapy should be initiated.
The prognosis is generally excellent. About one-third of children will experience recurrence, especially those with early onset, a family history of seizures, or lower fever at the time of the initial episode. The risk of developing epilepsy later in life is only slightly increased compared to the general population, particularly in children with simple febrile seizures and normal neurologic development. Most children can be safely discharged if they return to baseline, have a normal neurologic examination, and the source of fever is identified and manageable. Parental reassurance and education are essential, as febrile seizures are typically benign. Aggressive fever control does not prevent recurrence, and long-term anticonvulsant prophylaxis is generally not recommended.
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Infectious Disease and Microbiology – Cat-Scratch Disease (Bartonella Infections)
Cat-scratch disease is typically a self-limited acute illness characterized by regional lymphadenopathy with or without fever and constitutional symptoms. It is most commonly caused by Bartonella henselae. In addition to classic lymphadenitis, B. henselae infection may present as fever of unknown origin, hepatosplenic granulomatous disease, neuroretinitis, or encephalopathy. Other medically important Bartonella species include Bartonella quintana, the historical cause of trench fever and a cause of persistent bacteremia, endocarditis, and bacillary angiomatosis, and Bartonella bacilliformis, which causes Oroya fever.
In the United States, the incidence of cat-scratch disease is approximately 10 cases per 100,000 person-years, with an estimated 24,000 recognized cases annually. The disease occurs worldwide and shows seasonal variation in temperate climates, with most cases occurring between August and January. It primarily affects immunocompetent individuals, and about 80% of cases occur in persons younger than 21 years. Most cases are self-limited and present with regional adenopathy. Bartonella quintana is globally endemic and associated with body louse transmission, while Bartonella bacilliformis is transmitted by sand flies and occurs at altitudes above 1 km in the Andes.
Cats, particularly kittens and those infested with fleas, are the natural reservoir of B. henselae. A history of cat exposure is reported in approximately 90% of patients, and about 60% recall a scratch. B. quintana outbreaks are associated with homelessness and poor socioeconomic conditions, where infestation with the human body louse (Pediculus humanus) facilitates transmission. Immunocompromised patients, especially those with advanced HIV infection, are at increased risk of chronic Bartonella infections such as bacillary angiomatosis and bacillary peliosis.
Prevention includes limiting exposure to cat scratches, bites, and licks, especially in immunocompromised individuals. Control of cat fleas may reduce transmission risk.
Bartonella species are fastidious, gram-negative, aerobic rod-shaped bacteria capable of intracellular survival. Infection leads to an inflammatory response and granuloma formation, particularly in lymph nodes. B. henselae accounts for the vast majority of cat-scratch disease cases.
The illness often begins with a small erythematous papule or pustule at the site of inoculation that may persist for weeks. Regional lymph nodes draining the affected area enlarge and become tender. Fever and malaise occur in approximately 30% of patients. Neuroretinitis may present with decreased visual acuity, and encephalopathy may manifest as mental status changes. In HIV-infected individuals, B. henselae bacteremia may cause progressive fatigue, weight loss, recurrent fevers, and hepatomegaly.
Trench fever due to B. quintana typically presents after an incubation period of 3–38 days with sudden onset of chills and fever. Symptoms such as headache, retro-orbital pain, and conjunctival injection are nonspecific. Bacillary angiomatosis presents with red-to-purple papules or nodules that may ulcerate or bleed. Bacillary peliosis involves blood-filled cystic lesions within visceral organs such as the liver or spleen.
On physical examination, regional lymphadenopathy is the most common finding. A primary inoculation lesion may be visible. Parinaud’s oculoglandular syndrome, characterized by conjunctivitis, conjunctival granuloma, and preauricular lymphadenopathy, occurs in approximately 5% of patients. Visceral involvement may produce hepatosplenomegaly.
Laboratory findings may show mild leukocytosis with occasional eosinophilia. Bartonella species can be isolated from blood using specialized culture techniques. Serologic testing with indirect fluorescence assays is widely used; titers greater than 1:256 strongly suggest active infection. If results are equivocal, repeat serology after two weeks may assist diagnosis. Ultrasound of enlarged lymph nodes may help evaluate suppuration and guide aspiration. Histopathology typically shows granuloma formation, and special stains such as Warthin–Starry or tissue PCR may support the diagnosis.
Diagnosis is suggested by compatible clinical findings, history of cat exposure, positive serology, characteristic histopathology, and exclusion of other causes of lymphadenopathy, particularly mycobacterial infections and suppurative adenitis.
Although cat-scratch disease is usually self-limited, antibiotics may hasten resolution. Azithromycin (500 mg orally once, then 250 mg daily for four additional days) is commonly used in adults. Neuroretinitis has been treated with doxycycline plus rifampin, although evidence is limited. Bacillary angiomatosis limited to the skin is treated with erythromycin (500 mg four times daily) or doxycycline (100 mg twice daily) for 8–12 weeks. Bacteremia requires at least four weeks of therapy, and longer courses (2–3 months) are recommended in HIV-infected patients, persistent fever, or endocarditis. Alternative agents include rifampin, ciprofloxacin, and trimethoprim-sulfamethoxazole.
If lymph node suppuration occurs, needle aspiration is preferred over incision and drainage to relieve pain and promote recovery. Valve replacement may be required in Bartonella endocarditis.
Lymphadenopathy usually resolves over several months. One episode appears to confer lifelong immunity. In immunocompetent individuals, fever resolves promptly with or without therapy. In HIV-infected patients, resolution of fever may take longer, although bacteremia typically clears within a week of treatment.
Uncomplicated cat-scratch disease has an excellent prognosis. Complications, including retinitis, encephalopathy, or severe systemic disease, occur in 5–14% of cases. Encephalopathy typically develops weeks after the acute illness and may present with seizures; recovery is usually rapid. Inflammatory organ involvement without angiomatosis or peliosis has been reported in patients with AIDS, affecting the liver, spleen, lymph nodes, heart, and bone marrow.
Cat-scratch disease is typically a self-limited acute illness characterized by regional lymphadenopathy with or without fever and constitutional symptoms. It is most commonly caused by Bartonella henselae. In addition to classic lymphadenitis, B. henselae infection may present as fever of unknown origin, hepatosplenic granulomatous disease, neuroretinitis, or encephalopathy. Other medically important Bartonella species include Bartonella quintana, the historical cause of trench fever and a cause of persistent bacteremia, endocarditis, and bacillary angiomatosis, and Bartonella bacilliformis, which causes Oroya fever.
In the United States, the incidence of cat-scratch disease is approximately 10 cases per 100,000 person-years, with an estimated 24,000 recognized cases annually. The disease occurs worldwide and shows seasonal variation in temperate climates, with most cases occurring between August and January. It primarily affects immunocompetent individuals, and about 80% of cases occur in persons younger than 21 years. Most cases are self-limited and present with regional adenopathy. Bartonella quintana is globally endemic and associated with body louse transmission, while Bartonella bacilliformis is transmitted by sand flies and occurs at altitudes above 1 km in the Andes.
Cats, particularly kittens and those infested with fleas, are the natural reservoir of B. henselae. A history of cat exposure is reported in approximately 90% of patients, and about 60% recall a scratch. B. quintana outbreaks are associated with homelessness and poor socioeconomic conditions, where infestation with the human body louse (Pediculus humanus) facilitates transmission. Immunocompromised patients, especially those with advanced HIV infection, are at increased risk of chronic Bartonella infections such as bacillary angiomatosis and bacillary peliosis.
Prevention includes limiting exposure to cat scratches, bites, and licks, especially in immunocompromised individuals. Control of cat fleas may reduce transmission risk.
Bartonella species are fastidious, gram-negative, aerobic rod-shaped bacteria capable of intracellular survival. Infection leads to an inflammatory response and granuloma formation, particularly in lymph nodes. B. henselae accounts for the vast majority of cat-scratch disease cases.
The illness often begins with a small erythematous papule or pustule at the site of inoculation that may persist for weeks. Regional lymph nodes draining the affected area enlarge and become tender. Fever and malaise occur in approximately 30% of patients. Neuroretinitis may present with decreased visual acuity, and encephalopathy may manifest as mental status changes. In HIV-infected individuals, B. henselae bacteremia may cause progressive fatigue, weight loss, recurrent fevers, and hepatomegaly.
Trench fever due to B. quintana typically presents after an incubation period of 3–38 days with sudden onset of chills and fever. Symptoms such as headache, retro-orbital pain, and conjunctival injection are nonspecific. Bacillary angiomatosis presents with red-to-purple papules or nodules that may ulcerate or bleed. Bacillary peliosis involves blood-filled cystic lesions within visceral organs such as the liver or spleen.
On physical examination, regional lymphadenopathy is the most common finding. A primary inoculation lesion may be visible. Parinaud’s oculoglandular syndrome, characterized by conjunctivitis, conjunctival granuloma, and preauricular lymphadenopathy, occurs in approximately 5% of patients. Visceral involvement may produce hepatosplenomegaly.
Laboratory findings may show mild leukocytosis with occasional eosinophilia. Bartonella species can be isolated from blood using specialized culture techniques. Serologic testing with indirect fluorescence assays is widely used; titers greater than 1:256 strongly suggest active infection. If results are equivocal, repeat serology after two weeks may assist diagnosis. Ultrasound of enlarged lymph nodes may help evaluate suppuration and guide aspiration. Histopathology typically shows granuloma formation, and special stains such as Warthin–Starry or tissue PCR may support the diagnosis.
Diagnosis is suggested by compatible clinical findings, history of cat exposure, positive serology, characteristic histopathology, and exclusion of other causes of lymphadenopathy, particularly mycobacterial infections and suppurative adenitis.
Although cat-scratch disease is usually self-limited, antibiotics may hasten resolution. Azithromycin (500 mg orally once, then 250 mg daily for four additional days) is commonly used in adults. Neuroretinitis has been treated with doxycycline plus rifampin, although evidence is limited. Bacillary angiomatosis limited to the skin is treated with erythromycin (500 mg four times daily) or doxycycline (100 mg twice daily) for 8–12 weeks. Bacteremia requires at least four weeks of therapy, and longer courses (2–3 months) are recommended in HIV-infected patients, persistent fever, or endocarditis. Alternative agents include rifampin, ciprofloxacin, and trimethoprim-sulfamethoxazole.
If lymph node suppuration occurs, needle aspiration is preferred over incision and drainage to relieve pain and promote recovery. Valve replacement may be required in Bartonella endocarditis.
Lymphadenopathy usually resolves over several months. One episode appears to confer lifelong immunity. In immunocompetent individuals, fever resolves promptly with or without therapy. In HIV-infected patients, resolution of fever may take longer, although bacteremia typically clears within a week of treatment.
Uncomplicated cat-scratch disease has an excellent prognosis. Complications, including retinitis, encephalopathy, or severe systemic disease, occur in 5–14% of cases. Encephalopathy typically develops weeks after the acute illness and may present with seizures; recovery is usually rapid. Inflammatory organ involvement without angiomatosis or peliosis has been reported in patients with AIDS, affecting the liver, spleen, lymph nodes, heart, and bone marrow.
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Infectious Disease and Microbiology – Chancroid
Chancroid is a sexually transmitted infection characterized by painful genital lesions that may progress to ulceration. The primary lesion typically begins as a tender papule or pustule measuring 2–20 mm in diameter, which rapidly becomes an excavated ulcer with undermined, ragged, or irregular edges.
In the United States, chancroid is rare and usually occurs in localized outbreaks. Only a small number of cases are reported annually, although underdiagnosis is possible. Globally, the disease remains prevalent in parts of Africa, Asia, and Latin America, and in lower socioeconomic populations in the United States.
Risk factors include multiple sexual partners, sexual contact with an infected partner, and travel to or sexual contact with persons from endemic regions. Chancroid increases both transmission of and susceptibility to HIV infection. Approximately 10% of individuals with chancroid acquired in the United States are coinfected with Treponema pallidum (syphilis) or herpes simplex virus (HSV).
The causative agent is Haemophilus ducreyi, a small, fastidious gram-negative rod. The incubation period ranges from 1 to 14 days.
Clinically, tender papules develop at the site of inoculation and become pustular, eroded, and ulcerated within 1–2 days. Multiple lesions may coalesce to form a large ulcer exceeding 2 cm in diameter. Approximately 40% of patients develop tender inguinal lymphadenopathy (“buboes”), which may suppurate and rupture spontaneously. The combination of painful genital ulcers and suppurative inguinal lymphadenopathy is characteristic.
Diagnosis is often based on clinical and epidemiologic features, but misdiagnosis is common; laboratory confirmation is important. Definitive diagnosis requires isolation of H. ducreyi on specialized culture media, which are not widely available. Culture has approximately 80% sensitivity. Polymerase chain reaction (PCR) testing has higher sensitivity (about 95%) but is not widely available and is not FDA approved in many settings. Diagnosis also requires exclusion of syphilis (negative dark-field examination or serology performed more than 7 days after ulcer onset) and genital herpes (negative clinical and laboratory findings). All patients should be tested for HIV at diagnosis.
For specimen collection, the lesion may be gently abraded to obtain exudate, which can be applied to slides for microscopy when indicated. Histopathologic examination typically shows superficial purulent exudate and mononuclear cell infiltrates in the dermis; neutrophil infiltration may be reduced in HIV-infected patients.
The differential diagnosis includes genital herpes, syphilis, acute HIV infection, lymphogranuloma venereum, granuloma inguinale (donovanosis), mycobacterial or fungal infections, parasitic infections, venereal warts, scabies, molluscum contagiosum, folliculitis, and plague. Noninfectious causes such as malignancy, trauma, fixed drug eruptions, Behçet syndrome, and dermatitis herpetiformis should also be considered.
First-line treatment options include azithromycin 1 g orally as a single dose or ceftriaxone 250 mg intramuscularly as a single dose. Alternative regimens include ciprofloxacin 500 mg orally twice daily for 3 days or erythromycin base 500 mg orally four times daily for 7 days. Ciprofloxacin is contraindicated in pregnancy and lactation. Some isolates with intermediate resistance to ciprofloxacin or erythromycin have been reported. Sexual partners within 10 days preceding symptom onset should be evaluated and treated with the same regimen.
Fluctuant buboes may require needle aspiration through intact skin or incision and drainage; incision and drainage may reduce the need for repeated procedures.
Patients should be reexamined within 3–7 days to assess clinical improvement. Complete healing depends on ulcer size; larger ulcers may require more than two weeks to resolve. Healing may be slower in uncircumcised men with subpreputial ulcers. Failure to improve may indicate incorrect diagnosis, coinfection (e.g., syphilis), reinfection, antimicrobial resistance, noncompliance, or HIV infection. Patients with initial negative HIV or syphilis tests should be retested approximately three months later.
Complications include secondary ulcers, draining fistulas following lymph node rupture, and increased risk of HIV transmission. Education on safe-sex practices is essential to prevent recurrence and transmission.
Chancroid is a sexually transmitted infection characterized by painful genital lesions that may progress to ulceration. The primary lesion typically begins as a tender papule or pustule measuring 2–20 mm in diameter, which rapidly becomes an excavated ulcer with undermined, ragged, or irregular edges.
In the United States, chancroid is rare and usually occurs in localized outbreaks. Only a small number of cases are reported annually, although underdiagnosis is possible. Globally, the disease remains prevalent in parts of Africa, Asia, and Latin America, and in lower socioeconomic populations in the United States.
Risk factors include multiple sexual partners, sexual contact with an infected partner, and travel to or sexual contact with persons from endemic regions. Chancroid increases both transmission of and susceptibility to HIV infection. Approximately 10% of individuals with chancroid acquired in the United States are coinfected with Treponema pallidum (syphilis) or herpes simplex virus (HSV).
The causative agent is Haemophilus ducreyi, a small, fastidious gram-negative rod. The incubation period ranges from 1 to 14 days.
Clinically, tender papules develop at the site of inoculation and become pustular, eroded, and ulcerated within 1–2 days. Multiple lesions may coalesce to form a large ulcer exceeding 2 cm in diameter. Approximately 40% of patients develop tender inguinal lymphadenopathy (“buboes”), which may suppurate and rupture spontaneously. The combination of painful genital ulcers and suppurative inguinal lymphadenopathy is characteristic.
Diagnosis is often based on clinical and epidemiologic features, but misdiagnosis is common; laboratory confirmation is important. Definitive diagnosis requires isolation of H. ducreyi on specialized culture media, which are not widely available. Culture has approximately 80% sensitivity. Polymerase chain reaction (PCR) testing has higher sensitivity (about 95%) but is not widely available and is not FDA approved in many settings. Diagnosis also requires exclusion of syphilis (negative dark-field examination or serology performed more than 7 days after ulcer onset) and genital herpes (negative clinical and laboratory findings). All patients should be tested for HIV at diagnosis.
For specimen collection, the lesion may be gently abraded to obtain exudate, which can be applied to slides for microscopy when indicated. Histopathologic examination typically shows superficial purulent exudate and mononuclear cell infiltrates in the dermis; neutrophil infiltration may be reduced in HIV-infected patients.
The differential diagnosis includes genital herpes, syphilis, acute HIV infection, lymphogranuloma venereum, granuloma inguinale (donovanosis), mycobacterial or fungal infections, parasitic infections, venereal warts, scabies, molluscum contagiosum, folliculitis, and plague. Noninfectious causes such as malignancy, trauma, fixed drug eruptions, Behçet syndrome, and dermatitis herpetiformis should also be considered.
First-line treatment options include azithromycin 1 g orally as a single dose or ceftriaxone 250 mg intramuscularly as a single dose. Alternative regimens include ciprofloxacin 500 mg orally twice daily for 3 days or erythromycin base 500 mg orally four times daily for 7 days. Ciprofloxacin is contraindicated in pregnancy and lactation. Some isolates with intermediate resistance to ciprofloxacin or erythromycin have been reported. Sexual partners within 10 days preceding symptom onset should be evaluated and treated with the same regimen.
Fluctuant buboes may require needle aspiration through intact skin or incision and drainage; incision and drainage may reduce the need for repeated procedures.
Patients should be reexamined within 3–7 days to assess clinical improvement. Complete healing depends on ulcer size; larger ulcers may require more than two weeks to resolve. Healing may be slower in uncircumcised men with subpreputial ulcers. Failure to improve may indicate incorrect diagnosis, coinfection (e.g., syphilis), reinfection, antimicrobial resistance, noncompliance, or HIV infection. Patients with initial negative HIV or syphilis tests should be retested approximately three months later.
Complications include secondary ulcers, draining fistulas following lymph node rupture, and increased risk of HIV transmission. Education on safe-sex practices is essential to prevent recurrence and transmission.
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Infectious Disease and Microbiology – Chickenpox (Varicella)
Chickenpox is a highly contagious febrile illness of childhood caused by primary infection with the varicella zoster virus (VZV). It is characterized by a generalized pruritic rash in which lesions at different stages of development—macules, papules, vesicles, pustules, and crusts—are present simultaneously.
The disease occurs worldwide and shows seasonal peaks in late winter and early spring. Before widespread vaccination, the primary attack rate in susceptible individuals was approximately 90%, with household secondary attack rates of 70–90%. Since implementation of routine vaccination programs, incidence, hospitalizations, complications, and deaths have declined dramatically. Both sexes are equally affected. Most cases occur in young children, although an increasing proportion of cases now occur in adolescents and adults in partially vaccinated populations.
Risk factors include close contact with an infected individual (varicella or herpes zoster), lack of immunity, and increasing age at exposure. Approximately 90% of individuals aged 15 years or older are immune, but 10% remain susceptible. Adults, pregnant women, and immunocompromised individuals are at higher risk for severe disease and complications.
Varicella zoster virus is a DNA virus (human herpesvirus 3) in the Herpesviridae family. Humans are the only known reservoir. Transmission occurs via respiratory droplets or direct contact with vesicular fluid. The virus enters through the respiratory mucosa, replicates in regional lymph nodes, and spreads through primary viremia. A secondary viremia disseminates the virus to the skin and other organs. After primary infection, the virus remains latent in sensory ganglia and may reactivate later in life as herpes zoster.
The incubation period is typically 10–14 days (range 10–21 days). Patients are infectious from approximately 48 hours before rash onset until all lesions have crusted. In immunocompetent children, mild prodromal symptoms such as malaise and low-grade fever may precede the rash by 1–2 days. The rash begins on the face and trunk and spreads centrifugally, sometimes involving mucous membranes. Lesions evolve rapidly from macules to papules and then to clear vesicles on an erythematous base. Vesicular fluid becomes turbid, crusting follows, and new crops of lesions appear over 2–4 days. Crusts fall off within 1–2 weeks and usually do not scar unless secondarily infected.
In immunocompromised patients, lesions are more numerous, may have hemorrhagic bases, and heal more slowly. Visceral complications occur in 30–50% of severe cases and can be fatal. Adults typically experience more severe illness and have a higher risk of complications. Pregnant women are at increased risk of varicella pneumonia and may transmit infection to the fetus. Perinatal varicella (maternal infection 5 days before to 2 days after delivery) can result in severe neonatal disease with high mortality. Congenital varicella syndrome, occurring when infection develops in the first two trimesters, may cause limb hypoplasia, skin scarring, ocular abnormalities, and central nervous system impairment.
Diagnosis is primarily clinical, based on the characteristic rash. Laboratory confirmation can be obtained by PCR detection of VZV DNA from lesion specimens, which is the most reliable method. Viral culture, direct immunofluorescence staining, Tzanck smear (showing multinucleated giant cells), and serologic testing may also be used. Chest radiographs may show nodular or interstitial infiltrates in cases of varicella pneumonia.
Differential diagnosis includes disseminated herpes simplex infection, enteroviral rashes, scabies, dermatitis herpetiformis, folliculitis, atypical measles, and rickettsialpox.
Oral acyclovir is recommended within 24 hours of rash onset to reduce disease severity. In immunocompetent children aged ≥2 years and weighing ≤40 kg, the dose is 80 mg/kg/day divided into four doses (maximum 3200 mg/day) for 5 days. Older children and adults may receive 3200 mg/day in four divided doses for 5 days. Adequate hydration is important. Valacyclovir and famciclovir are alternatives for older children and adults. Supportive care includes bathing, antipruritic agents, trimmed fingernails to prevent excoriation, and acetaminophen for fever. Aspirin should be avoided due to the risk of Reye’s syndrome.
Passive immunization with varicella zoster immunoglobulin is recommended for high-risk susceptible individuals after exposure, including immunocompromised persons, pregnant women, certain premature infants, and newborns exposed perinatally. It should be administered ideally within 72 hours and no later than 96 hours after exposure.
Active immunization with the live attenuated varicella vaccine is part of routine childhood vaccination schedules. Two doses are recommended, beginning at 12–15 months of age, with a second dose at 4–6 years. Unvaccinated adolescents and adults without evidence of immunity should receive two doses separated by at least 28 days. Vaccination is contraindicated during pregnancy.
Prognosis in healthy children is excellent, and infection usually confers lifelong immunity. However, severe disease may occur in adults, immunocompromised patients, pregnant women, and neonates. The virus remains latent and may later reactivate as herpes zoster.
Complications include secondary bacterial skin infections (commonly due to staphylococci or group A streptococci), varicella pneumonia (particularly in adults and pregnant women), cerebellar ataxia, encephalitis, meningitis, transverse myelitis, myocarditis, nephritis, hepatitis, bleeding disorders, and Reye’s syndrome. Mortality is low in the post-vaccination era but remains significant in high-risk groups.
Chickenpox is a highly contagious febrile illness of childhood caused by primary infection with the varicella zoster virus (VZV). It is characterized by a generalized pruritic rash in which lesions at different stages of development—macules, papules, vesicles, pustules, and crusts—are present simultaneously.
The disease occurs worldwide and shows seasonal peaks in late winter and early spring. Before widespread vaccination, the primary attack rate in susceptible individuals was approximately 90%, with household secondary attack rates of 70–90%. Since implementation of routine vaccination programs, incidence, hospitalizations, complications, and deaths have declined dramatically. Both sexes are equally affected. Most cases occur in young children, although an increasing proportion of cases now occur in adolescents and adults in partially vaccinated populations.
Risk factors include close contact with an infected individual (varicella or herpes zoster), lack of immunity, and increasing age at exposure. Approximately 90% of individuals aged 15 years or older are immune, but 10% remain susceptible. Adults, pregnant women, and immunocompromised individuals are at higher risk for severe disease and complications.
Varicella zoster virus is a DNA virus (human herpesvirus 3) in the Herpesviridae family. Humans are the only known reservoir. Transmission occurs via respiratory droplets or direct contact with vesicular fluid. The virus enters through the respiratory mucosa, replicates in regional lymph nodes, and spreads through primary viremia. A secondary viremia disseminates the virus to the skin and other organs. After primary infection, the virus remains latent in sensory ganglia and may reactivate later in life as herpes zoster.
The incubation period is typically 10–14 days (range 10–21 days). Patients are infectious from approximately 48 hours before rash onset until all lesions have crusted. In immunocompetent children, mild prodromal symptoms such as malaise and low-grade fever may precede the rash by 1–2 days. The rash begins on the face and trunk and spreads centrifugally, sometimes involving mucous membranes. Lesions evolve rapidly from macules to papules and then to clear vesicles on an erythematous base. Vesicular fluid becomes turbid, crusting follows, and new crops of lesions appear over 2–4 days. Crusts fall off within 1–2 weeks and usually do not scar unless secondarily infected.
In immunocompromised patients, lesions are more numerous, may have hemorrhagic bases, and heal more slowly. Visceral complications occur in 30–50% of severe cases and can be fatal. Adults typically experience more severe illness and have a higher risk of complications. Pregnant women are at increased risk of varicella pneumonia and may transmit infection to the fetus. Perinatal varicella (maternal infection 5 days before to 2 days after delivery) can result in severe neonatal disease with high mortality. Congenital varicella syndrome, occurring when infection develops in the first two trimesters, may cause limb hypoplasia, skin scarring, ocular abnormalities, and central nervous system impairment.
Diagnosis is primarily clinical, based on the characteristic rash. Laboratory confirmation can be obtained by PCR detection of VZV DNA from lesion specimens, which is the most reliable method. Viral culture, direct immunofluorescence staining, Tzanck smear (showing multinucleated giant cells), and serologic testing may also be used. Chest radiographs may show nodular or interstitial infiltrates in cases of varicella pneumonia.
Differential diagnosis includes disseminated herpes simplex infection, enteroviral rashes, scabies, dermatitis herpetiformis, folliculitis, atypical measles, and rickettsialpox.
Oral acyclovir is recommended within 24 hours of rash onset to reduce disease severity. In immunocompetent children aged ≥2 years and weighing ≤40 kg, the dose is 80 mg/kg/day divided into four doses (maximum 3200 mg/day) for 5 days. Older children and adults may receive 3200 mg/day in four divided doses for 5 days. Adequate hydration is important. Valacyclovir and famciclovir are alternatives for older children and adults. Supportive care includes bathing, antipruritic agents, trimmed fingernails to prevent excoriation, and acetaminophen for fever. Aspirin should be avoided due to the risk of Reye’s syndrome.
Passive immunization with varicella zoster immunoglobulin is recommended for high-risk susceptible individuals after exposure, including immunocompromised persons, pregnant women, certain premature infants, and newborns exposed perinatally. It should be administered ideally within 72 hours and no later than 96 hours after exposure.
Active immunization with the live attenuated varicella vaccine is part of routine childhood vaccination schedules. Two doses are recommended, beginning at 12–15 months of age, with a second dose at 4–6 years. Unvaccinated adolescents and adults without evidence of immunity should receive two doses separated by at least 28 days. Vaccination is contraindicated during pregnancy.
Prognosis in healthy children is excellent, and infection usually confers lifelong immunity. However, severe disease may occur in adults, immunocompromised patients, pregnant women, and neonates. The virus remains latent and may later reactivate as herpes zoster.
Complications include secondary bacterial skin infections (commonly due to staphylococci or group A streptococci), varicella pneumonia (particularly in adults and pregnant women), cerebellar ataxia, encephalitis, meningitis, transverse myelitis, myocarditis, nephritis, hepatitis, bleeding disorders, and Reye’s syndrome. Mortality is low in the post-vaccination era but remains significant in high-risk groups.
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Infectious Disease and Microbiology – Cryptosporidiosis
Cryptosporidiosis is caused by Cryptosporidium parvum and related species, including Cryptosporidium hominis and Cryptosporidium meleagridis. Cryptosporidium is an intracellular protozoan parasite that infects epithelial cells, primarily in the gastrointestinal tract. In immunocompetent individuals, infection typically causes self-limited watery diarrhea. However, in patients with advanced HIV infection or other forms of immunosuppression, it can lead to severe, protracted, and potentially life-threatening diarrhea.
Cryptosporidium is ubiquitous worldwide. In the United States, more than 300,000 cases occur annually. Seroprevalence rates reach up to 25% in industrialized countries and as high as 75% in developing regions. Transmission increases during warmer months in temperate climates. Large waterborne outbreaks have occurred, including the well-known outbreak in Milwaukee, Wisconsin, linked to contaminated municipal water. Contamination is often associated with agricultural runoff from dairy farms or other livestock operations. The organism is resistant to routine chlorination, making waterborne transmission particularly problematic.
Risk factors include advanced HIV infection (especially CD4 count <100 cells />mu;L), other immunosuppressive states, young children in developing countries, and animal handlers. Transmission occurs primarily through ingestion of oocysts in fecally contaminated water or food. Oocysts are highly resilient and may survive in the environment for up to 18 months. Ingestion of fewer than 1,000 oocysts can result in disease. Person-to-person transmission occurs in childcare centers, hospitals, through sexual contact, and via contaminated swimming pools. Transmission from pets and farm animals is also possible.
After ingestion, oocysts release sporozoites that infect epithelial cells of the small intestine. The parasite resides intracellularly but extracytoplasmically at the brush border of the mucosa. The entire life cycle occurs within a single host. Infection leads to impaired absorption, resulting in watery diarrhea and malabsorption. In immunocompromised patients, dissemination may occur, involving the biliary tree or respiratory tract.
The incubation period ranges from 7 to 10 days. In immunocompetent individuals, watery diarrhea lasts from several days up to one month and may be accompanied by crampy abdominal pain and low-grade fever. Vomiting is less common than with other causes of gastroenteritis. In patients with advanced immunosuppression, especially those with CD4 counts below 50 cells/μL, diarrhea may be profuse and exceed 10–15 liters per day. Weight loss, malnutrition, and recurrent disease occur in up to 40% of cases. Respiratory involvement may produce dyspnea.
Physical examination findings are nonspecific. In severe or chronic cases, signs of dehydration and wasting due to malabsorption may be evident.
Diagnosis is made by stool examination. Modified acid-fast staining demonstrates red or pink oocysts against a blue-green background. Immunofluorescent antibody staining is considered the gold standard. Enzyme-linked immunosorbent assays (ELISA), immunochromatographic tests, and PCR assays are also available and more sensitive than routine microscopy. Leukocytosis is uncommon, and fecal leukocytes or erythrocytes are typically absent. Malabsorption may result in abnormal D-xylose testing, elevated alkaline phosphatase levels, and low vitamin B12 levels. Imaging studies are nonspecific but may show ileus patterns or bowel wall edema. Biliary involvement may present with dilated intrahepatic or extrahepatic bile ducts, and respiratory involvement may show bilateral pulmonary infiltrates. Small intestinal biopsy reveals organisms attached to the brush border.
The differential diagnosis includes bacterial enteric infections such as Salmonella, Shigella, and Campylobacter; Clostridioides difficile infection; viral gastroenteritis; mycobacterial infections; other protozoal infections such as Giardia, Cyclospora, Isospora, and Microsporidia; and cytomegalovirus colitis.
Treatment depends on immune status. In immunocompetent individuals, the illness is typically self-limited and supportive care with hydration is sufficient. Nitazoxanide is effective in treating diarrhea in non-immunocompromised patients and is given for three days. In immunocompromised patients, especially those with AIDS, antiparasitic drugs have limited proven efficacy. Nitazoxanide has been used compassionately in such cases for extended durations. Paromomycin has shown variable results and may be combined with antimotility agents. Macrolides such as azithromycin and clarithromycin have some activity. Combination regimens (e.g., paromomycin plus azithromycin) and rifaximin have also been used.
Supportive care is critical, particularly in HIV-infected patients. Antimotility agents such as loperamide or diphenoxylate/atropine may help reduce stool frequency. Octreotide can decrease stool output without eradicating the organism. Immune reconstitution with effective antiretroviral therapy is the most important intervention in patients with AIDS and often leads to clinical improvement. Hospitalization and intravenous fluids are required for severely dehydrated patients, especially children.
Prognosis is generally good in immunocompetent individuals and in patients with HIV infection whose CD4 counts exceed 150 cells/μL. In advanced HIV infection, however, protracted diarrhea can be life-threatening. Complications include acalculous cholecystitis, sclerosing cholangitis, pancreatitis, tracheitis, and bronchitis when the respiratory tract is involved.
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Infectious Disease and Microbiology – Cholera
Cholera is a severe watery diarrheal illness caused by Vibrio cholerae. It is characterized by rapid fluid loss that can lead to life-threatening dehydration if untreated.
Globally, cholera remains a major public health problem, with an estimated 3–5 million cases and 100,000–120,000 deaths annually. It is one of the leading causes of death in children under five years of age worldwide. The disease is most common in areas with poor sanitation, contaminated water supplies, and during humanitarian crises or natural disasters.
Risk factors include exposure in endemic or epidemic settings, malnutrition, and immunosuppression, which may lead to a more severe clinical course. Individuals with blood group O are at increased risk of severe disease, particularly with the El Tor subtype. In pregnancy, cholera increases the risk of miscarriage or premature delivery, especially in the third trimester.
Transmission occurs through the fecal–oral route, typically via contaminated water, food, or undercooked seafood. The incubation period ranges from 12 hours to 5 days but may be as short as a few hours in cases of high bacterial inoculum or reduced gastric acidity. The organism produces cholera toxin, which consists of one A subunit and five B subunits. The toxin binds to enterocytes in the small intestine and stimulates chloride ion secretion, resulting in massive water loss into the intestinal lumen.
Vibrio cholerae is a gram-negative rod with over 190 serotypes based on the O antigen. Only serogroups O1 (El Tor and classical biotypes) and O139 are responsible for epidemic cholera. Non-O strains typically cause milder diarrheal illness.
Clinically, cholera presents with sudden onset of profuse watery diarrhea, often described as “rice water stools” with a fishy odor. Stool output can exceed one liter per hour in severe cases (cholera gravis). Patients may experience abdominal cramping but typically do not have fever. Vomiting may occur. Signs of dehydration include poor skin turgor, dry mucous membranes, tachycardia, hypotension, and in severe cases, shock.
Initial laboratory evaluation includes assessment of electrolytes, glucose, and renal function. Severe dehydration may lead to metabolic acidosis, hypokalemia, and acute kidney injury. Diagnosis can be confirmed by stool culture using selective media such as thiosulfate citrate bile sucrose (TCBS) agar. Gram stain may reveal motile gram-negative rods. However, treatment should not be delayed while awaiting confirmation.
The cornerstone of therapy is prompt and aggressive rehydration. Oral rehydration solution (ORS) is highly effective because sodium and glucose absorption remain intact despite toxin activity. Approximately 80% of patients recover with oral hydration alone. Intravenous fluids, preferably isotonic solutions such as lactated Ringer’s, are indicated in patients with severe dehydration or inability to tolerate oral intake.
Antibiotics are adjunctive to rehydration and reduce stool volume by approximately 50% and shorten the duration of bacterial shedding. First-line options include tetracycline (500 mg orally four times daily for 3 days) or a single 300 mg dose of doxycycline. Alternative agents include azithromycin, ciprofloxacin, or norfloxacin. In children and pregnant women, erythromycin or azithromycin is preferred; quinolones and tetracyclines should be avoided in these groups.
Zinc supplementation (30 mg daily) has been shown to decrease stool output and duration of illness, particularly in children. Close monitoring of electrolyte status, acid–base balance, glucose levels, and volume status is essential. Hospital admission is indicated for severe dehydration, inability to tolerate oral fluids, or signs of shock.
Preventive measures include safe water practices such as boiling, chlorination, filtration, and strict hand hygiene. Vaccination with killed whole-cell oral vaccines is recommended by the World Health Organization in endemic areas and provides approximately 78% protection for up to two years. Herd immunity significantly reduces transmission, and partial population vaccination can substantially lower case numbers.
Prognosis is excellent with timely rehydration. However, untreated cholera carries a mortality rate of up to 50%. Complications include arrhythmias due to electrolyte disturbances and acute renal failure secondary to severe dehydration.
Rapid recognition of acute watery diarrhea and immediate initiation of rehydration are critical. Management should focus on fluid and electrolyte replacement first, with antimicrobial therapy administered once the patient is stabilized. Suspected cases should be reported promptly to public health authorities to prevent outbreaks.
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Infectious Disease and Microbiology – Coccidioidomycosis
Coccidioidomycosis is a pulmonary and/or extrapulmonary infection caused by the dimorphic fungus Coccidioides immitis. The disease is commonly referred to as “Valley fever” and may range from a mild respiratory illness to severe disseminated infection.
In the United States, the incidence is approximately 91 cases per 100,000 people. C. immitis is endemic to the southwestern United States, particularly California, Arizona, and Texas, as well as parts of Mexico and Central and South America. Increasingly, cases are recognized outside endemic areas, typically in travelers or due to reactivation of latent infection. Periodic outbreaks with sharp increases in case numbers occur.
Risk is increased in individuals with immunosuppressive conditions or therapies, including AIDS, solid-organ transplantation, lymphoma, and glucocorticoid or other immunosuppressive treatments. Among patients with AIDS, infection is especially likely when CD4 counts are below 250 cells/mm³. Even distant past exposure to endemic regions is important, as latent infection may reactivate during immunosuppression. Pregnancy, particularly in late stages, and diabetes mellitus are also associated with increased risk of severe disease.
The fungus resides in soil and grows optimally at approximately 30°C but also proliferates at body temperature (37°C). Infection occurs through inhalation of airborne arthroconidia. In most cases, acute pulmonary infection resolves spontaneously. However, some patients develop progressive pneumonia or chronic pulmonary infection. Approximately 5% of infected individuals develop asymptomatic residual lung nodules or thin-walled cavities. Disseminated disease occurs in roughly 1 in 200 infected individuals and most commonly involves the meninges, bones, joints, skin, and soft tissues. Extrapulmonary disease usually develops within one year of primary infection but may occur later if immunity declines.
Symptoms typically appear 1–3 weeks after exposure. The most common presentation is a lower respiratory tract infection accompanied by systemic symptoms such as fever, cough, sputum production, chest pain, weakness, anorexia, sweating, and arthralgias. Erythema nodosum or erythema multiforme may occur. Extrapulmonary disease presents with focal symptoms depending on the organ involved. Skin lesions often appear as wart-like nodules. Joint involvement is usually unilateral. Meningitis often affects the basilar meninges and may present with headache as the most prominent symptom.
Physical examination should include careful evaluation of the skin and neurologic system to identify extrapulmonary involvement. Coccidioidal lesions are typically focal and produce localized symptoms such as swelling, ulceration, or discomfort.
Diagnosis relies on culture, serologic testing, and coccidioidal skin testing. Accurate travel history is essential, particularly outside endemic areas. Serum IgM antibodies are detectable in approximately 75% of primary infections early in the course of illness. IgG antibodies develop later and usually decline if the infection resolves. False-positive serologic results are uncommon. Skin testing becomes positive soon after symptom onset in primary infection and rarely cross-reacts with other infections. Cerebrospinal fluid in meningitis typically shows mononuclear pleocytosis, elevated protein, and low glucose. Chest radiographs may demonstrate infiltrates, pleural effusion, or hilar adenopathy.
The role of antifungal therapy in mild or moderate primary pulmonary infection remains uncertain. Treatment decisions are individualized. Oral fluconazole (400 mg daily) or itraconazole (200 mg twice daily) may be given for 3–6 months, particularly in patients with severe symptoms, high antibody titers, extensive lung involvement, significant weight loss, persistent symptoms, or increased host susceptibility. Disseminated disease always requires prolonged antifungal therapy.
Fluconazole and itraconazole are the drugs of choice for meningeal disease. Lifelong suppressive therapy may be required in some cases. Amphotericin B is reserved for patients who fail azole therapy or have severe disseminated infection. Surgical intervention may be necessary for complications such as severe hemoptysis, enlarging cavities, empyema, bronchopleural fistula, or restricted lung expansion.
Approximately 5–10% of infections result in residual sequelae. Early diagnosis of meningitis is critical, as untreated cases carry a 90% mortality within 12 months. Chronic pulmonary disease may develop, particularly in diabetic or immunocompromised patients.
Complications are more common and severe in immunocompromised individuals, especially those with extrapulmonary disease. Organ transplant recipients are at highest risk during the first year after transplantation. Meningitis may occur within six months of primary infection and may lack classic meningeal signs seen in bacterial meningitis. Symptoms can include headache, fever, confusion, seizures, diplopia, ataxia, vomiting, and focal neurologic deficits.
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Infectious Disease and Microbiology – Common Cold
The common cold is an acute, self-limiting viral infection of the upper respiratory tract. It is one of the most frequent infectious illnesses worldwide and primarily affects the nose and throat.
Adults typically experience 2–4 colds per year, while children average 6–10 episodes annually. In the United States alone, approximately 1 billion cases occur each year. The higher incidence in children is partly due to immature immune systems and close contact in school or daycare settings.
Risk factors include close contact with infected individuals, especially in enclosed environments, and direct hand contact with contaminated secretions. Cold transmission increases during winter months when people cluster indoors. Smoking, psychological stress, and immunocompromised states also increase susceptibility.
The common cold is caused by several respiratory viruses. Rhinoviruses account for 30–50% of cases, followed by coronaviruses (10–15%) and influenza viruses (5–15%). Less common causes include parainfluenza virus, respiratory syncytial virus (RSV), adenovirus, enterovirus, and metapneumovirus. Transmission occurs through respiratory droplets, aerosols, and contaminated hands contacting the nose, mouth, or eyes.
Symptoms typically include rhinorrhea, sneezing, nasal congestion, throat irritation, cough, and mild chilliness. Some patients may experience conjunctivitis, muscle aches, fatigue, headache, shivering, and decreased appetite. Fever is uncommon in adults but may occur in children. The illness is usually mild and resolves spontaneously within 7–10 days.
Physical examination generally shows signs of upper respiratory tract infection, such as nasal congestion and mild pharyngeal erythema. Laboratory tests and imaging are not required unless complications or alternative diagnoses are suspected.
The differential diagnosis includes pneumonia, pertussis (whooping cough), sinusitis, influenza, and allergic rhinitis. Persistent or worsening symptoms may warrant further evaluation.
There are no antiviral medications with established benefit for routine treatment of the common cold. Management is symptomatic. Analgesics and antipyretics such as acetaminophen or ibuprofen can relieve aches and fever. Aspirin should be avoided in children and adolescents due to the risk of Reye’s syndrome. Nasal decongestants may provide short-term relief, but topical nasal sprays should not be used for more than three days due to rebound congestion. Ipratropium bromide nasal spray may help reduce nasal discharge. Dextromethorphan can be used for cough in adults, and first-generation antihistamines may relieve symptoms. In children, codeine, dextromethorphan, and antihistamines are generally not recommended.
Complementary therapies such as echinacea, vitamin C, and zinc have not consistently demonstrated clear benefit for active treatment.
Prevention focuses on frequent handwashing, avoiding hand-to-face contact, minimizing exposure to infected individuals, and covering the mouth and nose with the elbow when coughing or sneezing. Sharing towels or utensils should be avoided. Patients should be educated that antibiotics are not effective against viral infections and are not indicated for uncomplicated colds.
The prognosis is excellent, as the disease is generally mild and self-limiting. However, complications can occur, including otitis media, sinusitis, pneumonia, pharyngitis, acute bronchitis, and exacerbations of asthma, chronic bronchitis, or obstructive sleep apnea. A cough persisting longer than three weeks should prompt evaluation for pneumonia or pertussis.
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Infectious Disease and Microbiology – Dengue
Dengue is a mosquito-borne viral illness that causes a severe flu-like disease and, in some cases, progresses to life-threatening dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), with a fatality rate of approximately 1–5% in severe cases. It is caused by the dengue virus, a single-stranded RNA virus of the Flaviviridae family, with four antigenically distinct serotypes (types 1–4). Humans and nonhuman primates serve as reservoirs. Transmission occurs through the bite of infected Aedes aegypti and Aedes albopictus, which are predominantly day-biting mosquitoes.
Globally, about 2.5 billion people—nearly two-fifths of the world’s population—are at risk of infection. An estimated 50–100 million infections occur annually, including approximately 500,000 cases of DHF and hundreds of thousands of deaths worldwide. Transmission increases during the rainy season due to mosquito breeding in stagnant water. Dengue is endemic in around 100 countries across Asia, the Pacific, the Americas, Africa, and the Caribbean, primarily in tropical and subtropical urban and suburban areas.
The principal risk factor is travel to or residence in endemic regions. No genetic predisposition has been clearly identified. Prevention focuses on avoiding mosquito bites and vector control. Measures include using insect repellents containing at least 30% DEET, wearing long-sleeved clothing (preferably treated with permethrin), and eliminating mosquito breeding sites such as stagnant water. Indoor insecticide use and larvicidal agents may reduce mosquito populations. Bed nets are of limited value because Aedes mosquitoes bite during the daytime.
After inoculation, the incubation period ranges from 3 to 14 days, most commonly 4–7 days. The virus initially replicates in dendritic cells and then spreads to reticuloendothelial cells, hepatocytes, and endothelial cells. Immune mediators contribute to the acute febrile illness, which typically lasts 5–7 days, with full recovery within 7–10 days in uncomplicated cases. Prior infection with a different serotype increases the risk of DHF and DSS due to immune-mediated mechanisms.
DHF and DSS usually develop between days 3 and 7 of illness, often at the end of the febrile phase. Increased capillary permeability leads to plasma leakage, hemoconcentration, pleural effusions, and ascites. Thrombocytopenia, capillary fragility, and disseminated intravascular coagulation (DIC) can result in hemorrhage ranging from petechiae to life-threatening gastrointestinal bleeding. Liver involvement may cause hepatitis and coagulopathy, which can be fatal in severe cases. Mother-to-child transmission has been documented.
Clinically, patients present with fever, headache, chills, myalgias, bone pain, rash, nausea, vomiting, abdominal pain, and anorexia. Cutaneous hyperesthesia and changes in taste may occur. Hemorrhagic manifestations include bruising, epistaxis, gum bleeding, menorrhagia, and gastrointestinal bleeding. A careful travel history is essential.
On physical examination, fever is common. Rash may appear in two phases: an initial generalized blanching macular rash followed by a morbilliform maculopapular rash that typically spares the palms and soles. Conjunctival and pharyngeal injection are frequent findings. Signs of shock—tachycardia, hypotension, and delayed capillary refill—indicate severe disease. Hepatomegaly, lymphadenopathy, mucosal bleeding, and altered mental status (suggesting encephalopathy or intracranial hemorrhage) may also be present.
Laboratory evaluation often reveals leukopenia, lymphopenia, elevated hematocrit (reflecting hemoconcentration), and thrombocytopenia. Liver transaminases are commonly elevated, and albumin may be low. Electrolyte disturbances such as hyponatremia and metabolic acidosis may occur. Coagulation studies may show prolonged PT and APTT, low fibrinogen, and elevated fibrin degradation products in DIC. Serologic testing (ELISA for IgM and IgG) is commonly used for diagnosis. Imaging may demonstrate pleural or pericardial effusions and ascites. Head CT is indicated in patients with altered consciousness.
The differential diagnosis includes malaria, yellow fever, rickettsial infections, leptospirosis, typhoid fever, viral hepatitis, meningitis, bacterial sepsis, and other viral illnesses such as influenza or chikungunya.
There is no specific antiviral treatment for dengue, DHF, or DSS. Management is supportive. Aspirin and nonsteroidal anti-inflammatory drugs should be avoided due to bleeding risk. Adequate analgesia, antipyretics (such as acetaminophen), and careful fluid management are essential. In severe cases, aggressive fluid resuscitation using isotonic crystalloids or colloids is required to maintain adequate blood pressure and organ perfusion. Advanced life support protocols should be followed in patients with shock. Blood products may be necessary in cases of severe hemorrhage. Close monitoring of fluid balance, urine output, electrolytes, and coagulation status is critical.
Hospital admission is indicated for patients with hemodynamic instability, DHF, DSS, or significant bleeding. Intensive care is required for hypotension, DIC, or organ failure. Discharge is appropriate once the patient is hemodynamically stable and has recovered clinically.
Prognosis is generally excellent for uncomplicated dengue fever, with most patients recovering fully. Those who survive the critical phase of DHF or DSS usually recover without long-term sequelae. However, complications may include neurological manifestations (encephalopathy, seizures, Guillain–Barré syndrome, transverse myelitis), myocarditis, and liver failure. Cases should be reported to public health authorities, and patients should be informed that infection with a different serotype in the future increases the risk of severe disease.
