Infectious Disease and Microbiology – Caliciviruses and Calici-like Viruses

Overview

Human caliciviruses are important causes of acute viral gastroenteritis. The two major genera that infect humans are Norovirus and Sapovirus. Norovirus is especially important because it is one of the most common causes of outbreaks of vomiting and diarrhea in both children and adults.

These viruses spread easily and can produce large outbreaks in households, schools, hospitals, nursing homes, cruise ships, and other crowded settings.

Microbiologic Characteristics

Caliciviruses are:

• Small viruses

• Nonenveloped

• Single-stranded, positive-sense RNA viruses

• Icosahedral in symmetry

Because they are nonenveloped, they are relatively resistant to environmental conditions compared with many enveloped viruses.

Important Human Genera

The two major human calicivirus genera are:

Norovirus

and

Sapovirus

Norovirus is by far the more important cause of widespread gastroenteritis outbreaks.

Sapovirus also causes gastroenteritis, particularly in children, but outbreaks may occur in adults as well.

Incubation Period

Symptoms generally begin within:

12 hours to 4 days

For norovirus, onset is often relatively rapid, frequently within about 1–2 days after exposure.

Epidemiology

Norovirus infection is extremely common worldwide.

Transmission occurs through several routes:

• Fecal–oral spread

• Contaminated food

• Contaminated water

• Contaminated surfaces and fomites

• Direct person-to-person contact

Vomiting can also generate droplets and contaminated environmental material, which may contribute to spread during outbreaks.

A very small infectious dose is sufficient to cause illness, which helps explain the high transmissibility.

Common Outbreak Settings

Norovirus is especially associated with outbreaks in:

• Cruise ships

• Nursing homes

• Hospitals

• Schools

• Daycare centers

• Restaurants

• Military or institutional settings

• Households

Closed or semi-closed environments make rapid person-to-person spread especially likely.

Clinical Infection

Caliciviruses cause acute gastroenteritis in children and adults.

Typical symptoms include:

• Sudden nausea

• Vomiting

• Watery diarrhea

• Abdominal cramps

• Malaise

• Low-grade fever in some patients

• Headache or myalgias in some cases

Vomiting is often particularly prominent with norovirus.

Clinical Course

The illness is usually self-limited.

Symptoms commonly last:

1–3 days

Some patients, particularly young children, older adults, and immunocompromised individuals, may experience a more prolonged course.

The main clinical risk is dehydration.

Dehydration

Patients with significant fluid loss may develop:

• Dry mucous membranes

• Tachycardia

• Reduced urine output

• Orthostatic dizziness or hypotension

• Weakness

• Lethargy in severe cases

Infants, older adults, and medically fragile patients are at greatest risk.

Diagnosis

In most uncomplicated cases, a specific viral diagnosis is not required because treatment is supportive.

Diagnosis is often based on:

Acute vomiting and watery diarrhea

• ●

Compatible outbreak or exposure history

• ●

Short duration of illness

RT-PCR

The most useful laboratory method for confirming norovirus or sapovirus is:

Reverse-transcription polymerase chain reaction (RT-PCR)

This detects viral RNA in stool and is especially useful during outbreak investigations.

Antigen and Serologic Testing

Enzyme immunoassays can detect viral antigen in stool or antibodies in serum, but these methods are generally less sensitive or less useful than molecular testing.

Electron Microscopy

Direct visualization of viral particles in stool by electron microscopy has historically been used, but this is not routinely performed in most clinical laboratories.

Differential Diagnosis

Calicivirus gastroenteritis may resemble:

• Rotavirus

• Adenovirus gastroenteritis

• Astrovirus

• Food poisoning from preformed toxins

• Enterotoxigenic bacterial diarrhea

• Other acute viral gastroenteritis

A classic epidemiologic clue is:

Sudden vomiting and diarrhea affecting many people in a closed setting

→ Think norovirus.

Treatment

Supportive Care

There is no routinely effective specific antiviral therapy.

Treatment is therefore primarily supportive.

The cornerstone is:

Fluid and electrolyte replacement

Patients with mild disease can usually be managed with oral fluids.

Oral rehydration solutions are particularly useful in children and patients with significant fluid losses.

Severe Dehydration

Intravenous fluids may be necessary when there is:

• Inability to tolerate oral fluids

• Persistent vomiting

• Severe dehydration

• Hypotension

• Significant electrolyte disturbance

Symptom Control

Antiemetics may be used in selected patients when vomiting interferes with oral hydration.

Antibiotics are not useful because the disease is viral.

Prevention

Prevention requires careful infection-control practices.

Important measures include:

• Frequent hand washing with soap and water

• Careful cleaning of contaminated surfaces

• Safe food preparation

• Avoidance of food handling while symptomatic

• Appropriate disposal of vomitus and stool

• Contact precautions in healthcare settings when indicated

Soap-and-water hand washing is particularly important because alcohol-based hand sanitizers may be less reliable against norovirus than they are against many other pathogens.

Contact Precautions

For diapered or incontinent children, contact precautions should be used during the illness.

In healthcare outbreaks, isolation or cohorting of affected patients may help reduce transmission.

Food Handlers

People with active vomiting or diarrhea should not prepare food for others.

Because viral shedding may continue after symptoms resolve, local public-health or workplace guidance should be followed regarding return to food-handling duties.

High-Yield Clinical Pattern

Cruise ship or nursing-home outbreak

• ●

Sudden vomiting

• ●

Watery diarrhea

• ●

Illness lasting 1–3 days

→ Think norovirus

Exam Essentials

Virus group: Caliciviruses

Major human genera: Norovirus and Sapovirus

Genome: Positive-sense single-stranded RNA

Envelope: None

Symmetry: Icosahedral

Incubation: Approximately 12 hours to 4 days

Major disease: Acute gastroenteritis

Classic norovirus clue: Prominent vomiting with outbreak in a closed setting

Transmission: Fecal–oral, contaminated food/water, fomites, direct contact

Best confirmatory test: Stool RT-PCR

Routine diagnosis required? Usually no

Treatment: Supportive care and rehydration

Specific antiviral therapy: None routinely available

Prevention: Hand washing, environmental cleaning, food hygiene, and contact precautions when appropriate

Key clinical pearl: A short, explosive outbreak of vomiting and watery diarrhea affecting many people at once is highly characteristic of norovirus.

Infectious Disease and Microbiology – Burkholderia Species

Overview

Burkholderia species are aerobic Gram-negative bacilli that can cause opportunistic and healthcare-associated infections. Among the most clinically important organisms are members of the Burkholderia cepacia complex, particularly because of their association with chronic respiratory infection in patients with cystic fibrosis.

Other clinically relevant species include Burkholderia gladioli and organisms historically referred to as B. pickettii.

Important Species

The B. cepacia complex contains several closely related species, including:

• B. cenocepacia

• B. multivorans

• B. dolosa

• B. cepacia

• Several additional species

B. gladioli can also produce respiratory and opportunistic infections.

The organism formerly called Burkholderia pickettii is now generally classified as Ralstonia pickettii.

Microbiology

These organisms are aerobic, nonfermenting Gram-negative bacilli. They are environmentally hardy and can survive under conditions that inhibit many other bacteria.

An important clinical characteristic of the B. cepacia complex is its substantial intrinsic antimicrobial resistance, which can make treatment difficult.

Epidemiology

Burkholderia organisms are widely distributed in the environment, particularly in soil and water.

The B. cepacia complex has special importance in patients with cystic fibrosis (CF). Certain strains can spread from one patient to another, making infection-control measures particularly important in CF clinics and other settings where patients with cystic fibrosis interact.

Healthcare-associated transmission can also occur through contaminated medical products, solutions, or equipment.

Respiratory Infection in Cystic Fibrosis

The B. cepacia complex is best known for causing persistent respiratory colonization and infection in patients with cystic fibrosis.

Clinical manifestations range from relatively stable chronic airway colonization to:

• Bronchitis

• Recurrent pulmonary exacerbations

• Progressive decline in pulmonary function

• Pneumonia

• Bacteremia and sepsis

B. gladioli can produce a similar respiratory infection in susceptible patients.

Cepacia Syndrome

A particularly severe manifestation is sometimes called cepacia syndrome.

This is characterized by rapid clinical deterioration with:

• High fever

• Severe necrotizing pneumonia

• Respiratory failure

• Bacteremia

• Sepsis

The syndrome is especially associated with certain members of the B. cepacia complex and can be life-threatening.

Other Infections

Outside cystic fibrosis, Burkholderia species may cause opportunistic infections such as:

• Wound infections

• Catheter-associated infections

• Bacteremia

• Sepsis

These infections occur particularly in immunocompromised or severely ill hospitalized patients.

Contaminated Medical Products

Organisms historically identified as B. pickettii, now generally classified as Ralstonia pickettii, have been associated with outbreaks of bloodstream infection resulting from contaminated intravenous fluids or other medical products.

Their ability to persist in aqueous environments contributes to their importance in healthcare-associated outbreaks.

Diagnosis

Diagnosis is established primarily through culture of appropriate clinical specimens.

Depending on the clinical syndrome, specimens may include:

• Sputum or other respiratory secretions

• Blood

• Wound material

• Catheter-related specimens

Accurate species identification is particularly important in patients with cystic fibrosis because different members of the B. cepacia complex may differ in epidemiology, transmissibility, resistance, and clinical significance.

Treatment

Treatment can be challenging because B. cepacia complex organisms frequently demonstrate multidrug resistance.

Antimicrobial susceptibility testing should therefore guide definitive treatment whenever possible.

Potentially active agents include:

Ceftazidime

Cefepime

Meropenem

Imipenem

Piperacillin-tazobactam

Ciprofloxacin

Trimethoprim-sulfamethoxazole

Susceptibility varies substantially among isolates.

Combination Therapy

For severe infection, particularly pneumonia, bacteremia, or rapidly progressive disease, combination antimicrobial therapy may be considered based on susceptibility results and specialist guidance.

Isolates recovered from patients with cystic fibrosis may have higher minimum inhibitory concentrations and greater resistance than environmental strains, making individualized susceptibility testing particularly important.

Aminoglycosides

Although aminoglycosides have historically been considered among possible treatment options, members of the B. cepacia complex commonly demonstrate substantial intrinsic resistance. They should therefore not be assumed to be active without susceptibility evidence.

Inhaled Antimicrobial Therapy

In selected patients with chronic pulmonary infection, particularly those with cystic fibrosis, inhaled antimicrobial therapy may sometimes be incorporated into a broader treatment strategy.

Nebulized agents such as tobramycin or meropenem have been studied or used in selected circumstances, although effectiveness depends heavily on the infecting species and its susceptibility pattern.

Prevention

Preventing transmission is especially important in the cystic fibrosis population.

Patients with CF who are not colonized with the B. cepacia complex should avoid close contact with individuals known to be infected or colonized.

Healthcare facilities caring for patients with cystic fibrosis use strict infection-prevention practices and patient segregation measures to reduce person-to-person transmission.

Appropriate handling and quality control of intravenous solutions, medications, and medical equipment are also important for preventing healthcare-associated outbreaks.

High-Yield Clinical Pattern

A patient with cystic fibrosis who develops chronic or rapidly worsening pulmonary infection with a multidrug-resistant Gram-negative nonfermenting bacillus should raise suspicion for the Burkholderia cepacia complex.

Rapid deterioration accompanied by necrotizing pneumonia, bacteremia, and sepsis suggests the severe form known as cepacia syndrome.

Exam Essentials

Genus: Burkholderia

Microbiology: Aerobic, nonfermenting Gram-negative bacilli

Important group: B. cepacia complex

Important species: B. cenocepacia, B. multivorans, B. dolosa, B. cepacia, and B. gladioli

Major association: Cystic fibrosis

Major infection: Chronic respiratory infection and pneumonia

Severe manifestation: Cepacia syndrome

Other infections: Wound infection, catheter-associated bacteremia, and sepsis

Diagnosis: Culture with accurate species identification and susceptibility testing

Treatment: Individualized according to antimicrobial susceptibility; potentially active drugs include TMP-SMX, ceftazidime, meropenem, and other selected agents

Major treatment problem: Multidrug resistance

Prevention: Strict infection-control measures and avoidance of transmission between patients with cystic fibrosis

Important classification pearl: B. pickettii is an older designation; the organism is now generally known as Ralstonia pickettii.

Infectious Disease and Microbiology – Burkholderia pseudomallei

Overview

Burkholderia pseudomallei is an aerobic Gram-negative bacillus that causes melioidosis, an infection that may remain localized or progress to pneumonia, bacteremia, sepsis, and multiple internal abscesses. The organism is closely associated with soil and surface water in endemic tropical regions.

Melioidosis is especially important because it can present either shortly after exposure or after a prolonged latent period.

Microbiologic Characteristics

B. pseudomallei is:

• Gram negative

• Bacillary

• Aerobic

• An environmental saprophyte

It can survive in soil and water and is well adapted to tropical environments.

Incubation Period

The incubation period is highly variable.

Some patients develop symptoms within only:

2–3 days

However, infection can also remain latent, with disease emerging:

Months or even years later

This delayed presentation is a classic feature of melioidosis.

Epidemiology

B. pseudomallei is found naturally in:

• Soil

• Mud

• Surface water

• Ponds

• Contaminated environmental material

Melioidosis is especially important in Southeast Asia and northern Australia, with Thailand historically representing one of the major endemic regions.

In highly endemic areas, it can account for a substantial proportion of community-acquired sepsis.

Transmission

Humans most commonly acquire infection through environmental exposure.

Important routes include:

• Inoculation of contaminated soil or water into cuts and abrasions

• Inhalation of contaminated dust or aerosols

• Aspiration or ingestion of contaminated water

Direct person-to-person transmission is extremely uncommon.

Risk Factors

Severe melioidosis is particularly associated with:

• Diabetes mellitus

• Chronic kidney disease

• Heavy alcohol use

• Chronic lung disease

• Immunosuppression

• Older age

Diabetes is one of the strongest recognized risk factors.

Clinical Manifestations

Melioidosis has a remarkably broad clinical spectrum.

It may present as:

• Localized skin infection

• Lymphadenitis

• Pneumonia

• Bacteremia

• Sepsis

• Disseminated infection with multiple abscesses

Localized Skin Disease

Inoculation through broken skin may produce:

• Papules

• Pustules

• Ulcers

• Abscesses

• Regional lymphadenitis

Localized infection can remain limited or progress hematogenously.

Pneumonia

Pneumonia is one of the most important forms of melioidosis.

Symptoms may include:

• Fever

• Cough

• Sputum production

• Dyspnea

• Pleuritic chest pain

• Hemoptysis in severe disease

Radiographic appearances can resemble bacterial pneumonia, tuberculosis, or lung abscess.

Bacteremia and Sepsis

Severe disease can produce:

• High fever

• Hypotension

• Septic shock

• Multiorgan dysfunction

• High mortality if treatment is delayed

Melioidosis should be considered in a patient from an endemic region with community-acquired sepsis and no obvious source.

Disseminated Abscesses

One of the classic features of systemic melioidosis is the formation of abscesses in multiple organs.

Potential sites include:

• Liver

• Spleen

• Prostate

• Kidney

• Bone

• Brain

• Soft tissues

A characteristic radiologic pattern of multiple small abscesses in the liver and spleen is sometimes described as a “honeycomb” appearance.

Neurologic Disease

Neurologic melioidosis is less common but serious.

Possible manifestations include:

• Brainstem encephalitis

• Cranial nerve abnormalities

• Weakness

• Ataxia

• Meningitis

• Brain abscess

These cases require prolonged therapy and careful specialist management.

Bone and Joint Disease

Melioidosis may cause:

• Osteomyelitis

• Septic arthritis

Persistent focal pain or swelling should prompt imaging and assessment for deep infection.

Diagnosis

Culture

The definitive diagnosis is usually made by culture of B. pseudomallei from:

• Blood

• Sputum

• Pus

• Urine

• Abscess fluid

• Other sterile-site specimens

Because the organism has important laboratory biosafety implications, the microbiology laboratory should be informed when melioidosis is suspected.

Serology

Serologic testing may provide supportive information in endemic areas, but its specificity can be limited because background antibody positivity may occur.

A positive antibody test alone does not establish active disease.

Molecular and Antigen Detection

In endemic regions, locally developed molecular or antigen-detection methods may support diagnosis.

PCR can be useful where available, particularly in difficult or rapidly progressive cases.

Treatment

Management has two distinct phases:

1. Intensive therapy

followed by

2. Eradication therapy

Both phases are essential because relapse can occur if treatment is stopped too early.

Intensive Phase

For severe melioidosis, the source lists:

Ceftazidime 2 g IV every 6 hours

or

Meropenem 1 g IV every 8 hours

or

Imipenem 1 g IV every 6 hours

for at least:

10–14 days

Longer intensive treatment may be required for deep-seated or complicated disease.

Role of TMP-SMX

Trimethoprim-sulfamethoxazole may be added, especially when there is:

• Neurologic disease

• Cutaneous disease

• Bone infection

• Prostatic infection

This is particularly important because tissue penetration and eradication of residual organisms are critical in these sites.

Eradication Phase

After the intensive phase, prolonged oral therapy is required.

The classic regimen is:

TMP-SMX orally for at least 3 months

This phase reduces the risk of relapse.

Older regimens sometimes combined TMP-SMX with doxycycline, though current practice often favors TMP-SMX alone when tolerated and active.

Abscess Drainage

Large abscesses may require:

• Percutaneous drainage

• Surgical drainage

• Debridement when necessary

Source control is particularly important for large or poorly responding collections.

Additional Treatment Options

Amoxicillin-clavulanate can sometimes be used as an alternative when first-line oral eradication therapy cannot be used.

Fluoroquinolones have historically been considered, but they are generally less reliable and are not preferred when more effective agents are available.

Antimicrobial Resistance

Susceptibility patterns vary.

A notable concern is resistance to TMP-SMX in some isolates, particularly in regions where resistance has historically been reported.

For this reason, serious infections should be managed according to current susceptibility testing whenever possible.

Prevention

There is no widely available human vaccine.

Risk reduction includes:

• Avoiding direct contact with soil or muddy water when skin is broken

• Wearing protective footwear and gloves in endemic areas

• Covering cuts and abrasions

• Avoiding exposure to contaminated surface water, particularly in high-risk individuals

• Using safe water practices during severe weather and flooding

People with diabetes or other major risk factors should be particularly cautious in endemic areas.

High-Yield Clinical Pattern

Endemic-area exposure

• ●

Diabetes

• ●

Community-acquired pneumonia or sepsis

• ●

Multiple internal abscesses

→ Think melioidosis due to Burkholderia pseudomallei

High-Yield Treatment Pattern

Severe melioidosis:

→ Ceftazidime or a carbapenem

then

→ Prolonged TMP-SMX eradication therapy

Exam Essentials

Organism:

→ Burkholderia pseudomallei

Microbiology:

→ Aerobic Gram-negative bacillus

Disease:

→ Melioidosis

Reservoir:

→ Soil and surface water

Major geographic association:

→ Southeast Asia and northern Australia

Incubation:

→ Days to years

Important risk factor:

→ Diabetes mellitus

Major clinical forms:

→ Skin infection, pneumonia, bacteremia, sepsis, disseminated abscesses

Diagnosis:

→ Culture

Initial intensive treatment:

→ Ceftazidime, meropenem, or imipenem

Eradication treatment:

→ Prolonged TMP-SMX

Important additional management:

→ Drain large abscesses

Major clinical pearl:

→ Melioidosis can relapse or present long after the initial environmental exposure, so both adequate intensive therapy and prolonged eradication therapy are essential.

Infectious Disease and Microbiology – Burkholderia mallei

Overview

Burkholderia mallei is an aerobic Gram-negative bacillus that causes glanders, an uncommon zoonotic infection primarily affecting horses, donkeys, and mules. Human disease is rare and usually follows close occupational or environmental exposure to infected equine animals. Infection can range from localized skin disease to severe pneumonia and life-threatening sepsis.

Microbiology

B. mallei is a small Gram-negative bacillus belonging to the genus Burkholderia. It is closely related to Burkholderia pseudomallei, the organism responsible for melioidosis.

The two organisms cannot reliably be distinguished by routine microscopic examination alone, so definitive identification requires specialized laboratory methods.

Incubation Period

The incubation period is generally short, with symptoms commonly developing approximately 1–5 days after exposure, although the clinical timing can vary according to the route and intensity of exposure.

Epidemiology

B. mallei primarily causes disease in equine animals, particularly horses, donkeys, and mules. Humans are accidental hosts and usually acquire infection through direct contact with infected animals or their contaminated secretions.

Historically, human glanders has been reported in parts of Asia, Africa, the Middle East, and Central and South America. Human cases are now uncommon because of improved veterinary surveillance and control of glanders among equine populations.

People with frequent occupational contact with horses and other equids are at greatest risk, including veterinarians, animal handlers, farmers, and laboratory personnel working with the organism.

Transmission and Pathogenesis

Human infection generally occurs when contaminated animal secretions enter through broken skin or mucous membranes. Infection may also occur through inhalation of contaminated droplets or aerosols.

Following entry into the body, the organism can produce localized infection or spread through the lymphatic and bloodstream systems. Dissemination can lead to multiple abscesses, pneumonia, bacteremia, and septic illness.

Clinical Infections

The characteristic disease caused by B. mallei is glanders. Clinical presentation depends partly on the route of infection.

Cutaneous glanders may begin with papules, pustules, or ulcerative lesions at the inoculation site. Regional lymphatic involvement can produce lymphangitis and suppurative lymphadenitis.

Pulmonary glanders can develop after inhalation or hematogenous dissemination. Patients may experience fever, cough, chest pain, dyspnea, and pneumonia.

Septicemic or disseminated glanders represents severe systemic infection and may cause high fever, hypotension, multiple organ involvement, abscess formation, and sepsis.

Diagnosis

Direct microscopic examination of wound or lesion exudates is generally insensitive, and the organism may not be readily identified.

Furthermore, microscopic morphology cannot reliably differentiate B. mallei from the closely related B. pseudomallei.

Definitive diagnosis is primarily based on culture and specific laboratory identification of the organism. Serologic testing has also been used as supportive evidence in appropriate epidemiologic settings.

Because B. mallei poses an important laboratory-acquired infection risk, the laboratory should be informed promptly when glanders is suspected so that appropriate biosafety procedures can be followed.

Treatment

Clinical evidence regarding the optimal antimicrobial regimen for glanders is limited because human infection is exceptionally rare.

Historically, combinations involving imipenem and doxycycline have been used. Other agents with laboratory activity against B. mallei include ceftazidime and certain other antimicrobials.

Management of suspected or confirmed glanders should involve infectious disease and public health specialists because treatment generally requires prolonged antimicrobial therapy and careful assessment for disseminated infection.

Surgical Management

Antibiotic therapy may need to be combined with surgical treatment when localized collections of infection develop.

In particular, suppurative lymphadenitis or abscesses may require drainage or other appropriate surgical management in addition to systemic antimicrobial therapy.

Prevention

The most effective preventive strategy is control and eradication of glanders in equine animals. Individuals handling potentially infected horses, donkeys, or mules should use appropriate protective measures and avoid direct exposure to animal secretions.

Suspected human infection also requires appropriate infection-control and public health precautions. Although person-to-person transmission is uncommon, exposure to infected secretions should be minimized.

High-Yield Clinical Pattern

Consider glanders when a patient with significant horse, donkey, or mule exposure develops an acute febrile illness accompanied by ulcerative skin lesions, lymphadenitis, pneumonia, or sepsis.

The key organism is Burkholderia mallei, and definitive diagnosis requires specialized culture or laboratory identification rather than microscopy alone.

Exam Essentials

Organism: Burkholderia mallei

Microbiology: Aerobic Gram-negative bacillus

Major disease: Glanders

Primary reservoir: Horses, donkeys, and mules

Transmission: Contact with infected animals or their secretions; inhalational exposure may occur

Typical incubation: Approximately 1–5 days

Clinical forms: Cutaneous/lymphatic, pulmonary, and septicemic or disseminated disease

Diagnosis: Culture with specific organism identification; serology may provide supportive evidence

Important distinction: Microscopy alone cannot reliably distinguish B. mallei from B. pseudomallei

Treatment: Requires specialist-guided antimicrobial therapy; regimens may include agents such as ceftazidime or a carbapenem followed by appropriate eradication therapy

Surgery: May be necessary for abscesses or suppurative lymphadenitis

Prevention: Control of glanders in equine animals and appropriate precautions during animal and laboratory exposure.

Infectious Disease and Microbiology – Brugia Species

Overview

Brugia species are filarial nematodes (roundworms) that infect the human lymphatic system. The two principal human pathogens are Brugia malayi and Brugia timori. Infection can produce lymphatic filariasis, characterized by recurrent lymphatic inflammation, progressive lymphedema, and, in advanced disease, elephantiasis.

Microbiology

Brugia species are tissue-dwelling nematode helminths. Adult worms reside primarily within the lymphatic vessels, where they produce microscopic larvae known as microfilariae.

The microfilariae circulate in peripheral blood and can be detected microscopically. Their circulation may demonstrate periodicity, with higher concentrations in the bloodstream during particular times of the day or night.

Important Species

Brugia malayi is the most widely distributed human Brugia species and is an important cause of lymphatic filariasis in parts of Asia.

Brugia timori has a more geographically restricted distribution and is primarily associated with infection in Indonesia.

Epidemiology

B. malayi occurs predominantly in South and Southeast Asia and parts of the western Pacific, including areas of India, China, Malaysia, Vietnam, Cambodia, Thailand, Indonesia, Papua New Guinea, and several Pacific islands.

B. timori is mainly found in Indonesia, particularly in parts of the Lesser Sunda Islands.

Transmission occurs through the bite of infected mosquito vectors, which introduce infective larvae into humans during feeding.

Pathogenesis

After transmission by a mosquito, infective larvae migrate to the lymphatic system and mature into adult worms. Their presence within lymphatic vessels causes inflammation and progressive disruption of normal lymphatic drainage.

Repeated episodes of lymphangitis and chronic lymphatic obstruction can eventually result in persistent swelling and marked tissue enlargement.

Clinical Infections

The major manifestation is lymphatic filariasis. Some infected individuals remain asymptomatic despite having circulating microfilariae.

Symptomatic disease may include recurrent fever, lymphangitis, lymphadenitis, and painful swelling of affected areas.

Chronic lymphatic damage can lead to persistent lymphedema. Severe longstanding disease may progress to elephantiasis, in which affected tissues become markedly enlarged and thickened.

Secondary bacterial infections and abscess formation may further aggravate lymphatic damage and swelling.

Diagnosis

Diagnosis traditionally relies on demonstrating microfilariae in peripheral blood. Fresh blood, concentrated specimens, or filtration techniques can increase the likelihood of detecting the parasites.

Thick blood smears stained with Giemsa or hematoxylin and eosin (H&E) can help visualize the microfilariae and distinguish Brugia from other filarial species based on their morphologic characteristics.

Because microfilariae may demonstrate nocturnal periodicity, the timing of blood collection should correspond to the expected period of maximum peripheral circulation in the geographic strain involved.

Serologic testing for antifilarial antibodies may provide supportive evidence, although antibody positivity does not necessarily distinguish active from previous infection.

Treatment

Diethylcarbamazine (DEC) has traditionally been the principal treatment for brugian lymphatic filariasis.

The source regimen lists diethylcarbamazine 2 mg/kg orally every 8 hours for 12 days. In patients with significant microfilaremia, treatment may be introduced gradually with lower initial doses before increasing to the full therapeutic dose.

The gradual regimen described begins with 50 mg on day 1, followed by increasing doses over the subsequent days until the therapeutic regimen is reached and treatment is completed over 12 days.

Treatment should be individualized according to current regional recommendations, coinfections, contraindications, and the patient’s clinical condition.

Treatment Reactions

Patients can develop inflammatory or allergic-type reactions, particularly during the first several days of treatment. These reactions occur largely because of the host inflammatory response to dying microfilariae.

Manifestations can include fever, headache, myalgias, rash, lymph node tenderness, and worsening lymphatic inflammation. Patients with substantial microfilaremia may experience more prominent reactions.

Additional Treatment

Alternative regimens described for filarial infection include a single dose of diethylcarbamazine 6 mg/kg.

Ivermectin 150 micrograms/kg as a single dose can reduce circulating microfilariae, although its effect on adult worms and chronic lymphatic disease is limited. Treatment strategies may differ substantially among endemic regions and elimination programs.

Management of Chronic Lymphatic Disease

Antiparasitic therapy does not necessarily reverse established lymphatic damage. Patients with chronic lymphedema therefore require careful skin hygiene, limb care, treatment of secondary bacterial infections, exercise, and measures that improve lymphatic drainage.

These interventions are important for preventing repeated inflammatory episodes and limiting progression of lymphedema and elephantiasis.

Prevention

Prevention centers on reducing transmission by mosquito vectors. Measures include insecticide-treated bed nets, mosquito control, protective clothing, and repellents where appropriate.

In endemic regions, population-based treatment programs can substantially reduce circulating microfilariae and interrupt community transmission.

High-Yield Clinical Pattern

Consider Brugia infection in a person from an endemic region of South or Southeast Asia who develops recurrent lymphangitis followed by chronic lymphedema or elephantiasis.

The diagnosis is supported by finding characteristic microfilariae on a properly timed peripheral blood smear.

Exam Essentials

Genus: Brugia

Species: B. malayi and B. timori

Organism: Filarial nematode

Transmission: Mosquito bite

Major disease: Lymphatic filariasis

Chronic manifestation: Lymphedema and elephantiasis

B. malayi distribution: Primarily South and Southeast Asia and parts of the western Pacific

B. timori distribution: Primarily Indonesia

Diagnosis: Peripheral blood examination for microfilariae

Stain: Giemsa or H&E on thick blood smear

Important diagnostic point: Blood collection may need to be timed according to microfilarial periodicity

Traditional treatment: Diethylcarbamazine

Additional microfilaricidal therapy: Ivermectin

Important treatment issue: Inflammatory reactions may occur as microfilariae die.

Infectious Disease and Microbiology – Borrelia Species

Overview

Borrelia species are spirochetal bacteria responsible for several vector-borne infections. The species B. recurrentis, B. hispanica, B. mazzottii, and B. venezuelensis are particularly associated with relapsing fever, an illness characterized by recurrent episodes of high fever separated by periods of clinical improvement.

Microbiology

Borrelia organisms are long, thin, helically shaped spirochetes rather than typical rod-shaped bacilli. They possess an outer membrane with structural features resembling those of Gram-negative bacteria, although they are not reliably visualized with a routine Gram stain.

Because of their relatively large size among spirochetes, organisms causing relapsing fever may be visible directly in peripheral blood during periods of high spirochetemia.

Important Species

Several Borrelia species are associated with relapsing fever, including B. recurrentis, B. hispanica, B. mazzottii, and B. venezuelensis. Other species can produce similar tick-borne infections in different geographic regions.

B. recurrentis is particularly important because it causes louse-borne relapsing fever, whereas several other Borrelia species are transmitted by ticks.

Incubation Period

Symptoms generally develop 4–18 days after exposure, with an average incubation period of approximately 7 days.

Epidemiology and Transmission

Relapsing-fever Borrelia infections occur in different regions worldwide. Transmission depends on the particular species.

Louse-borne relapsing fever is primarily caused by B. recurrentis and is transmitted by the human body louse.

Tick-borne relapsing fever is transmitted primarily by infected soft ticks, particularly Ornithodoros species. Rodents and other small mammals frequently serve as reservoirs for tick-borne organisms.

Crowding, poor hygienic conditions, and situations that promote body-louse infestation increase the risk of louse-borne disease.

Clinical Infection

The principal clinical syndrome is relapsing fever. Patients typically develop an abrupt episode of high fever accompanied by chills, severe headache, myalgias, arthralgias, and generalized weakness.

The initial febrile episode resolves, but fever subsequently returns. These alternating periods of fever and improvement result from changes in the organism’s surface antigens, allowing Borrelia to temporarily escape the host immune response.

Multiple relapses are particularly characteristic of tick-borne disease.

Diagnosis

Diagnosis is most useful during a febrile episode, when large numbers of spirochetes may circulate in the bloodstream.

Peripheral blood smears stained with Giemsa or Wright stain may demonstrate the characteristic spirochetes. Dark-field microscopy can also visualize the organisms, although this technique is not routinely available in many laboratories.

PCR provides another method for detecting and identifying Borrelia and may be especially useful when microscopy is negative or species identification is required.

Serologic testing can be performed, but its usefulness is limited by imperfect sensitivity and specificity.

Treatment

Treatment differs somewhat between louse-borne and tick-borne relapsing fever.

For louse-borne relapsing fever, the traditional source regimen includes a single oral dose of tetracycline 500 mg or doxycycline 100 mg.

For tick-borne relapsing fever, treatment generally requires a longer course. The source regimen lists tetracycline 500 mg orally every 6 hours or doxycycline 100 mg orally every 12 hours for 7 days.

Current antibiotic choice and duration should be individualized according to the specific Borrelia infection, patient characteristics, disease severity, and contemporary guidelines.

Additional Treatment

Erythromycin may be used as an alternative in selected circumstances.

When infection is complicated by meningitis or encephalitis, an antibiotic with reliable central nervous system penetration is required. Intravenous penicillin G or ceftriaxone may be used.

Important Treatment Reaction

A major clinical consideration is the Jarisch–Herxheimer reaction, which may develop shortly after antibiotic therapy is started.

Rapid destruction of spirochetes can trigger an acute inflammatory response characterized by fever, chills, rigors, headache, myalgias, tachycardia, and hypotension. Patients, particularly those with severe infection, should therefore be monitored after the first antimicrobial dose.

High-Yield Clinical Pattern

Think of relapsing-fever Borrelia when a patient has repeated episodes of abrupt high fever separated by afebrile periods, particularly when there is a history of body-louse exposure or soft-tick exposure.

Demonstration of spirochetes on a peripheral blood smear obtained during a febrile episode strongly supports the diagnosis.

Exam Essentials

Genus: Borrelia

Organism type: Spirochete

Major infection: Relapsing fever

Important species: B. recurrentis

Incubation: Approximately 4–18 days

Louse-borne disease: Primarily B. recurrentis

Tick-borne disease: Several Borrelia species transmitted by soft ticks

Diagnosis: Giemsa/Wright-stained peripheral blood smear, PCR; serology has limitations

Treatment: Usually doxycycline or tetracycline

CNS disease: Penicillin G or ceftriaxone may be required

Classic treatment complication: Jarisch–Herxheimer reaction

Key clue: Recurrent febrile episodes separated by periods of improvement.

Infectious Disease and Microbiology – Bordetella Species

Overview

Bordetella pertussis and Bordetella parapertussis are aerobic Gram-negative bacilli that cause respiratory tract infection, most notably whooping cough (pertussis). The organisms occur worldwide and can infect individuals who have previously been immunized, particularly adults, because protection against pertussis decreases over time.

Microbiology

Bordetella species are small, aerobic Gram-negative bacilli with a particular tropism for the respiratory tract. B. pertussis is the major cause of classic whooping cough, whereas B. parapertussis can produce a similar, often somewhat milder, pertussis-like respiratory illness.

Epidemiology

Pertussis has a worldwide distribution. Infection can occur even in previously vaccinated individuals because vaccine-induced immunity is not lifelong. Consequently, adolescents and adults may develop infection and can serve as sources of transmission to susceptible individuals, especially young infants.

Clinical Infection

The characteristic disease is whooping cough, a highly contagious respiratory infection. Illness typically begins with nonspecific upper respiratory symptoms and subsequently progresses to episodes of severe, repetitive coughing.

The classic paroxysmal stage is characterized by repeated coughing fits that may be followed by the characteristic inspiratory “whoop.” Post-tussive vomiting can occur. Infants may not develop the classic whoop and can instead present with apnea, cyanosis, or respiratory distress.

Diagnosis

Diagnosis can be established using a nasopharyngeal specimen. PCR provides rapid detection of Bordetella DNA and is commonly useful for confirming infection.

Culture may also be performed using specialized media, classically Bordet–Gengou medium. Culture is highly specific but becomes less sensitive as the illness progresses or after antibiotic therapy has begun.

Serologic testing may provide additional diagnostic information in selected patients, particularly later in the course of illness. Antigen detection from nasopharyngeal secretions has also been described, although molecular testing is generally more useful.

Treatment

Macrolide antibiotics are the principal antimicrobial therapy for pertussis. Treatment is most effective when started early and is particularly important for reducing transmission.

Traditional regimens include erythromycin, while azithromycin and clarithromycin are commonly used alternatives because they allow shorter treatment courses and are generally better tolerated.

The source regimen lists:

Erythromycin: 2 g/day orally in four divided doses for 14 days.

Azithromycin: 500 mg orally on day 1, followed by 250 mg once daily on days 2–5.

Clarithromycin: 500 mg orally every 12 hours for 7 days.

Antibiotics given after the paroxysmal cough has become established may have limited ability to shorten the cough itself, although treatment remains valuable for decreasing bacterial transmission.

Alternative Treatment

Trimethoprim–sulfamethoxazole (TMP-SMX) may be used when macrolides cannot be given. The listed adult regimen is one double-strength tablet orally every 12 hours for 7 days.

Antibiotic selection and dosing should be adjusted for age, pregnancy, contraindications, and current treatment guidelines.

Symptomatic Management

The cough associated with pertussis can persist for weeks despite appropriate antimicrobial treatment. Conventional cough suppressants, including codeine, are generally ineffective and are not routinely useful for controlling the characteristic paroxysmal cough.

Supportive management is particularly important in infants and patients with severe disease, who may require monitoring for apnea, hypoxemia, dehydration, or feeding difficulties.

Prevention and Infection Control

Pertussis is highly transmissible through respiratory droplets. Appropriate respiratory/droplet isolation precautions are therefore important during the infectious period.

Vaccination remains the major preventive strategy. Because immunity decreases with time, booster immunization is important for maintaining population protection and reducing transmission to vulnerable infants.

High-Yield Clinical Pattern

A patient with an initial mild upper respiratory illness followed by recurrent paroxysms of severe cough, inspiratory whooping, and post-tussive vomiting should raise strong suspicion for pertussis.

The diagnosis is supported by nasopharyngeal PCR or culture, and treatment is generally with a macrolide antibiotic.

Exam Essentials

Organisms: Bordetella pertussis and Bordetella parapertussis

Microbiology: Aerobic Gram-negative bacilli

Distribution: Worldwide

Major disease: Whooping cough (pertussis)

Transmission: Respiratory droplets

Classic symptom: Paroxysmal cough with inspiratory whoop

Diagnosis: PCR or specialized culture; serology may assist in selected cases

Classic culture medium: Bordet–Gengou medium

Treatment: Macrolides, especially azithromycin, clarithromycin, or erythromycin

Alternative: TMP-SMX

Cough suppressants: Usually ineffective

Prevention: Vaccination and appropriate respiratory/droplet precautions

Important pearl: Previous vaccination does not completely exclude pertussis, particularly in adolescents and adults whose immunity has waned.

Infectious Disease and Microbiology - Bordetella bronchiseptica

Basics

Bordetella bronchiseptica is an aerobic Gram-negative bacillus that primarily colonizes and infects the respiratory tracts of animals. Human infection is uncommon, but when it occurs it most often involves the respiratory system and may be more severe in immunocompromised patients or those with chronic lung disease.

Unlike Bordetella pertussis, which is a major human respiratory pathogen, B. bronchiseptica is principally a zoonotic organism.

Microbiologic Characteristics

Bordetella bronchiseptica is:

• Gram negative

• Bacillary in shape

• Aerobic

It belongs to the genus Bordetella, but its antimicrobial susceptibility pattern differs importantly from that of B. pertussis.

Epidemiology

The organism has a worldwide distribution.

It is commonly associated with animals, particularly mammals with respiratory tract colonization or infection. Human disease is rare and usually sporadic.

Exposure to infected or colonized animals may be relevant in some cases.

Risk Factors

Human infection is more likely in patients with:

• Immunosuppression

• Chronic pulmonary disease

• Structural lung abnormalities

• Frequent animal exposure

• Severe underlying illness

Although healthy individuals can occasionally be infected, clinically significant disease is more often reported in vulnerable hosts.

Clinical Infections

Pneumonia

Pneumonia is one of the most important manifestations of B. bronchiseptica infection.

Possible symptoms include:

• Fever

• Cough

• Sputum production

• Dyspnea

• Pleuritic discomfort

• Hypoxemia in severe disease

The infection may resemble other bacterial pneumonias and therefore requires microbiologic confirmation.

Tracheobronchitis

The organism may also cause tracheobronchial infection.

Patients may develop:

• Persistent cough

• Increased respiratory secretions

• Wheezing

• Chest discomfort

• Fever

This may be especially important in patients with preexisting respiratory disease.

Bacteremia

Although uncommon, B. bronchiseptica can invade the bloodstream.

Bacteremia is more likely in:

• Immunocompromised patients

• Patients with severe pulmonary infection

• Patients with major underlying disease

Clinical findings may include:

• Fever

• Chills

• Hypotension

• Sepsis in severe cases

Diagnosis

Diagnosis is primarily established by:

Culture

Depending on the clinical syndrome, specimens may include:

• Sputum

• Bronchoalveolar lavage fluid

• Tracheal secretions

• Blood cultures

Because the organism is unusual in humans, laboratory identification is important to distinguish it from other Bordetella species and from other Gram-negative respiratory pathogens.

Treatment

A key therapeutic point is that B. bronchiseptica differs from B. pertussis.

Macrolide Resistance

In contrast to B. pertussis, B. bronchiseptica is often resistant to macrolides.

Therefore, agents such as:

• Azithromycin

• Clarithromycin

• Erythromycin

should not automatically be assumed to be effective.

Antipseudomonal Beta-Lactams

Antipseudomonal beta-lactam agents are often active.

Potential options may include agents with activity against resistant Gram-negative bacilli, depending on susceptibility results.

Definitive therapy should ideally be guided by antimicrobial susceptibility testing.

Additional Treatment Options

Other potentially active agents include:

• Aminoglycosides

• Imipenem

• Doxycycline

The choice depends on the severity of infection, site involved, renal function, and susceptibility profile.

Severe Infection

For pneumonia with bacteremia or other invasive disease, intravenous therapy is generally appropriate initially.

Management should also include:

• Oxygen support when needed

• Fluid management

• Treatment of sepsis

• Evaluation of underlying immunosuppression

• Repeat cultures if bacteremia persists

Differential Diagnosis

Respiratory infection due to B. bronchiseptica may resemble:

• Bordetella pertussis

• Pseudomonas aeruginosa

• Haemophilus influenzae

• Enteric Gram-negative pneumonia

• Other bacterial causes of tracheobronchitis

A zoonotic exposure history and culture identification help distinguish the organism.

Prevention

Because human infection is uncommon, there are no routine human vaccination recommendations specifically for B. bronchiseptica.

General preventive measures include:

• Careful hygiene after animal contact

• Avoiding close exposure to sick animals when severely immunocompromised

• Appropriate veterinary care for infected animals

• Standard infection-control practices in healthcare environments

High-Yield Clinical Pattern

Immunocompromised patient

• ●

Animal exposure

• ●

Pneumonia or tracheobronchitis

• ●

Gram-negative Bordetella isolated in culture

→ Consider Bordetella bronchiseptica

High-Yield Treatment Distinction

B. pertussis

→ Macrolides commonly used

B. bronchiseptica

→ Often macrolide resistant

→ Antipseudomonal beta-lactams, imipenem, doxycycline, or aminoglycosides may be active

Exam Essentials

Organism:

→ Bordetella bronchiseptica

Microbiology:

→ Aerobic Gram-negative bacillus

Distribution:

→ Worldwide

Major infections:

→ Pneumonia, tracheobronchitis, bacteremia

Important host:

→ Often immunocompromised or chronically ill patients

Diagnosis:

→ Culture

Key antimicrobial distinction:

→ Often resistant to macrolides

Commonly active therapy:

→ Antipseudomonal beta-lactam

Other possible agents:

→ Aminoglycosides, imipenem, doxycycline

Key clinical pearl: Unlike B. pertussis, B. bronchiseptica should not be assumed to respond to macrolides; susceptibility-guided therapy is important.

Infectious Disease and Microbiology - Blastoschizomyces capitatus

Basics

Blastoschizomyces capitatus is a rare opportunistic yeast that can cause invasive fungal disease, especially in patients with profound neutropenia or other major defects in host immunity. It can also cause fungemia in intravenous drug users and occasionally infect surgical wounds or prosthetic heart valves.

Microbiologic Characteristics

Blastoschizomyces capitatus is a yeast that can form several different fungal structures, including:

• Arthroconidia

• Hyphae

• Blastoconidia

• Pseudohyphae

This variable morphology can make identification challenging and may require careful culture-based laboratory evaluation.

Epidemiology

Human infection is uncommon but appears to occur worldwide.

The highest-risk patients are those with:

• Neutropenia

• Hematologic malignancy

• Intensive chemotherapy

• Severe immunosuppression

• Intravenous drug use in selected cases

• Prosthetic cardiac material

Clinical Infections

Disseminated Infection

The most serious manifestation is disseminated fungal disease, particularly in neutropenic patients.

Dissemination may involve:

• Bloodstream

• Skin

• Liver

• Spleen

• Other visceral organs

Patients may present with persistent fever despite broad-spectrum antibacterial therapy.

Fungemia

Fungemia is an important manifestation and may occur in severely immunocompromised patients or intravenous drug users.

Possible features include:

• Persistent fever

• Chills

• Sepsis

• Positive fungal blood cultures

• Evidence of metastatic infection

Once fungemia is identified, evaluation for deep organ involvement is important.

Skin Lesions

Disseminated disease may produce cutaneous lesions.

These may appear as:

• Papules

• Nodules

• Erythematous lesions

• Necrotic lesions in severe infection

Skin biopsy may provide a useful diagnostic specimen.

Visceral Abscesses

The fungus can seed internal organs and produce abscesses, especially in neutropenic patients.

Potential sites include:

• Liver

• Spleen

• Other deep tissues

Persistent fever, abdominal pain, or unexplained organ lesions in a high-risk patient should raise concern for invasive fungal disease.

Surgical Wound Infection

B. capitatus can occasionally infect postoperative wounds.

Clinical manifestations may include:

• Wound erythema

• Drainage

• Delayed healing

• Local tissue infection

Culture and tissue evaluation are important because routine antibacterial therapy will not treat the fungus.

Prosthetic Valve Endocarditis

A rare but serious manifestation is prosthetic valve endocarditis.

Possible findings include:

• Persistent fungemia

• Fever

• New or changing murmur

• Embolic events

• Prosthetic valve dysfunction

This form of infection may require both prolonged antifungal therapy and surgical intervention.

Diagnosis

Diagnosis is based primarily on:

• Fungal culture

• Histopathologic identification in tissue

Appropriate specimens may include:

• Blood

• Skin biopsy

• Abscess material

• Surgical wound tissue

• Cardiac valve tissue when relevant

Because the organism can produce multiple fungal forms, definitive laboratory identification may require specialized mycology methods.

Treatment

Amphotericin B

Intravenous amphotericin B has historically been a major treatment option for invasive Blastoschizomyces capitatus infection.

It is especially relevant in:

• Fungemia

• Disseminated infection

• Deep visceral disease

• Endocarditis

Treatment duration depends on the extent of infection and host recovery.

Flucytosine

Flucytosine has been used in combination with amphotericin B.

Combination therapy may be considered in severe invasive disease, particularly when susceptibility supports its use.

Fluconazole

The source lists:

Fluconazole 800 mg/day

as an additional treatment option.

However, susceptibility can vary, so antifungal therapy should ideally be guided by in vitro susceptibility testing and clinical response.

Additional Management

Neutropenia Recovery

In neutropenic patients, restoration of neutrophil function is a critical part of successful treatment.

Outcome is often better when:

• Neutropenia resolves

• Immunosuppressive therapy can be reduced

• Source control is achieved

Source Control

Depending on the infection site, management may require:

• Drainage of visceral abscesses

• Debridement of infected wounds

• Removal of infected foreign material

• Valve surgery in prosthetic valve endocarditis

Antifungal therapy alone may not be sufficient when infected prosthetic material remains in place.

Differential Diagnosis

In a neutropenic patient with fungemia or disseminated fungal disease, considerations include:

• Candida

• Trichosporon

• Geotrichum

• Aspergillus

• Other opportunistic yeasts and molds

Morphology and culture identification are essential for distinguishing these organisms.

High-Yield Clinical Pattern

Neutropenic patient

• ●

Persistent fever

• ●

Fungemia

• ●

Skin lesions or visceral abscesses

→ Consider Blastoschizomyces capitatus

Exam Essentials

Organism:

→ Blastoschizomyces capitatus

Type:

→ Opportunistic yeast

Morphologic forms:

→ Arthroconidia, hyphae, blastoconidia, pseudohyphae

Epidemiology:

→ Rare, probably worldwide

Major risk factor:

→ Neutropenia

Major infections:

→ Fungemia, disseminated disease, skin lesions, visceral abscesses

Other infections:

→ Surgical wound infection, prosthetic valve endocarditis

Diagnosis:

→ Culture and tissue biopsy

Main historical therapy:

→ Intravenous amphotericin B

Combination option:

→ Amphotericin B + flucytosine

Additional option:

→ High-dose fluconazole

Important management principle:

→ Recovery from neutropenia and source control improve outcomes

Key clinical pearl: In a neutropenic patient with persistent fungemia plus skin or visceral lesions, Blastoschizomyces capitatus should be considered among the invasive opportunistic yeasts.

Infectious Disease and Microbiology - Bipolaris Species

Basics

Bipolaris species are darkly pigmented, filamentous fungi that can cause a spectrum of infections ranging from localized corneal and subcutaneous disease to invasive pulmonary, cerebral, and disseminated infections.

Important species include:

• Bipolaris australiensis

• Bipolaris hawaiiensis

• Bipolaris spicifera

Both immunocompetent and immunocompromised individuals can develop infection.

⸻

Microbiologic Characteristics

Bipolaris species are dematiaceous fungi, meaning that melanin produces dark pigmentation within their fungal structures.

They are filamentous molds.

In culture, they typically:

• Grow relatively rapidly

• Produce cottony colonies

• Develop gray to black pigmentation

• Form hyphae at approximately 30°C

Within infected tissue, they may appear as:

• Yeast-like forms

• Pseudohyphae

• Septate pigmented hyphae

Infection caused by these pigmented fungi falls within the spectrum of phaeohyphomycosis.

⸻

Epidemiology

Bipolaris organisms are environmental fungi commonly found in:

• Soil

• Plants

• Decaying vegetation

Humans may acquire infection through:

Traumatic inoculation

Penetrating injuries can introduce fungal elements directly into the skin, subcutaneous tissues, or eye.

Another route is:

Inhalation of environmental conidia

This route is particularly relevant to sinus and pulmonary disease.

⸻

Clinical Infections

Fungal Keratitis

Bipolaris can cause mycotic keratitis, particularly after traumatic inoculation of the cornea.

Possible manifestations include:

• Eye pain

• Redness

• Photophobia

• Excessive tearing

• Corneal infiltrates

• Reduced visual acuity

Trauma involving soil or plant material is an important clinical clue.

⸻

Subcutaneous Infection

Traumatic implantation may produce localized infection of the skin and subcutaneous tissues.

Manifestations can include:

• Nodules

• Cysts

• Abscess-like lesions

• Slowly enlarging subcutaneous masses

These infections can occur even in otherwise healthy individuals.

⸻

Paranasal Sinusitis

Bipolaris is particularly associated with fungal sinus disease.

It may cause sinusitis in patients with:

• Allergic rhinitis

• Nasal polyposis

• Chronic sinonasal inflammation

It is an important cause of allergic fungal rhinosinusitis.

Patients may develop:

• Nasal obstruction

• Chronic sinus pressure

• Nasal discharge

• Recurrent sinusitis

• Nasal polyps

⸻

Pulmonary Infection

After inhalation, Bipolaris may occasionally produce pulmonary disease.

Pulmonary infection is more concerning in patients with impaired immunity and may manifest with:

• Fever

• Cough

• Dyspnea

• Pulmonary infiltrates or nodules

⸻

Central Nervous System Infection

Rarely, Bipolaris can cause severe CNS disease, including:

Meningoencephalitis

Cerebral involvement may result from direct extension or hematogenous dissemination.

Possible manifestations include:

• Headache

• Fever

• Altered mental status

• Seizures

• Focal neurologic deficits

⸻

Disseminated Infection

Immunocompromised patients are at greatest risk for invasive and disseminated disease.

The fungus may spread from a pulmonary or other primary site to multiple organs.

Disseminated infection is considerably more serious than localized cutaneous or corneal disease.

⸻

Diagnosis

Diagnosis requires demonstration and identification of the fungus in clinical specimens.

Methods include:

• Direct microscopic examination

• Histopathologic examination

• Fungal culture

⸻

Histopathology

The demonstration of pigmented septate hyphae within infected tissue is an important clue to infection by a dematiaceous fungus.

However, morphology alone may not reliably distinguish Bipolaris from other pigmented molds, so culture or molecular identification may be needed.

⸻

Culture

Fungal culture allows species identification.

Typical colonies are:

• Rapidly growing

• Cottony

• Gray to black

The dark pigmentation reflects the dematiaceous nature of the organism.

⸻

Treatment

Treatment depends heavily on the site and severity of infection.

Corneal Infection

For fungal keratitis, a traditional topical treatment is:

Topical natamycin

Urgent ophthalmologic management is important because progressive fungal keratitis can threaten vision.

⸻

Subcutaneous Infection

Localized subcutaneous infection may require:

Surgical excision

combined with antifungal therapy.

Agents that have historically been used include:

• Itraconazole

• Amphotericin B

• Ketoconazole

• Flucytosine

⸻

Systemic Infection

Severe systemic disease has traditionally been treated with:

Intravenous amphotericin B

Azole antifungals may also have activity, depending on the isolate and clinical syndrome.

⸻

Additional Antifungal Options

Other options include:

• Itraconazole

• Voriconazole

Clinical experience varies according to the infection site and species, so invasive disease should generally be managed with infectious-disease expertise and susceptibility information when available.

⸻

Surgical Management

Surgery is particularly important for accessible localized lesions.

Possible interventions include:

• Excision of subcutaneous lesions

• Debridement of infected tissue

• Surgical management of sinus disease

• Ophthalmologic procedures for severe ocular infection

Antifungal therapy and surgery are often complementary.

⸻

High-Yield Clinical Patterns

Corneal Disease

Eye trauma involving plant or soil material

Progressive keratitis

→ Consider Bipolaris fungal keratitis

Sinus Disease

Allergic rhinitis

Nasal polyps

Chronic fungal sinusitis

→ Consider a dematiaceous fungus such as Bipolaris

Immunocompromised Patient

Pulmonary infection

Neurologic manifestations or multiple organ involvement

→ Consider invasive or disseminated fungal infection

⸻

Exam Essentials

Organism:

→ Bipolaris species

Important species:

→ B. australiensis, B. hawaiiensis, B. spicifera

Type:

→ Dematiaceous filamentous fungus

Environmental reservoir:

→ Soil and plants

Transmission:

→ Traumatic inoculation or inhalation of conidia

Tissue morphology:

→ Pigmented septate hyphae

Culture:

→ Rapidly growing cottony gray-to-black colonies

Major infections:

→ Keratitis, subcutaneous infection, fungal sinusitis, pulmonary disease, meningoencephalitis, disseminated infection

Important sinus association:

→ Allergic rhinitis and nasal polyposis

Corneal treatment:

→ Topical natamycin

Localized disease:

→ Antifungal therapy + surgical excision when appropriate

Systemic disease:

→ Systemic antifungal therapy

Key clinical pearl: Bipolaris is a pigmented environmental mold that should be considered when fungal keratitis follows plant or soil trauma or when allergic fungal sinusitis occurs in a patient with nasal polyposis.