Infectious Disease and Microbiology – Schistosomiasis

Schistosomiasis, also called bilharziasis, is a parasitic infection caused by blood flukes of the genus Schistosoma. Humans serve as definitive hosts, while specific freshwater snails serve as intermediate hosts.

The major species causing human disease are Schistosoma mansoni, S. haematobium, S. japonicum, S. mekongi, and S. intercalatum. The species differ geographically and in the organs predominantly affected.

S. mansoni occurs predominantly in sub-Saharan Africa, parts of the Middle East, South America, and some Caribbean regions. It primarily produces intestinal and hepatosplenic schistosomiasis.

S. haematobium is found mainly in Africa and parts of the Middle East. It predominantly involves the urinary tract and is particularly associated with hematuria, urinary obstruction, bladder-wall calcification, and an increased risk of squamous cell carcinoma of the urinary bladder.

S. japonicum occurs primarily in parts of China, the Philippines, and Indonesia, while S. mekongi occurs around the Mekong River region of Cambodia and Laos. These species primarily cause intestinal and hepatosplenic disease and can occasionally produce neurologic complications.

The major risk factor is skin contact with contaminated freshwater in an endemic region. Swimming, bathing, washing, fishing, or wading in freshwater can expose a person to infective cercariae.

Infected humans release schistosome eggs in urine or feces. When the eggs reach freshwater, they hatch and release miracidia, which infect susceptible freshwater snails.

Within the snail, the parasite develops and eventually produces free-swimming cercariae. Cercariae are released into freshwater and can directly penetrate intact human skin.

After penetrating the skin, cercariae lose their tails and become schistosomula. They enter the circulation, migrate through the lungs and liver, mature into adult male and female worms, and subsequently migrate to their characteristic venous sites.

Adult S. mansoni, S. japonicum, and S. mekongi principally inhabit the mesenteric venous system, whereas S. haematobium predominantly inhabits the vesical venous plexus surrounding the urinary bladder.

Adult worms can survive for many years. Much of the disease is not caused directly by the adult worms but by the host’s granulomatous inflammatory response to deposited eggs.

Some eggs penetrate the intestinal or bladder wall and leave the body in feces or urine. Other eggs remain trapped within tissues or embolize to organs such as the liver, lungs, brain, or spinal cord, producing chronic inflammation and fibrosis.

The earliest manifestation following skin penetration can be cercarial dermatitis, commonly called swimmer’s itch. Patients may develop an itchy, erythematous, papular or urticarial eruption at sites of cercarial penetration.

Acute schistosomiasis (Katayama syndrome) usually develops several weeks after initial infection, particularly in previously unexposed individuals. It represents a systemic hypersensitivity response associated with maturation of the worms and initiation of egg production.

Katayama syndrome may produce fever, chills, headache, malaise, myalgia, cough, abdominal pain, diarrhea, urticaria, lymphadenopathy, and hepatosplenomegaly. Marked eosinophilia is an important laboratory clue.

Chronic intestinal schistosomiasis can cause abdominal pain, diarrhea, and gastrointestinal bleeding. Persistent deposition of eggs in the intestinal wall produces granulomatous inflammation and, in some patients, polyps and fibrosis.

Chronic hepatosplenic disease occurs particularly with S. mansoni and S. japonicum. Eggs reaching the liver induce granulomas and progressive periportal fibrosis.

The classic hepatic lesion is Symmers “pipestem” fibrosis, characterized by extensive periportal fibrosis. Importantly, hepatocyte function can remain relatively preserved until advanced disease or additional hepatic pathology develops.

Progressive periportal fibrosis causes portal hypertension, which can lead to splenomegaly, ascites, collateral circulation, and esophageal or gastric varices. Variceal hemorrhage can be a life-threatening complication.

Pulmonary schistosomiasis can occur when eggs reach the pulmonary circulation, particularly in patients with severe portal hypertension. Chronic pulmonary vascular disease may eventually cause pulmonary hypertension and cor pulmonale.

Urinary schistosomiasis is classically caused by S. haematobium. The characteristic presentation is hematuria, particularly terminal hematuria, together with dysuria or other urinary symptoms.

Chronic urinary disease can cause inflammation, ulceration, fibrosis, ureteric strictures, hydronephrosis, bladder dysfunction, and eventually obstructive uropathy or renal impairment.

Long-standing S. haematobium infection is strongly associated with squamous cell carcinoma of the bladder. This is one of the most important long-term complications of urinary schistosomiasis.

Genital schistosomiasis can affect both sexes. In women, lesions may involve the vulva, vagina, cervix, uterus, or fallopian tubes. In men, the prostate, seminal vesicles, epididymis, testes, or other genital structures may be affected.

Neuroschistosomiasis results from ectopic deposition of eggs in the central nervous system. Cerebral disease may produce headache, seizures, or focal neurologic deficits, whereas spinal involvement can cause myelopathy, weakness, sensory abnormalities, and paralysis.

Diagnosis is traditionally established by demonstrating characteristic schistosome eggs in stool or urine. Because egg excretion can be intermittent or low in light infections, repeated specimens and concentration techniques can substantially increase diagnostic sensitivity.

For suspected S. haematobium infection, urine microscopy is particularly important. Egg excretion often peaks around midday, so appropriately timed urine collection can improve detection.

The morphology of the eggs helps identify the species:

S. mansoni → prominent lateral spine

S. haematobium → prominent terminal spine

S. japonicum → small lateral knob/inconspicuous spine

Serologic antibody testing can be particularly useful in travelers or patients with light infections when microscopy is negative. However, antibodies can remain detectable after cure, so serology generally cannot reliably distinguish active from previously treated infection.

Antigen-detection assays and molecular methods such as PCR can provide additional evidence of infection where available. Their availability and routine use vary by setting.

Eosinophilia is common, especially during acute infection, but its absence does not exclude schistosomiasis. Anemia may develop because of chronic blood loss or advanced disease.

Abdominal ultrasonography can demonstrate periportal fibrosis, splenomegaly, portal hypertension, and other hepatosplenic abnormalities. Urinary ultrasonography can demonstrate bladder abnormalities, hydroureter, or hydronephrosis.

Chronic S. haematobium infection may cause characteristic bladder-wall calcification on imaging. CT or MRI of the brain or spinal cord is indicated when neuroschistosomiasis is suspected.

The differential diagnosis depends on the clinical syndrome and includes other helminthic infections, malaria, visceral leishmaniasis, viral hepatitis, inflammatory bowel disease, urinary tract disorders, and other causes of eosinophilia, portal hypertension, or hematuria.

Praziquantel is the treatment of choice for essentially all major forms of schistosomiasis. Treatment is recommended for infected patients because adult worms can survive for years and continue producing eggs.

For S. mansoni and S. haematobium, the usual praziquantel regimen is a total dose of 40 mg/kg orally in one day, generally divided into two doses.

For S. japonicum and S. mekongi, a total dose of 60 mg/kg orally in one day, generally divided into three doses, is commonly used.

Praziquantel acts most effectively against mature adult worms and is less effective against immature schistosomula. Therefore, patients treated very early after exposure may require repeat treatment after the parasites have matured.

Repeat praziquantel treatment may also be considered when viable eggs continue to be detected after treatment or when infection is believed to persist. Follow-up stool or urine examinations can be used to document parasitologic response when eggs were initially detectable.

Corticosteroids may be required as adjunctive therapy in severe acute Katayama syndrome and are particularly important when significant neurologic schistosomiasis produces inflammatory CNS disease.

Advanced complications require specific management in addition to antiparasitic therapy. Portal hypertension and esophageal varices may require gastroenterologic management, while obstructive urinary disease may require urologic intervention.

Prevention depends primarily on avoiding unsafe freshwater exposure in endemic regions, improving sanitation, reducing contamination of freshwater with human urine and feces, controlling snail populations where appropriate, and population-level treatment programs in endemic communities.

The prognosis is generally good when infection is diagnosed and treated before irreversible organ damage develops. Advanced portal hypertension, pulmonary hypertension, obstructive uropathy, neurologic disease, or malignancy can substantially worsen long-term outcomes.

A useful clinical pattern to remember is:

Freshwater exposure → cercarial skin penetration → eosinophilia/Katayama fever → egg deposition → chronic organ-specific disease.

S. mansoni / S. japonicum → intestine + liver → periportal “pipestem” fibrosis → portal hypertension

S. haematobium → urinary bladder → hematuria + calcification → squamous cell carcinoma