Over 200 million people worldwide are infected with schistosomiasis, with approximately 200,000 annual deaths. Transmission has been documented in over 75 tropical countries; it is considered the 2nd most devastating parasitic disease, behind malaria. Humans become infected by contact with blood flukes in contaminated freshwater, usually through bathing, swimming, fishing, or agricultural work. The infectious larval form of the worm, known as cercariae, penetrates the skin, develops into an adult, and deposits eggs causing a host immune response and tissue damage.
Acute symptoms are most often seen in nonimmune individuals such as travelers. Cercarial dermatitis, or swimmer’s itch, is a pruritic papular rash discovered immediately following exposure. Katayama fever may occur 2 to 8 weeks after exposure from systemic hypersensitivity and resembles serum sickness: fever, cough, chills, myalgias/arthralgias, diarrhea, lymphadenopathy, and hepatosplenomegaly. Resolution usually occurs after a few weeks, but patients can progress to coma or death.
Chronic symptoms are related to parasite load and involved organs. Intestinal symptoms (caused by Schistosoma mansoni) include abdominal pain, poor appetite, bloody diarrhea, with possible hepatosplenomegaly, periportal fibrosis, ascites, and portal hypertension. Urogenital disease (caused by Schistosoma haematobium) typically causes hematuria and dysuria, as well as genital lesions and vaginal bleeding in women. It can progress to fibrosis and calcification of the bladder and ureters, leading to hydroureter/nephrosis, infection, glomerular disease, and bladder cancer. Schistosomiasis is also associated with neurologic and pulmonary complications, including transverse myelitis, seizures, and pulmonary hypertension. Diagnosis is made through egg detection via stool or urine microscopy. Anemia and eosinophilia can be found in up to two-thirds of patients. Serology is useful for diagnosis in travelers.
Management and Disposition
Cercarial dermatitis usually resolves within 1 week; topical or systemic glucocorticoids may be considered. Therapy for Katayama fever is supportive, although glucocorticoids might be a useful adjunct. Praziquantel is effective against all forms of chronic disease but is not active on immature worms. Thus, patients with acute Katayama fever require delayed treatment in order to eliminate matured worms. Central nervous system disease is treated with a combination of praziquantel and steroids.
Hematuria is a common presenting symptom in areas endemic for S mansoni. These patients are often treated empirically with praziquantel.
Schistosomiasis can impact other infections, leading to more severe disease in patients with hepatitis B or C virus, HIV, and malaria.
Prevention includes avoiding high-risk waterways, wearing protective boots, and vigorous toweling/drying following contact/exposure.
Swimmer’s itch can occur as an initial manifestation of human schistosomal infection but is most often caused by avian schistosomes and is not otherwise infectious.
Schistosomiasis. Bladder thickening and calcification in a patient with S haematobium infection. (Used with permission from the J.D. MacLean Centre for Tropical Diseases at McGill University.)