Schistosoma Species - Schistosomiasis

Schistosomiasis, also known as bilharziasis, is a parasitic tropical disease found especially in sub-Saharan Africa that causes substantial morbidity and mortality. In developed countries, the disease is typically seen in nonimmune travelers returning from endemic areas.  Chronic infection is possible without treatment and may lead to increased risk of liver fibrosis or bladder cancer. Schistosomiasis is often asymptomatic, particularly in chronic disease. Laboratory testing strategies should be informed by careful review of travel and residence history, in addition to clinical evaluation. Schistosomiasis is generally diagnosed by detection of ova in stool and/or urine samples. Serology may also be useful in some situations, including for retrospective diagnosis. 

Quick Answers for Clinicians

Who should be tested for schistosomiasis?

Laboratory testing for schistosomiasis may be appropriate in patients with a history of travel to or residence in an endemic or high-risk area,  such as southern and sub-Saharan Africa. Individuals who have had contact with freshwater sources in these areas should be considered at risk for schistosomiasis. Schistosomiasis may present with rash, fever, headache, myalgia, respiratory symptoms, and eosinophilia.  When present, symptoms generally occur within 1-2 months of infection.  However, it is not uncommon for patients to be asymptomatic in both acute and chronic disease. Patients with a high-risk travel history should be considered for testing, even in the absence of symptoms.

What is the role of ova and parasite examination in the diagnosis of schistosomiasis?

Ova and parasite examination of stool or urine is the recommended laboratory test to diagnose schistosomiasis. Specimen type should be informed by the type of suspected schistosome, based on travel history. Refer to the CDC Yellow Book 2020  for more information on the geographic distribution of Schistosoma spp. Manual ova and parasite examination is a time- and resource-intensive process with variable sensitivity. Due to variable shedding cycles, collection of several samples during a 5- to 7-day period is recommended to maximize diagnostic accuracy. In cases of light infection, ova and parasite examination may not be sensitive enough to detect shedding.

When should serology be performed for schistosomiasis evaluation?

Serology is useful in several circumstances. Due to the intermittent nature of parasite shedding, the ova and parasite test may not be able to detect Schistosoma spp; therefore, it may be necessary to perform serologic testing. Serology can also be used to provide a retrospective diagnosis of past schistosomiasis or to identify asymptomatic people who may have been exposed during travel and could benefit from treatment. Serology cannot distinguish between past and current infection. Additionally, serologic tests are generally not effective until 6-12 weeks after initial exposure.

Indications for Testing

Laboratory testing for schistosomiasis may be appropriate in patients with a history of travel to or residence in an endemic area, such as southern and sub-Saharan Africa. The CDC provides detailed information about areas of high risk. 

Laboratory Testing

The CDC provides additional detailed information about the laboratory testing strategy for schistosomiasis on their Schistosomiasis page  and in their Yellow Book 2020: Health Information for International Travelers. 

Ova and Parasite Examination

Ova and parasite examination of stool or urine is the recommended laboratory test to diagnose schistosomiasis in most cases. Specimen type should be informed by the type of suspected schistosome, based on travel history. The CDC provides information about the geographic distribution of Schistosoma spp in their Yellow Book 2020: Health Information for International Travelers.  Stool specimens are preferred when Schistosoma mansoni or S. japonicum are suspected, whereas urine specimens are preferred when testing for S. haematobium. Ova are generally not detectable in stool or urine until 30-50 days after exposure. Due to variable shedding cycles, collection of several samples during a 5- to 7-day period is recommended to maximize diagnostic accuracy. In cases of light infection, ova and parasite examination may not be sensitive enough to detect shedding. Additional testing may be required to confirm diagnosis, given the low sensitivity of ova and parasite examination, although an artificial intelligence (AI)-based method with increased sensitivity and throughput was recently developed. 

Serology

Serology is useful for light infection (low parasite count), given that ova and parasite examination has particularly low sensitivity in such cases. Serology can also be used to provide a retrospective diagnosis of past schistosomiasis or to identify asymptomatic people who may have been exposed during travel and could benefit from treatment.  Serology cannot distinguish between past and current infection or between members of the Schistosoma spp. Additionally, serologic tests are generally not effective until 6-12 weeks after initial exposure.

ARUP Laboratory Tests

Use to detect ova and parasites in stool samples

Due to the various shedding cycles of many parasites, 3 separate stool specimens collected over a 5- to 7-day period are recommended for ova and parasite examination

A single negative result does not rule out the possibility of a parasitic infection

Use to detect extraintestinal parasites from other body fluids

Aids in the diagnosis of schistosomiasis

Positive results in patients from endemic areas may not represent active infection

Medical Experts

Contributor

Jackson

Brian R. Jackson, MD, MS
Associate Professor of Pathology (Clinical); Adjunct Associate Professor, Biomedical Informatics, University of Utah
Medical Director, Support Services, IT, and Business Development, ARUP Laboratories

References

  1. CDC - Schistomiasis Travel

    Montgomery S. Schistomiasis. In: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. CDC Yellow Book 2020: Health Information for International Travel. Oxford University Press; 2017.