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Schistosomiasis


Schistosomiasis is a helminth infection contracted when infective cercariae penetrate the skin of the human host during exposure to infected water (e.g. swimming and bathing). Fertilised females lay eggs which are shed via the lumen of the intestine (S. mansoni and S. japonicum) and of the bladder and ureters (S. haematobium), to spread via faeces or urine.

Symptoms may include fever, cough, abdominal pain, hepatosplenomegaly, eosinophilia and bloody diarrhoea for S. mansoni and S. japonicum or hematuria for S. haematobium.

 

Diagnosis of Schistosomiasis by microscopy

  • Sample type: terminal urine, stool samples and unfixed biopsy.

Definitive diagnosis is by demonstration of the characteristic ova in clinical material. Deposition of ova commences at about six weeks after exposure to the infection but their first appearance (e.g. in urine) may be delayed for several (typically three) months. Confirmation of Schistosomiasis by finding ova should be sought where possible.

For S. haematobium, a terminal urine sample (the last 10 to 20ml of urine passed) is required.

For S. mansoni (and S. japonicum) standard stool samples are the ideal specimens.

Given that S. mansoni and S. haematobium overlap in geographical distribution and can affect both genitourinary and alimentary systems both terminal urine and stool samples should be sent from all patients being investigated for schistosomiasis when serology is positive.

Biopsy material (unfixed) from rectum, sigmoid or bladder is valuable for the detection of ova by crush preparation and permits assessment of their viability. If biopsies are taken, fixed material should also be sent for histology. Rectal/sigmoid scrapings are also useful samples for the diagnosis of schistosomiasis. Such samples must be sent to the laboratory by prior arrangement only.


Diagnosis of Schistosomiasis by PCR

  • PCR is NOT available for these parasites within the Department of Clinical Parasitology.


Diagnosis of Schistosomiasis by serology

  • Sample type: A minimum of 0.5ml of serum is required.

The test should be requested on patients known to have been exposed to fresh water in endemic areas. It starts to become positive approximately six weeks after exposure. In asymptomatic individuals though to have acquired schistosomiasis, serological testing should be delayed until three months post exposure.

The ELISA is reported to detect about 96% of Schistosoma mansoni and 92% of Schistosoma haematobium infections. The test does not distinguish active from treated infections. The actual time taken to become seronegative post treatment varies, but in some patients the test may remain positive for over two years after treatment.

Positive results are reported at Levels 1 to 9. Levels 1 and 2 are regarded as weak positives; Levels 5 and over are strong positives.

It is known that patients may become seropositive through contact with cercaria from animal species of schistosome and probably when harbouring unisexual infection with human species.

The schistosomal egg antigen used in the ELISA may cross-react with the sera of trichinosis cases or with those of hepatitis cases in some instances.

Currently it is not possible to identify the infecting species using our Schistosomal ELISA test.

It is not recommended to retest patients post treatment for at least 18 months after the completion of treatment.