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Clinical Features


Bilharzia Introduction

Schistosomes are blood-dwelling flukes belong-ing to the class Trematoda, but differ from other trematodes having separate adult male and female parasites. Sexual reproduction happens in the definitive host (humans, cattle, etc), depending on the Schistosoma species and the asexual reproduction phase happens in the snail (intermediate host). Cercaria (released by specific snail species in the water) enters the human body through the skin. The young schistosomulum is most susceptible to immune damage. Employing certain evasion mech-anisms, the worm becomes refractory, or even immunologically unrecognisable to certain aspects of the host defense mechanism. Adult parasites may survive for many years in the host, even up to 40 years.

Approximately after 6 weeks post infection the adult worm-pairs start to lay eggs which penetrate the intestinal wall (S. mansoni, S. japonicum) or the bladder wall (S. haematobium) and excreted. A considerable proportion of eggs are not excreted but retained in the tissue inducing granuloma formation with subsequent complications to the different organs affected. The gastrointestinal duct of a schistosome is a cul-de-sac. The parasite has to regurgitate at regular intervals the undigested particulate as well as “parasitic gut associated glycoproteins”. One of the major antigens regurgitated by the parasites is CCA (Circulating Cathodic Antigen). Although Bilharzia eggs also release CCA antigen it is in minute quantities. The major source of CCA originates from live adult worms.

Clinical Significance

The urine-CCA cassette test is a fast and easy to perform methodology in the presumptive detection of Bilharzia in persons with clinical signs and symptoms consistent with an active Bilharzia infection. A positive test result indicates an active Bilharzia infection. The test should only be used as one of the aids for the treating physician in assisting the diagnosis and treatment of an active infection. Positive results are presumptive and should take in account diagnostic regimes such as clinical history, clinical findings, microscopy based diagnosis on urine or stool, serological testing, biopsy and or histological examination of tissue.

 

The test may be false negative in a low parasitic level of infection. The test result should be interpreted with caution during the premature developmental phases of a Bilharzial infection, usually in the first 4-8 weeks after infection which may render a false negative result.

 

Antibody detection against Bilharzia may further confirm the clinical suspicion. Antibodies against Bilharzia however may persist for many years, even after successful treatment making the diagnosis of a re-infection or the diagnosis of an unsuccessful treatment very cumbersome. Antibodies may also be absent in certain cases of chronic active infection. According to certain literature successful treatment after one standard recommended dose of chemotherapy is 65-85%. CCA rapidly declines after successful treatment and a positive test result usually becomes negative within 2-3 weeks after treatment. Re-infection may occur rapidly resulting in a positive test result within 6-10 weeks after initial successful chemotherapy.

Diagnosis of Bilharzia

Laboratory diagnosis of Bilharzia is usually performed by microscopical detection of eggs in urine or stool or by immunological methods (antibody or antigen detection).

 

Microscopic diagnosis is the established gold standard for detecting and confirmation of an active Bilharzia infection. However, expert microscopic diagnosis are often not available, or may also delay the diagnosis of treatment in clinical suspected patients, or may be unreliable or absent in remote areas. The sensitivity of microscopic examinations also depends on the severity of the infection. In low grade infections, the sensitivity of one microscopic examination may be as low as 20%. In clinically suspected cases up to 5 urine specimens (collected over midday), and or 5 stool specimens for microscopic examinations are recommended to increase the sensitivity of the tests.

 

Due to immune modulation the infected host may show a separate IgG, IgM, IgA and IgE antibody response, or a combination of these isotypes. As much as 14% of patients may not respond with any antibody formation. Depending on the methodology used and the timing in the post infected host, the sensitivity of current antibody assays is not optimal (ranging from 65 to 86%). Some of the commonly used methodologies are based on detection of antibodies directed against the soluble egg antigen (SEA). Due to the retention of eggs and the constant secretion of SEA by the deposited eggs, antibodies may be elicited for an indefinite period after the primary infection, irrespective of successful treatment.

 

The urine-CCA cassette test is an antigen detecting test which is present in all Schistosoma species, including animal species. The major portion of CCA released by the adult live parasite is secreted in the urine. A positive CCA test result on randomly collected midstream urine indicates an active Bilharzia infection.

Useful Links

Schistosomiasis Consortium for Operational Research and Evaluation (SCORE)
World Schistosomiasis Risk Chart
Centers for Disease Control and Prevention - Atlanta
World Health Organization - Bilharzia
Wikipedia - Bilharzia
Escargot - Bilharzia
icon SCI Schistosomiasis Control Initiative
icon Stanford University
icon The WHO Tropical Disease Research (TDR) Schistosomiasis Site

   
Schisto World Map
Mansoni
Haematobium
   
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