New approaches for the control and eradication of schistosomiasis in Venezuela

Abstract

Schistosomiasis in America with the exception of Brazil, behaves as a chronic mild disease with few clinical manifestations due to low parasite burden. These features restrict the clinical and parasitological diagnosis. The most commonly used stool examination method, Kato-Katz, becomes insensitive when the majority of individuals excrete less than 100 eggs/g of feces. In view that antigen-detecting techniques have not been able to reveal light infections, the antibody detecting assays remain as a very valuable diagnostic tool for epidemiological surveillance. The Venezuelan Schistosomiasis Research Group (CECOICE) has designed a mass chemotherapy strategy based on sero-diagnosis. Since blood sampling is one of the important limiting factors for large seroepidemiological trials we developed a simple capillary technique that successfully overcame most of the limitations of blood drawing. In this sense, ELISA seems to be the most adequate test for epidemiological studies. Soluble egg Schistosoma mansoni antigen (SEA) has been largely used in Venezuela. The sensitivity of ELISA-SEA in our hands is 90%, moreover its specificity reach 92% when populations from non-endemic areas but heavily infected with other intestinal parasites are analyzed. The Schistosomiasis Control Program is currently carrying out the surveillance of endemic areas using ELISA-SEA as the first screening method, followed by the Circumoval Precipitin test for validation assay. The results with these two serological techniques allowed us to defined the criteria of chemotherapy in populations of the endemic areas. On the search of better diagnostic technique, Alkaline Phosphatase Immunoenzyme Assay (APIA) is being evaluated in field surveys.

Laboratory diagnosis of Schistosomiasis in areas of low transmission: a review of a line of research

“…This test is now considered the gold standard test for schistosomiasis. Nevertheless, it is only recommended as a confirmatory test, since it is a labor-intensive technique, that requires the isolation and purification of alive S. mansoni eggs (Alarcón de Noya et al 1992b). Additionally, it is the only test based on the detection of antibodies that correlates with parasite activity (Hillyer et al 1979).…”

“…False positive reactions were observed by ELISA-SEA (15.3%) and 20.8% by ELISA-AWA (Colmenares et al 1993). Based on these results ELISA-SEA was considered to be the screening diagnostic test for the Schistosomiasis Control Program (SCP) of the Minister of Health in Venezuela (Alarcón de Noya et al 1992b). …”

“…In Venezuela, the SCP have monitored the endemic areas by screening the schoolchildren taking ear lobe blood with the micro-hematocrit capillary tube, analyzing the plasma with any of the referred immunoassays, ideally SMP-ELISA and APIA techniques. Those positives must be confirmed by stool examination (Kato-Katz) or by the COPT (Alarcón de Noya et al 1992b). …”

The Last Fifteen Years of Schistosomiasis in Venezuela: Features and Evolution

“…The SCP together with the SRG decided to incorporate serological studies as additional technique of diagnosis in the Control Program. This new strategy was based on the initial screening by ELISA with soluble egg antigen (ELISA-SEA) and confirmation of seropositives by circumoval precipitin-test (COPT) and KatoKatz (Alarcón de Noya et al 1992c). A summary of cross sectional studies carried out in the endemic area by different workers is shown in Table III.…”

“…Selective treatment was restricted to detected cases when COPT or Kato Katz prevalence were less then 20% and massive treatment when prevalence was higher than 20% (Alarcón de Noya et al 1992c). However, pressure for eradicate the disease plus political changes made massive treatments to be administrated twice or without previous evaluation of prevalence.…”