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Study of a bilharziose focus in Ayanabo (Gabon) |
Cahiers d'études et de recherches francophones / Santé . Volume 3, Number 2, 104-11, Mars-Avril 1993, Étude originale
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 Résumé
 Summary
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Author(s) : Michel Bouyssou, Yves Martin-Prevel, Martine Appriou, Jean Tribouley, Jeanne Tribouley-Duret, Christian Ripert, Laboratoire d’immunologie et de biologie parasitaire, Université de Bordeaux II, 146, rue Léo-Saignat, 33076 Bordeaux Cedex, France, Centre international de recherches médicales de Franceville, BP 769, Franceville, Gabon, Laboratoire de parasitologie médicale, Université de Bordeaux II, 146, rue Léo-Saignat, 33076 Bordeaux Cedex, France.. |
Summary : Human schistosomiasis due to S. intercalatum is limited to small foci confined to central Africa. The WHO strategy for the control of the disease is based on reducing morbidity and transmission by selective chemotherapy. This includes the development of efficient diagnostic and monitoring tools. We present a diagnostic aid, based on detecting an antigen characteristic of the genus Schistosoma, which is excreted in the urine. The results of this test are compared with those of the microscopic examination of stools and antibody detection. There were 375 permanent inhabitants in the study village and 323 were at home when the six successive surveys were carried out (August 1989 to July 1990). Three stool samples, one urine specimen and one serum specimen were obtained from each inhabitant. The age pyramid of the 323 persons (148 males and 175 females) showed a deficiency in young men. The prevalence of the disease, measured by egg excretion, was 27.5 %. The prevalences given by the antigen detection test were higher: 34.7 %, 52.3 %, and 35.3 %, respectively with IHAP, HAPr, and ELISA. The antibody detection method indicated that 73.1 % of the population was or had been infected. The coprological tests and immunological analysis did not show a significant difference in prevalence according to sex. Whatever the test used, the level of infection was highest for young people. The IHAP, HAP and ELISA tests were more sensitive and gave peak prevalences children less than 10 years old, while the coprologic analysis and immunologic tests placed it 10 to 14 years. Differences were found in the prevalence of infection according to the site of the houses in the village. Coprological analysis, antigen detection tests and seroimmunology confirmed that the village was separated into two areas, one in the East with a low prevalence and one in the West with a higher prevalence. Examination of the ponds and rivers visited by the inhabitants showed that those located in the West of the village contained the intermediate host of the parasite, Bulinus forskalii. Urinary antigen detection tests were more often positive than those of the coprological analysis. There were correlations between urinary antigen excretion and elimination of eggs in the stools (Chi 2 - 1 ddl \= 4.66 - P <\; 0.005), but the results of the two tests were not completely superimposed. The absence of antigen in the urine of patients voiding eggs in their stools was probably due to the fact that the antigen is only found when it is in excess relative to specific antibodies and that immune complexes escape detection when they are fixed to tissues (liver, kidney, etc.). Dissociation of the complexes should increase the sensitivity of the tests, especially in the case of polysacchande antigens.
Urinary antigen was sought for in 30 patients in a non-endemic zone who were hospitalised in a intensive care ward. Only one negative patient reacted and a brief investigation showed a probable interaction with a non-specific drug structure. In the same time, the Sm 10.27.12 monoclonal antibody reactivity was tested against several parasitic extracts in the IHAP test, and no cross-reactions occurred. In conclusion, the use of Sm 10.27.12 monoclonal anti-body for the detection of polysaccharide antigens in patients living in endemic areas seems to be a good supplementary tool for diagnosis. On its own, however, it cannot be used for diagnosis because of negative results is some patients who void eggs. A new immuno-epidemiological approach is however obtained, because the presence of antigen reflects the active period of the disease and the test shows that many people are infected, especially at the extreme ages of life. In combination with classical tests, it provides a better appreciation of the endemic situation. |
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