THE PROBLEM OF INTERPRETATION OF THE CATT IN MASS-SCREENING FOR TRYPANOSOMA-BRUCEI-GAMBIENSE SLEEPING SICKNESS

Mass screening for Gambiense sleeping sickness is usually done with the Card Agglutination Trypanosomiasis Test (CATT) in series (total blood CATT followed by a serum CATT if the first test is positive) and the search for trypanosomes in cervical adenopathies. At present, the double positives (blood CATT and serum CATT) as well as the subjects in whom the trypanosome was found (in the blood or gland juice) are treated. The existence of patients whose gland punction was proved positive whereas the total blood CATT remained negative, has led the authors to make a survey with the CATT in parallel (on blood and serum CATT) on a 2,030 subjects sample in the Boko Songho site (Congo - Bouenza area). Whereas the prevalence of the positive cases to blood CATT and serum CATT (CATT in series) is 6.8 %, the prevalence of the positive cases to at least one of both CATT's (CATT in parallel) is 19 %. The 12.2 % discordant results (blood + /serum - or blood-/serum +) have been reexamined a month later with the CATT and with the Indirect Fluorescent Antibody Test (IFAT). Among these, 22.3 % had become positive to blood and serum, whereas 30.6 % had become negative. The latter group presents the problem of cross-reactions. The authors performed an IFAT with Trypanosoma congolense (T.c.) as antigen on an 18 subjects sample with a discordant CATT. All IFAT performed with T.c. were positive to a 1/50th threshold whereas some of the serum remained negative with IFAT when Trypanosoma brucei gambiense had been used as an antigen. The authors evaluate the choice of diagnostic chain (CATT in series or CATT in parallel) and discuss the interpretation of the discordant results.