Parasitological detection of Trypanosoma brucei gambiense in serologically negative sleeping-sickness suspects from north-western Uganda.

Forty-five parasitologically confirmed cases of sleeping sickness were diagnosed in north-western Uganda using a combination of two or three techniques. Forty of the cases were positive by the card agglutination test for trypanosomiasis (CATT), four were negative and one was not screened by the CATT. Trypanosomes isolated from the four CATT-negative but parasitologically positive cases were propagated for detailed biochemical genetic analysis. The aim was to demonstrate whether these four stocks lacked the LiTat 1.3 gene which encodes the antigen on which the CATT is based. All the DNA extracts isolated from these CATT-negative stocks and from six CATT-positive stocks of Trypanosoma brucei gambiense were targeted for amplification by the three variable-surface-glycoprotein genes thought to be ubiquitous in T. b. gambiense. The LiTat 1.3 gene was shown to be present in all 10 stocks. Trypanosome carriers may be CATT-negative because the CATT is not sensitive enough, because their parasites lack the LiTat 1.3 gene, or because their parasites have this gene but do not express it. The four sleeping-sickness cases who gave negative CATT results in the present study have very important implications in the diagnosis of T. b. gambiense infections using the CATT. Following treatment of the CATT-positive cases, the CATT-negative carriers of the trypanosomes remain as human reservoir hosts for continuous infection of the population. Because CATT-negative individuals are rarely examined further, the general prevalence of parasitologically positive but CATT-negative cases is unclear. This study demonstrates the value of co-ordinated use of serological and parasitological techniques in the diagnosis of Gambian sleeping sickness.