Investigation of the specificity of the indirect fluorescent antibody test for the diagnosis of Trypanosoma congolense infection in mice

Abstract

The indirect fluorescent antibody test (I.F.A.T.) has been applied to the diagnosis of T. congolense infection in mice, with particular regard to the specificity of the reaction. Three methods of antigen preparation and several methods of fixation were investigated, Problems encountered included damaged trypanosome morphology loss of parasites after fixation, crystallisation on slides and nonspecific fluorescence. Blood smear T. congolense antigen fixed in N /25O H Cl in normal saline was found to be a sensitive, specific and reliable preparation. Fluorescence was associated with the surface of the parasite and determination of an "end-point" in antiserum titrations was possible. Antigenic quality was related to the level of parasitaemia in the mice from which the antigen was prepared. The limits of reproducibility of the I.F.A.T. using H Cl fixed blood smear antigens were found to be within a four-fold difference in antibody titre and the significance of this is discussed.

Antisera prepared from T. congolense infected mice were tested by the I.F.A.T. using the standard T. congolense antigen. Antibody was detected at low titre one week post—infection and subsequently developed to high levels.

Unexpected features of the pattern of antibody development were attributed to variant specificity and "eclipse" phenomena. Antibody levels in sera from mice infected with T. brucei and T. vivax were low when tested with T. congolense antigen, and it was therefore possible to distinguish between infections caused by different trypanosome species on the basis of higher titres obtained with homologous antisera. Antibodies in antisera from T. congolense infected mice which were cured by drug treatment early in the infection, were still detected 16 weeks after treatment, making it impossible to differentiate between infected and treated animals. Antisera to T. congolense tested with T. brucei and T, vivax antigens reacted at low titres and showed a different character of fluorescence. The significance of this is discussed. Cross reactions were not detected between antisera from mice infected with some non-trypanosomal protozoal infections, and the T. congolense antigen.