Trypanosomiasis in domestic animals: the problems of diagnosis.

Animal trypanosomiasis presents special problems with regard to diagnosis. The clinical signs are not pathognomonic and the standard techniques for the detection of trypanosomes are not sufficiently sensitive. Although significant improvements have been made in diagnosis, a high proportion of infections still remain undetected as the chronic, more common form of the disease, is often aparasitaemic. In the face of these constraints, alternative methods of diagnosis have been developed, most of which are for the detection of antibody responses to the antigens of the infecting trypanosomes. The most useful of these tests, in view of their sensitivity and specificity, are the indirect immunofluorescent antibody test, enzyme immunoassay (ELISA) and the card agglutination test for trypanosomiasis (CATT) which is used for the diagnosis of Trypanosoma evansi infections. However, there are several shortcomings in antibody detection tests: the antigens used are ill-defined, thus making standardisation of the tests rather difficult with regard to sensitivity and specificity. Furthermore, some of the tests are not applicable to the field. Moreover, the presence of antibody in the serum does not necessarily reflect an existing infection, as antibodies may persist for several months following recovery. Recently, development of assays for the detection of circulating trypanosomal antigens in the blood of infected animals has circumvented this problem since antigen-positivity indicates existing infection. These new assays have not yet been fully evaluated in the field, but the data generated so far do indicate that the diagnostic strategy for the future is likely to be a combination of one of the more sensitive standard trypanosome detection techniques with antigen-trapping ELISA.