Epidemiological investigations of bovine trypanosomiasis in the common fly belt of Zambia

Abstract

The causes of anaemia in cattle were reviewed and it was postulated that trypanosomiasis or malnutrition were the main factors in affecting herd mean packed cell volume (PCV). In 1995/6 18,000 cattle from 495 herds raised in the common tsetse fly belt of Zambia were examined haematologically, and PCV values were recorded with peripheral blood examined for trypanosomes via the buffy coat smear technique. Giemsa stained thick and thin dried blood smears (T&TS) were also examined. The results were entered into a tailor made Integrated Tsetse and Trypanosomiasis Database and were summarised as mean herd PCV, proportion of herd anaemic and parasitological prevalence. Using a linear regression model, 36% of the variance of the mean herd PCV could be accounted for by parasitological prevalence. A logistic regression of the data gave little improvement. The sensitivities of the buffy coat as well as T&TS examinations were calculated mathematically based on the Poisson distribution and these diagnostic techniques were reckoned to be probably more sensitive than previous work had suggested. The spatial distribution of bovine trypanosomiasis and the herd haematocrit values are displayed using a Geographical Information System (GIS). An indirect enzyme linked immunosorbent assay was developed for the detection of trypanosomal antibodies (Ab-ELISA) in serum using crude somatic antigen from T. congolense. The assay was further adapted to carry out tests using circles of dried blood held on filter paper. Inter and intra-assay sources of variation were investigated, as were the effects of sample storage and management. The assay was compared to the indirect fluorescent antibody test, and kinetics of anti-trypanosomal antibody were examined. Twelve thousand blood spot samples were assayed and the data were subject to a rigorous system of quality assurance, with the percentage positivity system of data expression being adopted.