The daily feeding rate of tsetse (Diptera: Glossinidae) on cattle at Galana Ranch, Kenya and comparison with trypanosomiasis incidence

At Galana Ranch, south-eastern Kenya, for 2 days each month from January to May 1993, Glossina pallidipes and G. longipennis were sampled around a heifer for 30 min every hour from 06:00 to 19:00. There was a seasonal decline in tsetse abundance; estimates of the total number attracted to the heifer in 1 day ranged from 556 G. pallidipes in January to 0 in May and 122 G. longipennis in February to 27 in May. The number of tsetse estimated to have fed on the heifer in 1 day during peak months was 260 G. pallidipes and 15 G. longipennis. Trypanosome infection rates of tsetse, obtained from trapped flies, suggest that the heifer received 1.1 T. congolense and 2.2 T. vivax-infected bites per day in January but only 0.007 T. congolense and 0.047 T. vivax-infected bites per day in May. These predictions are compared with the observed incidence of trypanosomiasis in a nearby herd of cattle. There was a linear relationship between the estimated daily rate of infected flies feeding on the heifer and the incidence of trypanosomiasis the following month. The slope of this relationship suggests that the transmission efficiencies of T. congolense and T. vivax from tsetse to cattle are 0.84 and 2.36%, respectively, considerably lower than has been reported elsewhere. Possible reasons for this are discussed and it is suggested that previous estimates may be too high.