a1 International Trypanotolerance Centre, PMB 14, Banjul, The Gambia
a2 Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK
The severity of the trypanosomiasis problem in a particular location is traditionally assessed in terms of a challenge index – the product of some measure of tsetse abundance and infection rate – which is assumed to be proportional to the force of infection. However, this index masks variation in the force of infection between herds and among individuals within herds. It is also not comparable between sites since the relative abundance of tsetse to hosts may vary. We have studied spatial distribution of herds of cattle in relation to tsetse in The Gambia and calculated an index of challenge based on the ratio of vectors to hosts over the livestock ranging area. This index is strongly correlated with estimates of the force of infection calculated from the incidence of infection in susceptible zebu; and it provides information on heterogeneity in exposure of different herds to tsetse.
(Received September 11 1993)
(Revised November 03 1993)
(Accepted November 08 1993)
p1 Institute for Animal Health, Pirbright Laboratory, Ash Road, Pirbright, Woking GU24 0NF, UK
* Reprint requests to: Dr P. J. M. Milligan, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK