Assessing the effect of insecticide-treated cattle on tsetse abundance and trypanosome transmission at the wildlife-livestock interface in Serengeti, Tanzania.

Jennifer S Lord,Liam J Morrison,Stephen J Torr,Glyn A Vale,Harriet K Auty,Rachel S Lea,Fiona K Allan,Edith Paxton,Mechtilda Byamungu,Furaha Mramba,John W Hargrove,David R Hall,Jessica Lingley

doi:10.1371/journal.pntd.0008288

Abstract

In the absence of national control programmes against Rhodesian human African trypanosomiasis, farmer-led treatment of cattle with pyrethroid-based insecticides may be an effective strategy for foci at the edges of wildlife areas, but there is limited evidence to support this. We combined data on insecticide use by farmers, tsetse abundance and trypanosome prevalence, with mathematical models, to quantify the likely impact of insecticide-treated cattle. Sixteen percent of farmers reported treating cattle with a pyrethroid, and chemical analysis indicated 18% of individual cattle had been treated, in the previous week. Treatment of cattle was estimated to increase daily mortality of tsetse by 5–14%. Trypanosome prevalence in tsetse, predominantly from wildlife areas, was 1.25% for T. brucei s.l. and 0.03% for T. b. rhodesiense. For 750 cattle sampled from 48 herds, 2.3% were PCR positive for T. brucei s.l. and none for T. b. rhodesiense. Using mathematical models, we estimated there was 8–29% increase in mortality of tsetse in farming areas and this increase can explain the relatively low prevalence of T. brucei s.l. in cattle. Farmer-led treatment of cattle with pyrethroids is likely, in part, to be limiting the spill-over of human-infective trypanosomes from wildlife areas.

Highlights

In East and Southern Africa, tsetse flies (Glossina spp) transmit Trypanosoma brucei rhodesiense, which causes Rhodesian human African trypanosomiasis (r-HAT)
Existing r-HAT foci are often associated with wildlife areas, the importance of cattle as reservoirs at the wildlife-livestock interface is unclear [1]
Ethical approval for this work was obtained from the SRUC Animal Experiments Committee and the Commission for Science and Technology (Costech) in Tanzania

Summary

Introduction

In East and Southern Africa, tsetse flies (Glossina spp) transmit Trypanosoma brucei rhodesiense, which causes Rhodesian human African trypanosomiasis (r-HAT). Tsetse transmit T. congolense, T. vivax and T. brucei, the causative agents of animal African trypanosomiasis (AAT) in livestock. Trypanosoma brucei s.l., T. congolense and T. vivax can circulate in transmission cycles involving livestock or wild mammals [1]. The extensive conservation areas of East and Southern Africa that support tsetse, as well as wildlife, can be foci for r-HAT and AAT. At the interface of wildlife and livestock areas, there is potential for trypanosomes to shift from a wildlife- to a livestock-dominated cycle of transmission [1]. Existing r-HAT foci are often associated with wildlife areas, the importance of cattle as reservoirs at the wildlife-livestock interface is unclear [1]

Objectives

Methods

Results

Conclusion