Evaluation of insecticide-treated cattle as a barrier to re-invasion of tsetse to cleared areas in northeastern Zimbabwe

A field trial in Zimbabwe investigated the efficacy of insecticide-treated cattle as a barrier to prevent the re-invasion of tsetse, Glossina morsitan s and G. pallidipes (Diptera: Glossinidae), into cleared areas. The origina l tsetse barrier consisted of insecticide-treated odour-baited targets, at an operational density of four to five targets per km(2), supported by inse cticide-treatments of cattle with either deltamethrin dip (Decatix(R), Coop ers) at two-weekly intervals, or deltamethrin pour-on (Spoton(R), Coopers) at monthly intervals, in a band approximate to 20 km wide from the reinvasi on front. Tsetse catch, and trypanosomiasis incidence in nine sentinel herd s was recorded for 7-8 months, respectively, before the targets were remove d, leaving only the insecticide treatment of the local cattle to stem the r e-invasion of tsetse. After the removal of the target barrier, the tsetse readily invaded the tri al area and the incidence of trypanosomiasis in sentinel herds increased, w hile their PCVs decreased. After seven months without the targets in place, trypanosomiasis prevalence in the local stock had reached alarmingly high levels; the trial was terminated prematurely and the target barrier re-depl oyed. Immediately after the redeployment of the target barrier, the tsetse catch in the trial area reverted to acceptable levels along the re-invasion front, and trypanosomiasis incidence in the sentinel cattle decreased. It is concluded that, under the conditions of the field trial, the insectic idal treatment of local cattle did not in itself form an effective barrier to tsetse reinvasion. By contrast, the target barrier performed as was pred icted by mathematical and experimental analysis, and readily cleared the ts etse infestation and reduced trypanosomosis incidence in the trial area.