A Trial to Control Tsetse at Galana Ranch: Examining the Issues

Abstract

We read with interest the article ‘Trypanosomiasis and tsetse control with insecticidal pour-ons—fact and fiction?’, by M. Baylis and P. Stevenson1xBaylis, M. and Stevenson, P. Parasitol. Today. 1998; 14: 77–82Abstract | Full Text | Full Text PDF | PubMed | Scopus (28)See all References, in which the authors presented the results of a small tsetse control trial using ‘pour-on’, from which they drew, in our opinion, too wide-ranging conclusions.First, the experimental and control areas were, according to our estimates, only about 60 km2 within Galana Ranch, Kenya – a tsetse-infested area covering some 6000 km2. Given that a tsetse barrier has to be at least 10 km in depth to prevent re-invasion, how can the authors have expected to have had any major, or even quantifiable, effect on the overall tsetse populations in this area from such a relatively small treatment zone?In addition, the tsetse data are likely to have been affected by the regular use of an unspecified acaricide for cattle in the non-intervention area; there is no guarantee that the product ‘is ineffective against tsetse’.Finally, the authors conclude that their experiment did not produce sufficient data to allow a coherent assessment of the impact of deltamethrin on tsetse populations at Galana, but then proceed to offer various hypotheses to explain the significantly lower incidence of trypanosomosis (AAT) in the treated area.We comment on two of their hypotheses:(1) The authors conclude that repellency is of no importance in explaining the observed effects of deltamethrin2xThomson, M.C. Trop. Pest. Man. 1987; 33: 329–335Crossref | Scopus (30)See all References, 3xThomson, J.W. et al. Trop. Anim. Health Prod. 1989; 23: 221–226Crossref | Scopus (29)See all References, 4xBauer, B. et al. Trop. Med. Parasitol. 1989; 40: 478–479PubMedSee all References, 5xBauer, B. et al. Trop. Med. Parasitol. 1992; 43: 38–40PubMedSee all References. We have tested more than 30 different pyrethroid formulations and published some of our findings6xBauer, B. et al. ISCTRC. 1993; 117: 276–279See all References. We have never observed any repellency of deltamethrins, but have observed irritant effects which, in conjunction with the defensive reactions of treated animals to tsetse landing, prevented a significant number of flies from probing and feeding.(2) The authors suggest that deltamethrin treatment would result in a reduction in tick burden leading to a general improvement in animal health and thereby improved resistance to trypanosomosis. Undeniably, tick control may have had a beneficial effect on the general health state of the treated animals. However, they supply no evidence to support the suggestion that there would be an impact on the incidence of AAT [eg. change in packed cell volume (PCV)]. Given that all animals in the non-intervention area were regularly treated with an acaricide, it is not entirely logical to suggest that tick-control reduces the incidence of AAT.Both target technology and live bait techniques can effectively control tsetse, but in an appraisal of the relative advantages/disadvantages of different tsetse control techniques, the essential issue is sustainability. Servicing traps or targets may be possible, but simultaneous and full cost-recovery is not, as was indicated by an investigation carried out in Cote d’Ivoire7xSee all References.Careful examination of the authors’ data for both tsetse and trypanosomosis shows very clearly that ‘pour-on’ treatment reduced incidence of AAT seasonally, when tsetse were relatively few, but not towards the end of 1990 and the end of 1991, when flies remained abundant. Such a finding is hardly surprising, given that tsetse suppression was carried out in a small plot amidst an ‘ocean’ of flies.