The role of host cues in the transmission of sleeping sickness

Tsetse (Glossina spp.) transmit species of Trypanosoma which cause trypanosomiases in livestock and humans. To improve the cost-effectiveness of baits used to control tsetse, studies were made of the host-oriented behaviour of the following Palpalis-group species: Glossina tachinoides and G. palpalis gambiensis in Burkina Faso, G. p. palpalis in Côte d’Ivoire, G. fuscipes quanzensis in the Democratic Republic of the Congo, and G. f. fuscipes in Kenya. In each country, electrocuting grids and traps were used to quantify the responses of tsetse to natural and artificial host stimuli. The results showed that riverine tsetse respond to certain natural host odours. For example, studies of the numbers of tsetse attracted to traps or grid baited with natural host odours showed that cattle odour doubled the catches of G. p. gambiensis and increased the numbers of G. tachinoides by five-fold; pig odour increased the catch of G. p. palpalis five-fold and doubled the numbers of G. f. quanzensis; and lizard odour doubled the catch of G. f. fuscipes. Responses of G. tachinoides and G. p. gambiensis to natural host odours were due largely to kairomones identified for savannah-tsetse (carbon dioxide, 1-octen-3-ol, acetone and 4-methylphenol). For instance, blends of 3-n-propylphenol, octenol, 4-methylphenol and acetone increased catches of G. tachinoides about five-fold, it doubled the catches of G. p. gambiensis and increased the catches of G. p. palpalis about 1.5-fold. Comparable catch ratios were obtained when acetone was removed from the blend; both G. tachinoides and G. palpalis were attracted by CO2. None of these chemicals was effective for G. f. fuscipes, suggesting that unidentified semiochemicals are present in lizard odour. For G. f. fuscipes, the response of female flies increased from 18% to 24% with lizard odour, but mammalian odours did not have any affect. For G. tachinoides the landing response increased significantly with cattle odour in one experiment only, and none of the odours had any effect in the landing responses for other species. The use of odours in control operations is discussed. Studies of visual stimuli showed that large targets (1m2) doubled the catches of G. p. palpalis and G. f. fuscipes compared to 0.25m2 targets, the smallest being eight times more cost-efficient. Horizontal oblongs were more attractive than vertical ones for G. f. quanzensis and vice versa for G. p. palpalis. For all species, square targets were as effective as the most attractive oblong. Landing responses were generally about 30%, and although consistently higher for larger targets, differences were not statistically significant. The addition of flanking nets increased the catches about four-fold. In conclusion, results suggest that cost-effective control of Palpalis-group tsetse could be achieved by using tiny targets (0.25×0.25m) flanked by nets of the same size