Ecology and vectorial capacity of glossina fuscipes fuscipes newstead 1910 on Rusinga Island and along the shores of Lake Victoria, Kenya

Abstract

Studies were carried out on Rusinga Island and the mainland in the Mbita Division of South Nyanza District, Kenya, from November 1987 to December 1989 to determine the distribution and die! activity patterns,population dynamics and responses of Glossina fusci pes Fusci pes Newstead 1910 to variovis odour attractants and trap types. The vectorial capacity of G,f.Fuscipes,a vector of both human and animal trypanosomiasis, was also examined in the field and in the laboratory. The details of the current tsetse/trypanosomiasis situation were assessed in view of the frequent outbreaks of human trypanosomiasis in the neighbouring regions. The distribution of G.F.Fuscipes was confined to strips of thick vegetation along the lake shores. Tsetse flies were caught all around the island. Apparent densities of G.F.fuscipes were found to be remarkably stable on both Rusinga Island and the mainland varying by a factor of five and seven, respectively, during the study period. Low trap catches were recorded during and immediately after the rainy season. In one of study sites on Rusinga island, the absolute population density was estimated at 301 male and 559 females per' hectare from mark-ease-recapture studies. The mortality rates for the fIies derived by two methods (i.e. ovarian age analysis and Moran curves technique) were not significantly correlated. Mortality rate estimates by ovarian age analysis showed a higher mortality on Rusinga island than on the mainland. This high mortality was recorded during the rainy season on the island. Reproductive abnormalities in female flies did not exceed 2 % indicating that the populations were not under stress. Using a random diffusion equation fly movement was estimated at 112 m per day. The short distance travelled implied that if control programmes involving traps were to be initiated they wouId have considerable effect on the tsetsefly populations in areas within tried vicinity of traps (e.g. villages).However, control of the flies from large areas would be difficult. Studies of odour attractants indicated that the use of cow and human urine, acetcne, 1-oc t en-3-ol , phenolic-fractions and washings from a monitor lizard and a goat at attractants did not enhance trap catches. Trap catches of females were inconsistent when acetone and cow urine were added to traps. At times the fly catches increased and at other times decreased. Females were strongly repelled by combinations of acetone, cow urine and phenolic fractions. The causal factors for this response discussed. Of the six trap design compared, the biconical trap was the most effective followed by the pyramidal trap, NG2B and NG2G, Vavoua and F3 traps.Trypanosome infection rates in wild-collected flies were very low (0 % on Rusinga island and 0.1 % on the mainland).Laboratory studies indicated that although G. f. fuscipes was significantl less susceptible to mature infection with T.congolense and T. brucei than G. pHllidipes there were no significant differences in immature infections between the two fly species.It was unlikely that an epidemic of human trypanosomiasis transmitted by G.f.fusaipos would occur on Rusinga island and the immediate area on the mainland. This is because of the confinement of the fly distribution to strips of vegetation along the lake shores, the nature of fly feeding patterns and the absence of trypanosome reservoir hosts in the area.