Studies on The Phlebotomine Sandflies of Rusinga Island and Their Potential as Vectors af Leishmaniasis

Abstract

This work involved a one-year study of the phlebotomine sandflies of rusinga island. The existence of sandflies had been reported in the vicinity by the medical vectors research programme of the international centre for insect science and physiology (icipe) in 1986 (anon. 1988) but no work had since been carried out on the flies of this region. Due to the toxicity, expense and lack of total effectiveness of drugs for treating leishmaniasis (lainson and shaw, 1978) control may have to depend on effective integrated management of reservoirs and/or insect vectors. It is thus important to study the sandfly population and to find out if known vectors and reservoirs of the disease do exist in places in which sandflies are known to exist and determine what potential they have as vectors and reservoirs of leishmaniasis. It is also necessary to find out if the disease has a local focus or whether it has the potential of being introduced. The objectives of the study were threefold:- 1. To identify the sandflies of rusinga island and determine their species incidence 2. To determine their seasonal incidence and climaticfactors affecting their abundance 3. To determine their importance as medical or veterinary vectors through dissection and bloodmeal analysis. The sticky trap (polythene sheet smeared with castor oil), mouth-suction trap and fan-suction light traps were used to trap the flies. A standard size of one metre by one metre was used for the sticky trap. The sticky trap and fansuction traps were set in the evenings between 1730 hours and 1845 hours and left overnight. They were removed the following morning between 0600 hours and 0730 hours. The traps were set in twenty-eight different sites(approximately 2 kilometres apart) in five sectors of the island. The flies were then washed and mounted. Identification was then done using keys developed by lewis (1973), abonnenc (1972), kirk and lewis (1951) and keys developed by mutinga (unpublished); all based on detailed morphological characteristics . Fresh female flies (those removed from the traps and worked upon immediately) were dissected and any• suspected promastigotes were cultured in novy, mac'neal and nicolle's (nnn) medium (taylor and baker, 1986) for five days. Fed female flies were used for bloodmeal analysis. This analysis was done by dr. C. Staak at therobert von ostertag institut (berlin, germany). Weather data were collected daily from the rusinga meteorological station. A total of twelve sandfly species were found on the island and eleven of them belonged to the serg~ntomyia genus whileone belonged to the phlebqtomus genus. When the anova test was done the difference between the species was found to be highly significant (f = 65.24, p < 0.0001, df = 8). The most abundant species was sargentomyia antenatus followed by sargentomyia bedfordi then s.serratus s, schwetzi s igrami s africanus s. Squamipleuris and s affinis descending order. Only two flies of the species s. Grainge,ri and s. Clydei and one fly each, of the species , s. Inermis, the mean number of female flies was found to be more than that of the male flies. Among the habitats from which the flies werecollected, they were most abundant in tree holes, followed by termite hills, toilets, outside houses, inside houses, cow-sheds and in the vegetation and rock crevices at the lakeshores. Statistical analysis showed that relative humidity, windspeed and hours of sunshine had a significant correlation with the abundance of the flies. Relative humidity had a significant negative correlation (r = -0.1009, p < 0.0001, df = 1898) as did wind-speed (r = -0.0488, p < 0.0019, df = 1898). During the month of april there was a negative significant relationship between sand fly abundance and rainfall. The "peak season" for the flies occurred during the months of june and October, that is, just after the rainy season and just before the onset of the short rains (november-december). There was a significant difference between the five sectors of the island as far as the population of flies in them was concerned. Three of them (on the southern side of the island) were different from the two sectors on the northern part. This was also reflected when correlation analysis was done for fly population and humidity in the different sectors. There was a significant negative correlation between humidity and sandfly population in three of the sectors and not in the other two, implying that the differences mentioned earlier could be due to differences in the humidity of the various sectors. Some species of the flies did not appear throughout the year, for example, sergentomyia affinis was only found during the short rains and the peak season for the sandflies (June and October). Other species were "perennial" and were caught throughout the year. None of the "flagellates" inoculated in the nnn medium pr?ved positive for leishmania parasites. The blood-meal analysis showed that the fed flies caught fed on chicken and monitor lizards; none was anthropophilic. They apparently preferred monitor lizards (66.67 %) to all other animals. Some of the fly species that were found are alternate zoonotic vectors elsewhere (for example sergentomyia ingrami is an alternate zoonotic vector of leishmania major in baringo district). However, at the moment the sandflies of rusinga island though having potential of being vectors of leishmaniasis do not pose any present danger of being vectors since they neither have the parasites nor are they anthropophilic. But if the parasites are transported from an endemic focus to the island, there could be a possibility of the disease occurring in the area, just in the same way that visceral leishmaniasis was introduced into kenya from sudan (adler, 1964).