Studies on Factors Affecting the Distribution and Abundance of Phlebotomine Sandflies in a Leishmaniasis Endemic Focus in Baringo District, Kenya

Abstract

Factors affecting the distribution and abundance of Phlebotornine sandf lies were studied during the period 1985 to 1987, in the Marigat leishrnaniasis endemic area, located in Baringo district, Kenya. It was found that the relative abundance of Phlebotomus species collected from burrows was 4-fold those from termite hills, whereas the Sergentomyia species recovered from termite hills was twice the number caught in animal burrows. Both termite hills and animal burrows were the main sandfly breeding and resting sites in the Marigat area. The Sergentomyia species were predominant in this area, representing 97 per cent of the total sandfly population, while the Phlebotomus constituted only 3 per cent of the population. Considerable seasonal variations were observed in both sites. However , sandfly relative abundance increased in the rainy season and decreased in the dry period. The Phlebotomine sandflies were grouped into perennial and seasonal species. Sergentomyia antennatus was the most collected sandfly species followed by~· bedfordi. Phlebotomus martini showed a high capture rate among the Phlebotomus species. Fluctuations in the sandfly population fr om all breeding and resting sites are discussed, in relation to environmental conditions. The abiotic factors, particularly the rainfall, appeared to have important effects er. the distribution and abundance of sandflies. Two major peaks of sandfly abundance were observed in April and August , corresponding to high rainfall. Significant positive correlation coefficients obtained between rainfall and sandfly vectors of leishmaniases, showed that high incidence of the disease occurred mainly during the rainy season. Collections of phlebotomine sandflies in differentes vegetation habitats demonstrated the existence of a high density of flies in large trees. Sergentomyia bedfordi seemed to be more closely associated with the vegetation habitats. The studies on sandfly vertical distribution in two different biotopes showed that only Serqentomyia bedfordi and antennatus reached a height of ten metres and beyond. Also, it was observed that two types of sandfly populations occur in the forested area: the lower species (Sergentomyia inqrarni, S. affinis, S. adleri, S. africanus and S· clydei ) , mainly flying from the ground level up to five metres and the species f l ying beyond six metres (Serqentomyia bedfordi and (antennatus). Relating the sandfly density and the height of flight , it was found that the number of both male and female sandf lies significantly decreased with the height. It was possible to relate the increase of phlebotomine sandflies in a particular habitat to the presence of a certain number of soil chemical and physical characteristics. It was found t hat the growth of t he immature stages depended on parameters such as organic carbon , calcium, potassium, sand and clay. However, other features such as sodium were found to have detrimental effects on sandfly abundance. The identification of bloodmeals , determined the range of possible hosts on which each sandfly feeds , and showed the degrees of anthropophily of the local sandflies . The bloodmeal analyses also provided the actual or potential sandfly hosts for the Leishmania parasite.