Infectivity rates of vectors of bancroftian filariasis during wet and dry seasons in Malindi and Kwale Districts of Coast Province, Kenya.

Abstract

Bancroftianfilariasis, a disease caused by Wuchereria bancrofti, is on the increase on the Kenyancoast with an estimated 2.5 million people at the risk of infection. 'Endemic zones on the Kenyan coast are Malindi, Kwale, Lamu, Tana River and Kilifi districts. Control of the ,disease is possible by chemotherapy , vector control and vector/man contact. avoidance. For effective vector control and vector / man avoidance in Kenya, there is needfor more enviromental and weather specific vector studies in the endemic zones. Suchstudies include enquiry in to the seasonal variation in infectivity rates of important vector species in a specified area so as to know when to protect oneself from infective mosquitobites and when it is most important to control the vector. This was the main issue of concern of this research. Two study sites on the Kenyan coast known to have bancroftian filariasis were selected. These were Gazi in Kwale district and Madunguni in Malindi district. Houses and compounds from which mosquitoes were to be sampled were selected by simple random sampling. Three methods of mosquito collection were employed; Day resting indoor collection [DRI], Pyrethrum spray catch [PSC] and light traps. The mosquitoes collected were morphologically sorted out into species , females dissected and any Wuchereria bancrofti third stage larvae present recorded. A total of 1832 female mosquitoes were dissected in this study in two phases, the the transmission [wet] season and the nontransmission [dry] season.

A significant difference [x2 =3.05 in Madunguni and 6.18 in Gazi,P<0.05] was found in the vector infectivity rates between the two seasons. The difference was greater in Gazi whose infectivity rate was zero during the non-transmission period than Madunguni whose infectivity rate was 0.21% during the same period. Anopheles gambiae 5.1 was the main vector in both study sites. The others were Culex quinquefasciatus and Anopheles funestus in order of importance. It was concluded that there is a difference in infectivity rates of bancroftian filariasis vectors between the transmission season and the nontransmission season. The abundance of An. gambiae s.s during the transmission season could be responsible for the increased infectivity rates of vectors in this season. *The wet and dry seasons in this thesis are specific for the year 1998 due to the EI Nino effect. The dry season in this case was in October and September whereas the wet season was in June and July 1998.