Studies On The Malaria Vector Mosquitoes (Diptera: Culicidae) In Kisumu Area, Kenya In Relation To Malaria Control Using Impregnated Bednets

Abstract

Malaria continues to be a health problem in Kenya, .. . due to the growing resistance to insecticides among the mosquito vectors (WHO, 1970) and increasing resistance to drugs by the malaria parasites. Parasite resistance to chloroquine as a common anti-malaria drug was first reported in Kenya in 1982. These two facts, coupled with inadequate national resources for mounting massive malaria control programme calls for a simple, cheap, sustainable and an appropriate vector control measure such as using mosquito nets. A field trial with mosquito bednets impregnated with 1% permethrin was carried out against malaria vectors in Kisumu to assess the potential of this method as a means of reducing vector densities thus reducing the risk of transmission of malaria. For effective control programmes to be conducted, the ecology and behaviour of the vector should also be understood. Therefore their breeding sites, seasonality and biting habits i. e. peak biting rates were investigated. Host preference was determined by bloodmeal analysis. The sporozoite rates were also determined by dissec~ion and ELISA for sporozoites methods. Six species of anopheline mosquitoes were identified in the area namely; Anopheles gambiae sensu stricto Giles, Anopheles .arabiaensis Patton, Anopheles funestus Giles, Anopheles pharoensis Theobald, Anopheles pretoriensis Theobald and Anopheles necuIipennis

Theobald. Collected data showed that the predominant mosquitoes in the region were An. gambiae s. s. and Anopheles funestus, and were singled out as the main malaria vectors in the area as earlier sited by (Surtees, 1970). Anopheles pretoriensis and An. maculipenis appeared only during the rainy seasons. The two species together with An. pharoensis were of no importance in malaria transmission. Use of, permethrin treated bed nets led to a marked decline in the mosquito densities when compared to the use of untreated bed nets or the no-nets situation. There was statistically, a significant difference found in the house resting densities of mosquitoes in the three study sites. Using the Analysis of variance F was 0.63, at degree of freedom (2,24), at probability level P< 0.05, based on Pyrethrum Spray Catch data. Rainfall had an influence on mosquito house resting densities. Data collected by both the PSC and night catch indicated a marked increase of both An. gambiae 5.1. and An funestus in site 2 and 3 with increased rains while in site 1 increase was negligible. This increase in sites 2 and 3 was attributed to the increased mosquito breeding sites. there was a decrease in the mosquito densities during the short rains and the dry season due to the limited breeding grounds in all the three sites. The bioassay experiments on -the permethrin treated nets showed that the insecticide had a long

lasting residual effect as it was found effective up to

9 months after impregnation 1.e. it could knock down and kill 70% of the exposed test mosquitoes and according to WHO this would be the correct time for re-impregnation. The malaria parasites found among 92 child~en examined were predominantly Plasmodium falciparum Welch, 1897, found in 56 children (64%), Plasmodium malariae Grassi and Feletti, 1890, found in 27 children (30%), and Plasmodium ovale Stephens, 1922, found in 8 children (6%). Children with P. 1!1alariae and P. ovale infections' also had concurred infections of P. falciparu1!1.Blood smears also showed a great decline of parasitaemia amongst children between 0-10 years of age in site 1 (the permethrin treated bed net site). At site 1 Plasmodium falciparum percentage prevalence was reduced from the original 98% during the baseline data to an average of 20% throughout the 11 subsequent months of the data collection period. Site 2 and 3 also had a reduction from 83% to an average 40% and from 90% to an average 60% respectively. There was a significant difference P < 0.05 noted between the percentage prevalence of P. falciparum in the three study sites where by the Chi square was and degree of freedom 2.