Malaria in Kakamega district: vectors species composition transmission patterns and the effect of mefloquine on sporogony
Résumé
An entomological study was conducted on vectors of malaria and their relative contribution to Plasmodium falciparum transmission in Mumias and Malava divisions in Kakamega district, western Kenya. Mumias supports large sugarcane plantations and lies at approximately 1500 metres above sea level (a.s.l.) Malava lies between 1650 m and 1800m. a.s.l., is forested and basically supports subsistence farming.
Mosquito sampling was carried out weekly for eleven months (May 1995 - March 1996) in human dwellings using pyrethrum spray collection (PSC). The female anopheline mosquitoes collected (n=3213) were identified to species as Anopheles gambiae sensu lato and Anopheles funestus based on morphological characteristic. An. gambiae s.i., the predominant vector species, respresented 83% (n=2667) of the total Anopheles mosquitoes collected in both study sites with An. funestus comprising only 17%. A larger proportion of the Anopheles mosquitoes (n=2524) were collected in Mumias as compared to Malava (n=689). The mean indoor resting densities in Mumias and Malava was 7.0 and 2.0 female anophelines per household respectively.
Polymerase chain reaction (PCR) identified all 802 specimens of the An. gambiae Giles complex tested as An gambiae sensu stricto, an indication that it may be the only sibling species represented in high altitude sites in western Kenya.
ELISA testes performed to determined the presence of Plasmodium falciparum circumsporozoite (CS) protein obtained sporozoite rates of 6.3% (133/2118) for An. gambiae s.l. and 9.5% (38/402) for An. funestus in Mumias. The sporozoite rates in Malava were 6.1% (33/542) and 3.6% (5/140) for An. gambiae s.l. and An. funestus respectively. None of 1600 mosquitoes tested for P. malariae sporozoite protein was positive ELISA tests of mosquito blood meals indicated a high tendency of anthropophagy, a behaviour contributing significantly to malaria transmission by the vector species. 94.5%, 3.74% and 0.2% of the mosquitoes tested had taken at least a blood meal on human, bovine and avian hosts respectively.
Malaria transmission intensity was low as revealed by the low entomological inoculation rates (EIR) recorded in both study sites. Inoculation rates were generally high for An. gambiae s.l. than An. funestus at both study sites. In Mumias the EIR for An. gambiae s.l. was 29.2 infective bites person per year (ib/p/year) and 17.5 ib/p/year for An. funestus. The EIR values in Malava were11 ib/p/year and 3 ib/p/year for An. gambiae s.l. and An. funestus respectively. The estimated time required for a person to receive a single infective bite was 8 days and 26 days in Mumias and Malava respectively.
Despite the low level of malaria transmission, high malaria infection rates were recorded among children in both study sites. In Mumias and Malava parasite rates peaked up at 58.3% and 48.3% during the wet season. While parasite rates were highest in the 5 -9 years age group (range 40.6% - 69%, the highest parasite densities were recorded in children aged 3 to 12 months. The predominant species, P. falciparum, was present in 85.4% of all positive slides, P. malariae in 5.5%, and 9.1% of the slides had both Plasmodium species. Gametocytes were present in 5.6% of the slides.
The results indicate that the intensity of malaria transmission in the high altitude sites is low but perennial with transmission being maintained by two vector species, An gambiae s.i. and An. funestus. They also underline the influence of local ecogeographical factors on vector population and malaria transmission, and hence the need for epidemiological studies in every malaria situation. Vector control measures and improvements in the availability of basic health facilities and services should be given a deserving priority in Kakamega district.
The effect of mefloquine on gametocyte infectivity and development of P. yoelii nigeriensis Killick-Kendrick (Coccidia: Plasmodiidae) in Anopheles stephensi Liston (Diptera: Culicidae) was tested. The drug was administered to mosquitoes either with the infectious bloodmeal or in metabolised form as a second bloodmeal during sporogony. As parameters of drug action in the vector, the number of oocytes on the midgut were counted on day 5-7 post infection and the sporozoite numbers estimated on day 15 p.i. Significant decrease in oocyst numbers was recorded when the drug was offered early in infection or when it was offered together with infective bloodmeal. The study indicates that mefloquine does not enhance gametocyte infectivity and sporogony and may limit disease transmission by interrupting parasite development in the vector.