Relationship between biting frequency and surface proteins of circum-sporozoite and merozoite stages in highlands of Western Kenya

Abstract

Vector control in the highlands of western Kenya has resulted in a significant reduction of malaria vectors. This has made the current entomological parameters being used to measure transmission to become insensitive and challenging. Antibodies to the Circum-sporozoite protein (CSP) have been associated with transmission intensity and have corresponded well with EIR. The study objective was to determine the association between the level of circum-sporozoite protein and merozoite surface protein antibodies and the entomological inoculation rates in the Western Kenya Highlands. Indoor resting adult mosquitoes and blood samples were collected in Western Kenya highlands in four selected villages categorized into two valley systems. These were the U shaped (Iguhu and Emutete) and the V shaped valleys (Marani and Fort Ternan). This study was done for eight months, from September 2009 to April 2010. Members of the Anopheles gambiae complex were identified by PCR. Blood samples were collected from children 6–15 years old and the densities categorized by site of home location and age of the children. Exposure to malaria was tested using Circum-sporozoite protein and Merozoite surface protein (MSP) immunchromatographic antibody test. Malaria infection was tested by microscopic examination of thick and thin smears. Sporozoite ELISA was conducted for detection of circum-sporozoite protein which was used to calculate the sporozoite rates and estimate the entomological inoculation rates. Overall indoor resting densities of Anopheles gambiae varied significantly between all the sites except between Emutete and Marani (P<0.05). Anopheles gambiae s.s. composed of 91.3% while Anopheles arabiensis composed 8.7%. Sporozoites were detected in only two of the Sites Iguhu and Emutete with an annual EIR of 3.7 and 2.05 respectively. Prevalence of CSP-MSP antibodies was 29.7%, 38.6%, 16.3 and 12.3% in Iguhu, Emutete, Marani and Fort Ternan Respectively. There was a significant correlation between indoor resting densities of Anopheles gambiae and the Circum-sporozoite and Merozoite surface protein antibodies prevalence in Iguhu and in Fort Ternan, (r2=0.567) and (r2=0.570), (P<0.05) respectively. The study indicates that the biting frequencies were very low with no sporozoites detected in the V shaped valleys. The presence of the Circum-sporozoite and merozoite surface protein indicated an ongoing transmission. This was detected two to three months earlier compared to the EIR. The changing vectorial system may favour Anopheles arabiensis which may pose challenges in vector surveillance due to its zoophilic and exophilic behaviour.