Spatial and temporal distribution of four malaria vector species and their relative contributions to Plasmodium falciparum transmission along the south-north transect of Benin, West Africa.

BACKGROUND

To help with planning malaria vector control in Benin, the National Malaria Control Program launched a study to update the distribution of major malaria vectors and their role in Plasmodium falciparum transmission. The study aimed to go beyond the standard entomological inoculation rate (EIR) by incorporating the average sporozoite load of the mosquitoes. This is because the parasite load is a key factor in a successful infection. The research proposed combining the average P. falciparum sporozoite load with EIR to better determine the vectors' true contribution to malaria transmission.

METHODS

The study was conducted across 18 communes in Benin. Within each commune, two villages were chosen for mosquito collection using human landing catches (HLC) and pyrethrum spray catches (PSC). Heads and thoraxes from female Anopheles gambiae s.l. and Anopheles funestus mosquitoes were analyzed for the Plasmodium falciparum circumsporozoite antigen using the ELISA/CSP method. The corresponding carcasses were used for species identification via PCR. The P. falciparum sporozoite load was quantified in CSP ELISA-positive mosquitoes using the NZYTech real-time PCR kit. The contribution of vectors to P. falciparum transmission was first estimated by considering both their infection and bite rates. Subsequently, the relative contribution to transmission was further assessed by correlating the P. falciparum sporozoite load of the primary vectors with EIR.

RESULTS

Anopheles coluzzii is responsible for 63.01% of malaria transmission, with an EIR of 87.7 infecting bites per person per year. This is followed by Anopheles gambiae, which accounts for 36.7% of transmission and has an EIR of 51.1 infecting bites per person per year. The contribution of Anopheles funestus is 0.24%. The study found that An. gambiae carries a higher load of Plasmodium falciparum sporozoites than An. coluzzii. Specifically, approximately 30% of An. gambiae individuals carried more than 10,000 sporozoites in their salivary glands, while less than 10% of An. coluzzii individuals had a comparable load.

CONCLUSION

This study clarifies the true contribution of malaria vectors to Plasmodium falciparum transmission by linking sporozoite load to the EIR. The findings will allow for a more accurate assessment of the vectors' role in P. falciparum transmission in Benin.