BACKGROUND

The World Health Organization (WHO) recommends mass distribution of insecticide-treated nets (ITNs) to prevent malaria transmission. Unfortunately, resistance to pyrethroids affects the efficacy of standard ITNs. To overcome this resistance and continue to protect the population, the WHO has recommended new types of ITNs that combine a pyrethroid insecticide with either a synergist (PBO) or a second insecticide, such as chlorfenapyr. This study examines the baseline characteristics of malaria vectors prior to the distribution of three types of insecticide-treated nets as part of a three-arm randomised controlled trial: Interceptor G2 (pyrethroid-chlorfenapyr), VEERALIN (pyrethroid-PBO), and MAGNet (pyrethroid only).

METHODS

The study was carried out in 40 villages (grouped into 33 clusters) of Tiébissou district in central Côte d'Ivoire. To assess biting rate and biting behaviour, human landing catches were conducted hourly indoors and outdoors in six randomly selected houses in each cluster, starting at 18:00 and continuing until 08:00 the next morning. Adult mosquitoes collected were morphologically identified, and a subset of Anopheles gambiae sensu lato (s.l.) and An. funestus s.l. were speciated by quantitative PCR (qPCR). Plasmodium sporozoite infections were detected by qPCR to estimate infection rates. The entomological inoculation rate was calculated as the product of the mosquito biting rate and the sporozoite infection rate.

RESULTS

Among the 10,698 mosquitoes collected, An. gambiae s.l. was the predominant species, accounting for 62.5% (n = 6683) of the catch, followed by An. funestus s.s., which accounted for 19.8% (n = 2120). Of the sub-sample of An. gambiae s.l. processed by PCR, 79.0% (n = 1291/1635) were An. coluzzii and the remaining were Anopheles gambiae s.s. Malaria vectors were highly aggressive, with an average of 14.8 bites/person/night for An. coluzzii, 2.0 b/p/n for An. gambiae s.s. and 5.4 b/p/n for An. funestus, representing an overall average of 22.2 b/p/n (95% CI 17.2-27.2 b/p/n). No significant difference was found in biting activity between indoor and outdoor environments (Z = -0.25, P = 0.803). Plasmodium sporozoite infection rate was 2.4% (95% CI 1.3-3.6%) for An. coluzzii, 1.5% (95% CI 0.3-2.6%) for An. gambiae s.s. and 2.7% (95% CI 1.2-4.3%) for An. funestus. The estimated overall entomological inoculation rate was 0.4 infected b/p/n (95% CI 0.3-0.6) and varied between 0.0 and 0.2 infective bites/person/night according to species. There was no difference observed in entomological infection rate (EIR) between capture locations (indoors versus outdoors; Z = 1.521, P = 0.128).

CONCLUSIONS

This study shows that An. coluzzii and An. funestus were the main malaria vectors and showed similar biting patterns both indoors and outdoors. Anopheles funestus was found in high density in a limited number of villages. Malaria transmission was high despite universal distribution of pyrethroid-ITN in the district.