Outdoor biting by Anopheles mosquitoes on Bioko Island does not currently impact on malaria control

Abstract

Background: There have been many recent reports that the rate of outdoor biting by malaria vectors has increased. This study examined the impact this might have on malaria transmission by assessing the association between exposure to outdoor bites and malaria infection on Bioko Island, Equatorial Guinea.

Methods: Responses to questions about time spent outside the previous night from a malaria indicator survey were combined with human landing catch measurements of hourly rates of outdoor and indoor biting for the whole island to estimate the number of outdoor and indoor bites received by each survey respondent. The association between RDT measured malaria infection status of individuals and outdoor bites received was investigated.

Results: The average number of bites received per person per night was estimated as 3.51 in total, of which 0.69 (19.7%) would occur outdoors. Malaria infection was not significantly higher in individuals who reported spending time outside between 7 pm and 6 am the previous night compared to those not spending time outside in both adults (18.9% vs 17.4%, p = 0.20) and children (29.2% vs 27.1%, p = 0.20). Malaria infection in neither adults (p = 0.56) nor in children (p = 0.12) was associated with exposure to outdoor bites, even after adjusting for confounders.

Conclusions: Malaria vector mosquitoes in Bioko do bite humans outdoors, and this has the potential to reduce the effectiveness of vector control. However, outdoor biting is currently not a major factor influencing the malaria burden, mainly because more than 95% of the population are indoors during the middle of the night, which is the peak biting period for malaria vector mosquitoes. The majority of resources should remain with control measures that target indoor biting and resting such as LLINs and IRS.

Figure 1

The indoor and outdoor biting rates averaged across Bioko Island in 2013. Biting rates recorded by indoor and outdoor human landing catches in 2013. The y-axis shows the mean number of times per hour a catcher was bitten by an anopheles mosquito.

Figure 2

Rate at which the population was bitten indoors, outdoors, and overall on Bioko in 2013. The hourly number of indoor and outdoor bites received per person during each hour of the night, Bioko 2013. The estimated total number of bites received by each Bioko residents in one night is 3.51 (area under green line). Of these, 0.69 (20%) were received outdoors (area under blue line), and 2.82 were received indoors (area under red line).

Figure 3

Association between outdoor biting and malaria prevalence by locality. Scatter graph and regression line showing the relation of prevalence of malaria infection in 2 – 14 year-olds and proportion of bites received outdoors at each of the 18 sentinel sites on Bioko in 2013. The slope of the line is 1.10 (95% CI: −1.55 to 3.76, p = 0.39), i.e. a 1% increase in the percentage of bites occurring outdoors corresponds to a 1.10% increase in malaria prevalence in children.