High exposure to malaria vector bites despite high use of bednets in a setting of seasonal malaria in southwestern Mali: the urgent need for outdoor vector control strategies.

BACKGROUND

Early evening and outdoor biting by vector mosquitoes undermines the effectiveness of insecticide-treated nets (ITNs), as users of nets are exposed to vector biting whilst not under a net, both outdoors and indoors. This study assessed exposure to malaria vector bites amongst users and non-users of ITNs in southwestern Mali.

METHODS

Using cross-sectional household survey data of human behaviour and malaria infection prevalence, along with mosquito human landing catch (HLC) data collected in 30 separate communities, the average number of Anopheles gambiae sensu lato (s.l.) mosquito bites per person per night (bppn) received outdoors and indoors were estimated for each survey respondent. The proportion of bites that were not preventable by using a net, the relative contributions of outdoor and indoor residual biting, and the risk factors for exposure to vector bites were estimated.

RESULTS

Despite very high use of nets (93.2%), malaria infection prevalence was 34% overall. A large proportion of respondents (78%) reported being outdoors at 8 pm, but by midnight, 98% were indoors. Net users were exposed to indoor biting for 1 h, on average, between going indoors and going to bed. For 91%, the net used was an ITN. Human biting rates peaked between 2 and 4 am, when most people (90%) were in bed. Individuals using a net received 11.2 bppn in total, of which 7.1 bppn (63%) occurred outdoors. Those not using a net received almost 10 times the number of bites indoors as net users (38.4 bppn versus 4.0 bppn). The total number of bites received by net users was about one third the total number of bites received by non-net users, indicating the proportion of bites not preventable by use of a net alone. Risk factors for biting exposure included not using a net, going indoors late, location near the river and age over 15 years.

CONCLUSIONS

ITNs substantially reduce exposure to indoor biting, but in this setting, net users still received a large number of Anopheles mosquito bites, giving rise to high malaria infection prevalence despite near-universal net use. Most residual biting occurred outdoors, but about a third still occurred with individuals indoors before going under a net. Effective interventions that reduce residual outdoor and indoor biting are necessary to reduce the high malaria burden in settings like southwestern Mali.