Behavioral adaptation of Anopheles funestus to long-lasting insecticide-treated nets.

TO THE EDITOR—Monitoring the behavioral changes of malaria vectors after implementation of vector control interventions is of great importance to public health, with 3.3 billion people at risk for malaria, 216 million malarial infections, and 655,000 deaths due to malaria in 2010 [1]. Moiroux et al recently showed that, after 3 years of universal long-lasting insecticide-treated net (LLIN) coverage in 2 villages in Benin, one of the most aggressive malaria vectors in Africa had evolved from a primarily nocturnal vector to a diurnal vector and shifted from indoor to outdoor biting [2]. The authors reported that the median catching time (MCT), measured using the human-landing collection (HLC)method, significantly shifted, from 2:00 AM at baseline to 5:00 AM at the final round of collection, and that, in one village (Tokoli), the rate of exophagy increased from 45.6% at baseline to 60.9% at the final round of collection [2]. These findings are of great concern because they have disastrous implications for LLINs as a control strategy aimed at avoiding human infections by targeting vectors indoors and at night [3]. However, there are a number of limitations that could explain the results, and perhaps the change in behavior from indoor to outdoor biting was not due to the LLINs. Moiroux et al addressed the environmental factors that can affect mosquito behaviors. However, they failed to comment on the limitations of the HLC method as a possible explanation for their results. Indeed, despite being a widely used and accepted method in malaria vector surveillance, HLC is not without limitations. The method is dependent on the skills and experience of the collectors and on the natural attraction of mosquitoes to humans [4]. The authors specified that they used volunteer collectors but failed to elaborate on the training and experience of these volunteers. The collectors’ individual behavior and their commitment and adherence to mosquito-collection protocols could have affected their use of the HLC method, which would explain the variability observed in the number of mosquitoes collected indoors versus outdoors and between rounds of collection, as reported by Moiroux et al in Table 3 of their article. An even greater oversight is that the authors failed to mention that, throughout the study period, one of the 2 villages, Tokoli, where the change from indoor to outdoor biting behavior was observed was randomized to receive indoor residual spraying with carbamate as part of another study performed by their team and published elsewhere [5]. With their results, Moiroux et al have simply reiterated the evidence that indoor residual spraying of insecticide is a malaria-control intervention that is effective at killing or repelling endophilic vectors [6]. In their conclusion, the authors advise that further research be performed to find new interventions that tackle malaria vectors outdoors and during the day. However, several studies have shown LLINs to be a cost-effective and proven intervention in preventing malaria transmission in countries of endemicity, and efforts are underway to provide nets to underprivileged families at risk for malaria [1, 7, 8]. The authors should abstain from discouraging the use of LLINs, until after the aforementioned limitations of their study are addressed.