Insecticide-Treated Nets Prove to be Effective Malaria Control

The findings of a new study published online in the Malaria Journal at BioMed Central Open Access reinforce the World Health Organization’s current malaria control strategy and the value of making insecticide-treated nets and indoor residual spraying of insecticides universally available—even in countries where the mosquitoes that transmit disease bite primarily outdoors.

The team of researchers, from Australia, assessed the ongoing malaria control activities in the Central Province of the Solomon Islands, where, they note, “significant reductions in [the disease] have been achieved in the past 20 years with insecticide-treated bed nets, indoor residual spraying, improved diagnosis, and treatment with artemisinin combination therapies.” Indeed, officials there have been able to achieve these reductions, the authors emphasize, in spite of the fact that the primary vector for transmission, the Anopheles farauti mosquito, prefers to feed outdoors in the early evening, thereby potentially evading indoor-targeted preventive measures.

“Successful control of malaria must be based on understanding the biology of the mosquito, the parasite, and the human host,” explained study co-author Tom Burkot, PhD, Professor and Tropical Leader, Australian Institute of Tropical Health and Medicine, James Cook University, Cairns, Australia. “Knowing the interactions between these three components will reveal vulnerabilities that can be exploited to control and eventually eliminate malaria. Given the funding shortfall between the resources needed to control and eliminate malaria and what is presently available, it is critically important to build the evidence base for additional control measures that will complement insecticide treated nets and indoor residual spraying.”

For their study, the authors estimated malaria transmission in Central Province using the entomological inoculation rate as well as the vectorial capacity of Anopheles farauti and examined the daily and seasonal biting behavior of the mosquitoes, noting the duration of the feeding cycle via mark-release-recapture. They found that Anopheles farauti was “highly exophagic,” with 72% captured by human landing catches outside of houses in the study area. More than 75% of blood feeding on humans was estimated to occur before 9:00 pm and the proportion of exposure to mosquito bites on humans occurring indoors was only 13%. In addition, they noted that peak densities of host-seeking Anopheles farauti occurred between October and January. The length of the feeding cycle was 2.1 days, during which female mosquitoes dine on five or six blood meals before the extrinsic incubation period is completed.

In their concluding remarks, the authors write that the short duration of the feeding cycle in large part explains the successful control of malaria in the Solomon Islands using insecticidal nets and indoor residual spraying, despite the fact that Anopheles farauti is “primarily exophagic.” The 2.1-day feeding period, and required blood meals during that time, they note, may “translate into substantial population-level mortality… before females would be infectious to humans with Plasmodium falciparum and Plasmodium vivax,” two of the parasites associated with malaria. However, they also emphasize that elimination of malaria in the region “will likely require vector control tools that target other bionomic vulnerabilities to suppress transmission outdoors and that complement the control provided by long-lasting insecticidal nets and indoor residual spraying.”

“Our findings provide an explanation for the seeming anomaly that indoor interventions can effectively control mosquitoes considered outdoor biting,” noted Dr. Burkot. “There are many parts of the malaria-endemic world where increasing the coverage and use of these control measures can reduce malaria rates further. Insecticide-treated nets and indoor residual spraying may not be capable of eliminating malaria on their own but they should be the foundation upon which a successful control program can build.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.