The effect of repeated washing of long-lasting insecticide-treated nets (LLINs) on the feeding success and survival rates of Anopheles gambiae

Abstract

Background: Insecticide-treated nets protect users from mosquito bites, thereby preventing transmissions of mosquito borne pathogens. Repeated washing of nets removes insecticide on the netting rendering them ineffective within a short period. Long-lasting insecticide-treated nets (LLINs) offer longer time protection against such bites because they are more wash resistant, and are preferred to conventionally treated nets. However, there is limited information on the effect of repeated washing of LLINs on the feeding success and survival of wild malaria vectors.

Methods: The current study evaluated the effect of repeated washing of four brands of LLINs on the feeding success and survival rates of Anopheles gambiae sl reared from wild strains. In this study, two- to five-day old F1s, reared from gravid mosquitoes collected from an area with a high coverage of LLINs were offered blood meals through protective barriers of the above LLINs. Mosquitoes were exposed for a period of 10 minutes each time. Nets were tested unwashed and subsequently after every 5th through wash 15. After exposure mosquitoes were sorted out according to their feeding status. They were then held under normal laboratory conditions for 24 hours and mortality was scored in both fed and unfed.

Results: It was observed that mosquitoes did not feed through a barrier of unwashed LLINs. However, the feeding success and survival rates increased with successive number of washes and were also net brand dependant. After 15 washes, 49% of vectors succeeded to feed through a protective barrier of PermaNet 2.0 and 50% of the fed died after 24 hrs while after the same number of washes 60% of vectors succeeded to feed through Olyset brand of LLINs and all of them survived. In general, more mosquitoes survived after feeding through Olyset compared to the other four brands that were evaluated. When efficacy of individual LLINs was compared by a t-test analysis to a conventionally treated net, the results were not significantly different statistically for Olyset (p = 0.239) and NetProtect (TNT) (p = 0.135). However, the results were highly significant when comparison was made with PermaNet and Interceptor (BASF); p values 0.015 and 0.025 respectively.

Conclusion: The result of this study shows that repeated washing of LLINs at short time intervals using local washing methods may render them infective within a short time in preventing local vectors from feeding.

Figure 1

An. gambiae feeding rates on the four brands of LLINs before and after repeated washing. shows the percentage of An. gambiae sl that succeeded in feeding on each brand of net after wash 0, 5, 10 and 15 respectively. The nets were hand washed in rain water using a local detergent OMO. Nets were air-dried outdoors under the shade.

Figure 2

Survival rates of fed mosquitoes. shows survival rates of blood fed mosquitoes after repeated washing, 24 hours after exposure.

Figure 3

Mortality rates among the unfed mosquitoes. shows the mortality rates among the mosquitoes that did not succeed in obtaining a blood meal through the four brands of LLINs after 24 hrs post exposure.