When Mosquitoes Laugh at Insecticides

Insecticide resistance is a rapidly growing threat to malaria control and elimination programs. Resistance to pyrethroids, which dominate both (1) the insecticide-treated bednet and (2) the indoor residual spraying markets, has been observed in all African countries.

Hunting for mosquito larvae outdoors.

But recent data on the status of insecticide resistance among key malaria mosquitoes in the district of Manhiça (southern Mozambique) are lacking. The latest available data date back 6 years, but indicated that insecticide resistance was an emerging problem in the district (Kloke et al. 2011). As the National Malaria Control Programme (NMCP) of Mozambique is currently designing and scaling up activities to eliminate malaria from southern Mozambique and insecticides play a crucial role, continuous mosquito resistance monitoring is critical.

Hunting for mosquitoes indoors. 

And for those reasons Katey Glunt and I spent 3 months at CISM (The Manhiça Health Research Centre) last year to assess the levels of insecticide resistance in local mosquito populations. This starts at the base: hunting for mosquitoes in the field, either through larval collections from water pools or by collecting adult mosquitoes off the walls inside houses.

Exposing mosquitoes to insecticides at the INS (Maputo).

These larvae and adults are reared to the next generation of adults, which are exposed to insecticides in either standard WHO tube bioassays or by placing them on actual insecticide-treated bednets. These tests showed that the main vector species in the area, Anopheles funestus, is extremely resistant to pyrethroids: only 5-20% of the mosquitoes died after a 1h exposure to the WHO-recommended discriminating dosages. In addition, six hours of exposure were required to kill 50% of the mosquitoes resting on an insecticide-treated bednet. Even though we acknowledge that the operational impact of resistance can only be assessed with clinical trials (looking at the effect on disease manifestation in humans), the results shocked us. The frontline insecticides cannot be used in spray campaigns any longer, and we urgently need to assess if we need to deploy the next-generation bednets in the area (those with an additional compound besides pyrethroids).

Exposing mosquitoes to insecticides at CISM (Manhiça).

These terrible implications aside, I am very proud of this paper: It is the first paper from the Entomology Platform at CISM since 2005. That last paper (Aranda et al. 2005) presented data collected during 1998-1999 (!) and showed that people in Manhiça received on average 15 malarious mosquito bites per person per year (note that  they got bitten another 1235 times by mosquitoes without malaria annually).

The revived Entomology Platform at CISM is determined to continue with the monitoring of insecticide resistance in Manhiça and the surrounding districts. But we won’t stop there:

We are determined to identify the vector species, both primary and secondary species. We are studying mosquito biting and resting behaviors, to identify those mosquitoes that bite outdoors and thus avoid contact with bednets and sprayed walls. We hope to be able to test new vector control products that are currently in development, to see how they perform against our mosquitoes in Southern Mozambique. We give courses to entomologists in the country, with topics ranging from mosquito collection methods to the design of control strategies using collected data. So our team, now led by Lucia Fernandez and our activities are growing rapidly. I am convinced the mosquito hunt will continue, for many years to come.

References:

Aranda et al. 2005. Entomological characteristics of malaria transmission in Manhiça, a rural area in Southern Mozambique. Journal of  Medical Entomology 42(2): 180-186

Glunt et al. 2015. Long-lasting insecticidal nets no longer effectively kill the highly resistant Anopheles funestus of Southern Mozambique. Malaria Journal 2015, 14:298

Kloke et al. 2011. Vectorial status and insecticide resistance of Anopheles funestus from a sugar estate in Southern Mozambique. Parasites & Vectors, 4:16

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