The resistance advantage – a field genetic background is important for survival of our Wolbachia mosquitoes in Malaysia and reduction of dengue

Banner image: Nancy with scientists from the Wolbachia dengue program at the Institute for Medical Research, Ministry of Health, Kuala Lumpur, Malaysia.

Words and photos: Nancy M. Endersby-Harshman

Our paper published two weeks ago in Insects is the result of a research collaboration between PEARG at the University of Melbourne, the Institute for Medical Research (IMR), Ministry of Health situated in Kuala Lumpur and the University of Glasgow. Controlling the vector mosquitoes through pesticides remains an important approach to reduce and contain outbreaks of the disease. Pyrethroid insecticides provide rapid knockdown of mosquitoes combined with relatively low mammalian toxicity. However, resistance to pyrethroids and other chemical groups is causing problems for mosquito control around the world. Almost three years ago I travelled across to KL to run a training workshop for the IMR scientists in screening for mutations associated with insecticide resistance in Aedes aegypti, the mosquito vector of dengue, Zika and chikungunya.

IMR team members with Nancy in the Wolbachia mosquito laboratory in Kuala Lumpur, Malaysia. Noor Afizah Ahmad (second from left) and Nancy (third from left) are joint corresponding authors on the collaborative paper published in Insects

The team at IMR are actively involved in implementing a Wolbachia mosquito release program for Ae. aegypti aimed at reducing the impact of dengue, a program which has now shown great success and does not rely on insecticides. Wolbachia is a naturally occurring bacterium that is found inside 60-70% of insects and nematodes, but NOT in Aedes aegypti. Once Wolbachia is introduced to Ae. aegypti, it turns the mosquitoes into incompetent vectors so they do not transmit dengue. The bacterium also gives infected female mosquitoes a reproductive advantage over non-Wolbachia females which leads to the Wolbachia mosquitoes replacing the wild population in the release area. Wolbachia mosquitoes are reared in the laboratory before release and must be able to survive in the field where they will encounter insecticides.

Scientists at IMR have expertise in running insecticide bioassays and, in combination with their new skills in detecting sodium channel mutations, we wrote this collaborative paper which provides information for future Wolbachia release programs on an important aspect of quality control. Our study demonstrates the benefits of crossing laboratory mosquitoes to those from the field over generations, so that the mosquito lines acquire field resistance characteristics (mutations in the sodium channel gene – the target site of pyrethroid insecticides). We demonstrate that resistance mutations provide a survival advantage to Wolbachia Ae. aegypti mosquitoes and so they must be maintained in laboratory lines by regular backcrossing, as they may be lost in a long term closed population.

We also describe appearance of a sodium channel mutation in Malaysian Ae. albopictus, a second species of mosquito that vectors dengue, which may indicate that pyrethroid resistance is increasing in this species, making development of non-insecticide control programs an important priority for this mosquito as well. Alternatively, it may be that the mutation has appeared in the Malaysian Ae. albopictus as a result of a genetic invasion (an incursion of mosquitoes which already had the mutation, entering the Malaysian population) meaning that biosecurity procedures may need to address mosquito movement. Distinguishing between these two sources of insecticide resistance mutations in Ae. albopictus is the next avenue of our research, following on from our success in tracking resistance allele movement in Ae. aegypti.

Lunch, Malaysian style, with IMR Wolbachia program scientists including Dr Nazni Wasi Ahmad (on right) and some of the other authors and collaborators on the recent paper