Australasian Science Magazine

By Stephen Luntz

Insecticide resistance has allowed diseases like malaria and dengue to kill millions, but its introduction could prove the key to their defeat according to Prof Ary Hoffman of the University of Melbourne’s Department of Genetics.

The Wolbachia bacterium infects many insects and other arthropods. It is symbiotic with some species, but Hoffmann has demonstrated that certain Wolbachia strains shorten mosquito lifespans (AS, August 2005, p.9). Infectious diseases require time to develop within mosquitos to become transmissible, so only older mosquitos spread disease. As Wolbachia only affects older members of the population it is unlikely that resistance to it will develop.

Hoffmann says that once Wolbachia becomes established in a mosquito population it tends to be maintained. “Wolbachia causes cytoplasmic incompatibility, where when an infected male mates with an uninfected female 100% of the embryos die,” he says.

Infected females pass the bacterium to their offspring irrespective of the father’s infection status. Once a threshold has been reached, the chances of uninfected males and females mating is low enough that the infected population maintains its dominance.

The challenge is to make Wolbachia the norm. One path lies with the wMelPop strain, which isn’t particularly effective at killing the older mosquitos and even seems to offer some advantage, allowing its rapid spread. However, it also blocks the transmission of dengue and other viruses, although malaria-carrying mosquito species have proven harder to infect.

In a study published in Proceedings of the Royal Society B, Hoffman has reported the introduction of genes for resistance to certain insecticides into Wolbachia-carrying mosquitos. These are passed onto the infected offspring, giving them an advantage in any area where the relevant insecticide is used.

“You wouldn’t apply resistance to all insecticides,” Hoffmann stresses. Instead, resistance to a particular chemical that is not heavily used in the relevant area could be induced. A couple of years of intensive use should establish the dominance of Wolbachia carriers even after insecticide spraying stops.

Hoffmann understands concerns about promoting resistance, but points out that this would be restricted to chemicals that mosquitoes have already evolved resistance against in other regions.

“The first step was to get approval to release Wolbachia infected mosquitoes and we have done that and the populations are beautifully stable. The next step is to get wMelPop into the population in countries where it can make a difference. We’ll then need to get public health officials to approve the release of resistant mosquitoes,” says Hoffman.