Sterile males are latest buzz in malaria battle

CARRIER: Malaria is caused by plasmodium parasites that are spread to people through the bites of infected anopheles mosquitoes, called 'malaria vectors', that bite mainly between dusk and dawn. Picture: THINKSTOCK
CARRIER: Malaria is caused by plasmodium parasites that are spread to people through the bites of infected anopheles mosquitoes. Picture: THINKSTOCK

REARED on a laboratory diet of dog biscuits and sugar water, a host of sterile male mosquitoes are soon to encounter wild females for the first time. They will be part of an experiment designed to show scientists at the University of the Witwatersrand (Wits) whether irradiating male Anopheles arabiensis mosquitoes can help stop the spread of malaria.

South Africa has made remarkable progress in containing malaria in the past decade, and looks set to meet its target of eliminating local transmission by 2018. One of the linchpins of the control programme has been a campaign to spray houses with insecticides that kill mosquitoes on contact.

Concern about insecticide resistance has prompted experts to find other ways to control malaria-carrying mosquitoes. The spraying campaign is also limited because it only works for mosquitoes that rest on surfaces indoors, such as Anopheles funestus. In South Africa, this leaves large numbers of Anopheles arabiensis unscathed, because they feed and rest inside and outside houses, says Prof Lizette Koekemoer of the Malaria Entomology Research Unit at Wits.

Her team is investigating whether sterilising male An. arabiensis could knock out the mosquitoes that spraying fails to reach. So far they have shown that their sterilised male Anopheles arabiensis will readily mate with females in a bucket in a laboratory, and that these females will go on to lay eggs. This demonstrates that irradiating Anopheles arabiensis does not compromise its capacity to mate, but they have yet to determine if a sterilised lab-reared male appears equally sexy to a wild female as its wild counterparts.

Using sterilisation as birth control for insects is not a new idea. It was used to eradicate the screw worm in the US and Mexico in the 1980s, and in South Africa farmers have tackled fruit flies and coddling moth in much the same way.

Male pupae are irradiated with a radioactive source such as Cobalt-60, and then grown to maturity in the lab. They are then released to mate with wild females, which lay unfertilised eggs that fail to hatch.

Further afield, scientists are exploring genetic engineering for controlling mosquito populations. The Oxford-based firm Oxitec is supplying scientists in the Florida Keys with transgenic male Aedes aegypti mosquitoes carrying a gene that will kill their offspring in the larval or pupal stage, in a bid to control dengue fever.


The males, which feed on nectar and do not bite humans, are expected to mate with females in the wild, but the US Food and Drug Administration has yet to grant approval for their release.

There is no genetic tool to render Anopheles arabeiensis sterile, so the only option at present is to do the job with radiation, says Prof Koekemoer.

The next step is a larger experiment in a laboratory enclosure, but more work is required before they can consider a field trial, she says. " And we also will need to inform communities before we release any mosquitoes, but we are nowhere near to that stage yet."

Since Anopheles arabiensis is just one of several species of mosquito found in South Africa, the scientists do not anticipate that eliminating them will have any significant effect on the environment.

"If you take them out of the ecosystem it still leaves many other species in the food chain for the geckos and spiders and dragon fly larvae," says Prof Koekemoer. And since male mosquitoes do not feed on blood, they pose no harm to humans.

A successful mosquito sterilisation programme would help prevent a resurgence of malaria, which scientists warn is possible if control measures lapse for even a few seasons.

"History has taught us that if you get close and don’t finish the job, then malaria will come roaring back," the World Health Organisation’s Global Malaria Programme director, Robert Newman, said earlier this month.