Bad mozzies, good bacteria

It’s not often that bacteria actually prevent disease, but that could be the case in North Queensland, where scientists have infected mosquitoes with the bacteria Wolbachia to try and control the spread of dengue fever.

Dengue fever is not yet endemic to northern Australia, being brought in each year by overseas visitors. But as the most common viral disease spread by insects – infecting about 50 million people worldwide each year – the danger is real and growing.

Symptoms of dengue fever range from a mild to very strong fever, with severe headache, pain behind the eyes, muscle and joint pain, and a characteristic rash. In some cases it leads to dengue haemorrhagic fever, which is much more serious, with a mortality rate of about 2.5% of cases.

Because of the high rate of infection, that translates to about 12,500 deaths annually. And in parts of South East Asia, dengue haemorrhagic fever is a leading cause of hospitalisation and death among children (for more information, see the World Health Organisation’s fact sheet).

Unfortunately there is no specific treatment for dengue and no vaccine, so a lot of work is being put into controlling the mosquitoes that transmit the virus: predominantly the species Aedes aegypti.

Aedes aegypti mosquito, with its distinctive white markings, sucking blood from a human
An Aedes aegypti mosquito, the most common culprit for dengue fever, showing its distinctive white markings (Photo by James Gathany, via Wikimedia Commons)
However, in two papers recently published in Nature, researchers led by Professor Scott O’Neill, formerly of the University of Queensland and now at Monash University in Victoria, announced success in using bacteria from the genus Wolbachiato prevent these mosquitoes spreading dengue in Far North Queensland.

See:

Wolbachia is a very common infection in insects, found in about 60% of species. It was first identified in 1924, tested, found not to affect mammals, and promptly ignored.

Well, not totally, because in 1971 an American PhD student, Janice Yen, found that when a male mosquito infected with Wolbachia mated with an uninfected female, they didn’t produce any viable eggs. It turns out that only infected females can mate with infected males: so not only does it affect who can pair off with who, but it’s quite effective at spreading through a population.

It also seems to have this same effect on some other insects, like fruit flies, but because mosquitoes are so frequently disease vectors, it was with them that the big potential lay.

The big breakthrough for Professor O’Neill and his team was when they discovered that a particular strain of Wolbachia – called wMel, found in Australian fruit flies – prevented the mosquitoes themselves from being infected with the dengue virus.

How it works isn’t exactly known; it might boost the mosquito’s immune system, or possibly it competes with the virus for food inside the mosquito. Either way, these Wolbachia-infected mosquitoes don’t carry the dengue virus. Not only that, but as we’ve seen they also have a reproductive advantage over uninfected mosquitoes.

Professor O’Neill’s team have so far released nearly 299,000 infected mosquitoes in the Cairns region – bred using volunteers who supply the blood the mosquitoes need – and they successfully spread through the wild mosquito population, passing the bacteria on to their offspring.

The research team are now currently working to get approval for similar trials in countries where dengue is endemic – like Vietnam, Thailand, Indonesia and Brazil – and where there’s greater potential for improving people’s health.

Which is all great, but an obvious question comes to mind: what about malaria? Malaria is one of the world’s biggest killers, responsible every year for about 781,000, or 2.23% of all deaths.

It is caused by a parasite carried by Anopheles mosquitoes, which are a different genus to the Aedes mosquitoes that spread dengue. And the trouble with using Wolbachia is that it doesn’t naturally infect Anopheles mosquitoes.

People are trying, though. A paper published in May this year showed that Wolbachia injected into Anopheles mosquitoes does indeed reduce the levels of the malaria parasite. However, it often also kills the mosquitoes, so it’s not suitable yet for introducing into a population the same way they’ve done in Queensland with Aedes. See Hughes GL, Koga R, Xue P, Fukatsu T & Rasgon JL 2011, “Wolbachia infections are virulent and inhibit the human malaria parasite Plasmodium falciparum in Anopheles gambiae“, PLoS Pathogens 7 (5): e1002043 DOI: 10.1371/journal.ppat.1002043.

So there’s a bit of a way to go there, but at least we’re making progress with dengue fever; and it’s good old Aussie scientists – and Aussie bacteria – leading the way.

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