Bacterial allies make dengue fever cases dive

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Recruiting a bacterial ally that infects mosquitoes has led to huge reductions in cases of dengue fever, trials around the world show.

Wolbachia bacteria make it harder for the insects to spread the virus, rather than kill them off.

Researchers say the findings are a “big deal” with cases falling by more than 70% in field trials.

New ways of controlling dengue are urgently needed as cases have exploded worldwide in the past 50 years.

What is dengue?

Dengue fever is caused by a virus that is spread from person to person by blood-sucking mosquitoes.

The symptoms vary wildly with some people showing no sign of infection, others have bad flu-like symptoms, while some are killed by dengue.

The disease is commonly known as “break-bone fever” because it causes severe pain in muscles and bones.

In the worst cases, people develop “dengue haemorrhagic fever”, which kills 25,000 people a year around the world.

“It doesn’t kill as many as malaria, but it causes an enormous amount of sickness and it is fundamentally a big problem,” said Prof Cameron Simmons, from the World Mosquito Programme.

The World Health Organization says cases have increased “dramatically”.

Rewind the clock to 1970 and only nine countries had faced severe dengue outbreaks. But dengue fever has spread so widely there are now more than 100 countries where the disease is present all the time, known as endemic dengue.

Around half of all people on the planet live in areas where dengue is a problem and there are thought to be 390 million infections each year.

This is in stark contrast to malaria, another mosquito-borne disease, where cases are falling.

Most of the countries affected are in tropical and sub-tropical climates, with 70% of cases in Asia.

“It is a really enormous concern,” Prof Steven Sinkins, from the University of Glasgow, told the BBC.

Wolbachia bacteria make it harder for the dengue virus to grow inside the mosquito.

It is thought the bacteria camp out in areas inside mosquitoes that the dengue virus needs to get into and that the bacteria use up resources that the virus needs.

If the dengue virus cannot replicate and increase its numbers in the mosquito then it is much less likely to be transmitted when the insect bites again.

Many species of insects are naturally infected by the bacterium, including the tiny fruit flies dancing around your kitchen.

But the main mosquito that spreads dengue fever – Aedes aegypti – is not normally affected.

So, researchers have been micro-injecting species of Wolbachia into tiny mosquito eggs to see which bacteria can thrive in different climates.

Some are already in large field trials; a team at the University of Glasgow has found a species that can cope in the very warm conditions (high 30Cs) where they would be trialled in Malaysia.

No, although that approach is also being investigated by scientists.

Wolbachia counts as biological control – using one species to control another.

“It is relatively uncontroversial: it is not infectious to humans and is perfectly safe,” Prof Sinkin said.

Prof Simmons said “enormous efforts” went into building community support before any trials went ahead and that one of the main complaints was a rise in the number of mosquito bites when fresh batches of insects are released.

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