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HEALTH: Battle Against Dengue Finds a New Front

Stephen de Tarczynski

MELBOURNE, Jan 26 2011 (IPS) - When an outbreak of dengue fever occurred in the hot and humid north of Australia’s Queensland state in late 2008, Nicola Strange was among hundreds of locals that contracted the mosquito-borne disease.

Now, together with her husband, Nicola Strange is volunteering their property for use in a scientific field trial that researchers hope will be the next step in an ambitious plan to eradicate dengue fever, an infection that leads to thousands of deaths in tropical areas of the world every year.

“I’ve never felt so unwell,” says Nicola, recalling her experience of type- two dengue.

Her fever lasted “for a good week”, with a high temperature accompanied by an intense headache, vomiting and severe pain behind her eyes and in her back.

“Even when all those symptoms had subsided, I was very weak for many weeks after,” she tells IPS.

According to the World Health Organisation (WHO), some 50 million people are infected with one of the four serotypes of dengue each year. The disease is endemic in more than 100 countries with around 2.5 billion people worldwide now at risk, particularly in tropical urban and semi-urban environments.

Southeast Asia and the Western Pacific account for 75 percent of all dengue cases, while areas of Africa, the Americas and the Eastern Mediterranean are also threatened.

A small percentage of people infected with dengue develop the potentially- fatal dengue haemorrhagic fever (DHF).

Children are particularly susceptible to DHF. First recognised in the 1950s in the Philippines and Thailand, DHF is now a leading cause of hospitalisation and death among youngsters in Asia.

Of the 500,000 DHF cases – mostly children – that the WHO estimates require hospitalisation each year, around 2.5 percent succumb to the disease. This fatality rate increases substantially without proper treatment.

Recent decades have seen a dramatic rise in dengue infections. Prior to 1970, only nine countries had experienced DHF epidemics. By 1995, more than four times that number had also seen similar outbreaks.

Such is the situation facing the international team of scientists from the ‘Eliminate Dengue’ project.

Hailing from Australia, Thailand, Vietnam and the U.S., the scientists are currently focusing their efforts on the Aedes aegypti mosquito, responsible for most of the world’s dengue infections, with separate research also being undertaken on the Aedes albopictus mosquito, a secondary dengue vector.

The project’s 12-week field trials got under way in the northern Queensland town of Cairns in early January.

“These trials are the next step in a long sequence of work we’ve been doing for many years where we’re moving out into the field. We’re all very excited in taking that next step,” says ‘Eliminate Dengue’ leader Professor Scott O’Neill from the University of Queensland.

O’Neill’s team is hopeful that a naturally-occurring bacterium called Wolbachia will spread through the Aedes aegypti populations in the trial areas by mating with mosquitoes already infected with the bacterium.

These mosquitoes were purposely infected with Wolbachia, which cannot be transmitted to humans, in a laboratory in Cairns and are being released in stages from the yards of residents like Nicola Strange.

The project’s organisers received regulatory approval from the Australian Pesticides and Veterinary Medicines Authority, a government agency, to carry out their research and their methods were given the lowest possible risk rating from the Commonwealth Scientific and Industrial Research Organisation.

The researchers had previously focused on Wolbachia’s ability to reduce the lifespan of mosquitoes. In 2009, the team found that Aedes aegypti’s lifespan of 30 days was cut by half when infected with certain strains of Wolbachia.

This was significant because Aedes aegypti is unable to transmit dengue until it is between 12 and 15 days old, drastically reducing its ability to transmit dengue to humans.

But cutting lifespan is no longer the focus of the project.

“In the course of our studies, we found out that the bacterium, when we put it in the mosquito, actually blocked the ability of dengue to replicate in the mosquito. And so we shifted away from life-shortening (strains) to strains of Wolbachia that don’t really do life-shortening but just interfere with dengue replication,” O’Neill tells IPS.

It is hoped that this will disrupt the transmission cycle in the wild mosquitoes, which do not naturally carry the dengue virus. Instead, Aedes aegypti must acquire the disease from a dengue-infected person before it can transmit dengue to another person.

After the Cairns field trials conclude, the researchers will spend several months trapping mosquitoes in order to determine how well the Wolbachia bacterium spreads into the populations.

If successful, the ‘Eliminate Dengue’ team will undertake larger-scale trials in Vietnam later this year, followed by further trials in Cairns in 2012.

“If those experiments are successful then we might expect to see full implementation and control of dengue in the Cairns region in a two to four- year timeframe. If we encounter unexpected difficulties, for example if we were to determine that the Wolbachia infection did not spread easily into wild mosquito populations, then it may take a longer time to fine-tune the technology until we are ready for full deployment,” says O’Neill.

For Nicola Strange, volunteering her yard for the trial was an easy decision.

Although she is now immune to type-two dengue, she remains at risk from the others and has an increased chance of developing DHF if she contracts dengue again.

“If they can find a way of eliminating dengue fever, then I’m all for it,” she says.

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