West Nile virus: what is the actual risk?

18 Aug 2012, 3:33 pm   -   Source: Debora MacKenzie, New Scientist
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Mosquitos are sorted at the Dallas County mosquito lab. (AAP)

Mosquitos are sorted at the Dallas County mosquito lab. (AAP)

The US city of Dallas has launched an aerial insecticide spraying after 200 known cases and 10 deaths from West Nile this summer. But how much of a threat is the virus really?

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The news this week is full of stories of West Nile virus, with Dallas, Texas declaring a health emergency and launching aerial insecticide spraying after 200 known cases and 10 deaths from West Nile this summer. But how much of a threat is the virus really? And could a minor bout of West Nile actually be your best protection for the coming years?

Anything that kills people, of course, is by definition a concern. What is most worrying about West Nile is that it appears to be on the rise – and virologists worry that this doesn't just mean more West Nile cases. It also means its nastier cousins, such as dengue, chikungunya and tick-borne encephalitis, could start booming as well.

West Nile virus normally infects birds, and is carried between them by mosquitoes. In temperate regions, the number of infected birds rises steadily after mosquitoes become active in spring. By late summer, so many birds have been infected that mosquito species that bite both birds and people occasionally carry the virus from a bird to a human. Horses are also severely affected.

Only one in five infected people develops any symptoms, and they are mostly flu-like. About one person in 150 – mostly the elderly or people with impaired immunity – develop serious, sometimes fatal, illness. Survivors of that may suffer kidney damage.

In Europe, there are outbreaks in humans most years, though they are sporadic, limited and "quite unpredictable", according to the European Centre for Disease Control and Prevention in Stockholm, Sweden. But the virus is becoming endemic in parts of southern Europe.

There were 93 reported cases in Europe last year, mostly in Greece. There may be more this year after the virus turned up a few weeks early in Greece, Italy and Israel, possibly due to a warm spring and more mosquitoes.

West Nile was unknown in the western hemisphere until 1999, when it was accidentally introduced in New York, possibly from a bird imported from Israel. In a few years it swept across the Americas and is now endemic, with annual outbreaks. This year, those outbreaks have also started early: the US Centers for Disease Control and Prevention in Atlanta reports 693 cases so far – the highest number seen by mid-August since 1999. Of those, 26 have died.

NATURAL CYCLE

The surge could be a natural cycle: when the virus is prevalent, many birds are infected and become immune. As they do not transmit the virus, it becomes scarcer. A generation of birds then grows up with less infection and less immunity. They permit more spread of the virus, so it booms again, and the cycle repeats.

But the boom this year could also be due to hot weather, which boosts mosquito numbers and makes the virus multiply faster. Moreover, winter normally kills off the mosquitoes that carry the virus. "This year, with a mild winter, more mosquitoes survived and so we're seeing a bumper crop," says George DiFerdinando at the University of Medicine and Dentistry of New Jersey in Piscataway.

What can we do? There are West Nile vaccines for horses, but no vaccine or treatment for humans. Franz Heinz of the Medical University of Vienna thinks vaccine companies lost interest as case numbers fell in the US after a peak in 2003.

"In theory, we know a vaccine is possible. Several companies are working on human vaccines," though none are now in human trials, says Ab Osterhaus of Erasmus University in Rotterdam, the Netherlands. "It may not be cost-effective to mass vaccinate," he says, because West Nile causes relatively few deaths. This may limit corporate interest in spending millions on clinical trials.

Meanwhile, what boosts West Nile boosts other insect-borne viruses. "Things are moving; there is more and more out there," says Osterhaus, who helps organise Vectorie, a Europe-wide programme to coordinate research on insect-borne viruses, their vectors, and human disease.

For now, all we can do to avoid West Nile is wear insect repellent, drain stagnant water where mosquitoes breed, spray insecticide – and be glad severe cases are rare in children. Those who unknowingly catch the virus this summer will have immunity and might be the best protected people as West Nile continues to boom.

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