West Nile Virus: Can the US fight it?

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Media captionScientists say people may eventually acquire immunity to West Nile Virus through exposure, but it could take several decades.

As the number of cases of West Nile Virus rises across the US, many Americans are wondering how they can protect themselves from one of the worst outbreaks on record. There's no vaccine - but scientists say we may eventually acquire immunity through exposure to the potentially deadly virus.

Some 134 people in the US have died from West Nile Virus (WNV) and more than 3,000 others have been sickened in one of the worst outbreaks on record. And scientists expect the death toll to increase until October as more cases are confirmed and the warm weather continues.

Carried by birds and spread to people by infected mosquitoes, the first case in the US was reported in New York in 1999.

Since then WNV has spread across the country and is now endemic, achieving what scientists call "ecological equilibrium". In other words, it's become a permanent part of the nation's eco-system.

That makes it extremely difficult to predict any specific threat to humans. Most people affected this year lived in the Dallas and Fort Worth region of Texas. Although an unusually mild winter and hot summer may have had some influence, the exact reason why this area, and not another, saw so many cases of WNV remains a mystery.

"Some trends have been observed, but even these are likely to be over-generalizations that limit their usefulness," says Roger Nasci, the chief of arboviral diseases branch of the Centers for Disease Control and Prevention.

Image caption The asian tiger mosquito is one of the most common carriers of West Nile Virus

"We don't know what specific suite of conditions resulted in the Dallas outbreak and why similar outbreaks didn't occur in other localities experiencing similar conditions."

Testing mosquitoes is the only way to detect an outbreak of WNV before humans become infected. State and county control units spend the summer months collecting samples and identifying virus hotspots, which are then treated with insecticides.

In Virginia this year the virus was detected several weeks earlier than usual, putting health officials on high alert.

"We're trying as many tactics and control measures as possible to fight the mosquitoes and that all starts with surveillance," says Tim McGonegal, head of Virginia's Prince William County Mosquito Control.

There are some 350 different species of mosquito in the US (1,200 worldwide), and not all of them are a threat to humans.

"There are mosquitoes that only feed on birds or amphibians so there's no public health threat there - even if they are capable of carrying the disease, they're not going to bite humans.

"So if you don't know what you have - what type of mosquito, how many - and you just start spraying and treating, you're wasting money and time."

Image caption The disease was first carried by birds

There's no commercially viable vaccine against WNV and the only protection is to prevent mosquito bites.

But even if somebody is bitten by a mosquito carrying the virus, they might not become ill.

Most people never experience any symptoms although a few will develop serious conditions, including meningitis which can be fatal.

In parts of Africa and the Middle East where WNV has been endemic for many centuries, people have acquired immunity due to prolonged exposure. Scientists say the same thing could happen in the US where so far only 3 to 5% of the population is immune.

"But that's going to take several years - probably decades for that to occur," says Dr Tony Fauci, director of the National Institute for Allergies and Infectious Diseases.

"If you look at the other side of the world where this has been around forever - places like Israel - a substantial portion of the population has this acquired immunity. Outbreaks are not very impressive in those countries.

"But in a country (such as the US) that is relatively naive to this infection, when you have a lot of cases in a season it's quite noticeable - and we are in the middle of a pretty bad season."

Over time, he says, enough people in the US will have been infected and recovered - or not fallen ill at all - to create a barrier of immunity against further outbreaks.

Although WNV is in the headlines this year and on track to become the worst outbreak on record, other insect-borne diseases are also threatening the US.

Image caption Prince William Mosquito Control are on the frontline in the battle against insect borne diseases.

Dengue fever has been reported in three US states and in Virginia, Mr McDonegal says they're looking out for Chikungunya and Rift Valley Fever (RVF), which both originated in Africa.

Also transmitted to humans by mosquitoes, they can cause severe fever, pain, vision impairment and in the case of RVF, internal bleeding leading to death.

"These make West Nile look like a pretty tame disease," says Mr McDonegal.

"And in Northern Virginia, with a lot of our international population, we're concerned that this might be one of the areas [in the US] where you start to see it first."

The World Health Organization says it's already concerned that Rift Valley Fever could extend to parts of Asia and Europe after it appeared in Saudi Arabia and Yemen in 2000 - the first time it occurred outside the African continent.

Controlling mosquito populations is part of the solution - but it's an ongoing battle.

Local authorities can treat public spaces and warn people about the dangers of leaving containers of water where mosquitoes can breed, but they have no control over private gardens or yards.

One clogged gutter or a neglected bucket of water can become the spawning ground for millions more mosquitoes.

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