HEALTH authorities in the United States have issued a nationwide alert over the West Nile virus, predicting an outbreak that could potentially claim hundreds of lives.
Unknown in the western hemisphere until it surfaced in New York four years ago, the mosquito-borne disease last year killed 284 people in the US, the highest annual toll so far. This year, say experts, there could be even deadlier consequences.
The spread of the disease, which affected 39 states last year and is predicted to hit all 50 this year, is of such concern that the Scottish National Blood Transfusion Service has joined the rest of the UK in barring donors from giving blood within 28 days of visiting the North American continent, in an attempt to protect stocks from infection.
"I think Britain’s policy is a necessarily cautious one," said Dr Paul Epstein, associate director of the Centre for Health and Global Environment at Harvard Medical School in Boston, Massachusetts.
"West Nile virus is becoming more and more of a concern," he added, emphasising that as well as infecting humans, the disease can jump species.
It has run rampant through wildlife while scientists struggle to fully understand its advance, he said.
"It is now in 230 species of animals, 138 species of birds and carried by 37 species of mosquito. We really don’t know where this disease is going," said Dr Epstein.
The virus was first identified by scientists in Uganda in 1937, but has made occasional leaps north since then, at times reaching Israel, Romania and even parts of France as mosquitoes’ habitat widens due to changing global weather patterns.
In 1999, it jumped the Atlantic, infecting 62 people and killing four in the New York borough of Queens.
Last year, a total of 4,146 people in the US became infected, around 7 per cent of them fatally. It was also detected in 15,500 horses.
Symptoms include headache, high fever and muscle weakness and, in severe cases, convulsions, paralysis and coma. While deaths had been limited to the frail and elderly in the past, last year saw an alarming change as the disease took its toll of younger and healthier victims.
Matthew Lee, 19, of rural North Platte in Nebraska, last year became West Nile’s youngest victim in the US.
It was also established for the first time that West Nile can be spread person-to-person through blood transfusions, organ transplants and possibly even breast milk.
"We are just at the initial stages of trying to understand and model this disease," said Dr Epstein. "Clearly, the surprise we have had in terms of how it’s transmitted is a new concern."
West Nile is expected to pose a far greater challenge to health authorities in America than SARS, which did not claim a single life in the US despite seriously affecting neighbouring Canada.
Key to its anticipated advance this summer will be the severe drought that has hit America’s western states.
Drought conditions mean that pools of water such as ponds and marshlands, are more shallow and stagnant, creating prolific breeding grounds for mosquitoes.
It also means there are fewer frogs and fish, which would ordinarily help keep the insect population in check.
Even the US space agency, NASA, has plans to help track the disease’s likely spread, using satellite imaging and weather monitoring to help identify possible breeding grounds.
Dr Epstein is concerned that New York, the first area affected four years ago, may be a particular area of concern again this year because it has not taken the measures to destroy the mosquitoes’ larvae as authorities in other areas of the country have done. The larvae can be killed by using a particular bacteria that is released into drains and other favoured breeding areas.
"As far as I’m concerned, that’s a big error on New York’s part," he said. "They have put a lot of emphasis on combating bio-terrorism, so that’s been a distraction from fighting the enemies we know."
In many areas, health surveillance authorities use flocks of "sentinel chickens" which undergo regular testing to establish whether the virus is present. But horses and wild birds such as blue jays and crows, which are particularly prone to mosquito bites, also provide a good indicator.
"So far in 2003 we have had West Nile activity in some 24 states," explained Bernadette Burden of the Centres for Disease Control and Prevention (CDC) in Atlanta, Georgia, yesterday. "It will likely continue its spread west, eventually reaching the Pacific coastline. Combating West Nile is a priority for the CDC."
This week has been dubbed Mosquito Awareness Week, during which the CDC and state health authorities will amplify their West Nile prevention message to the public under the slogan "Fight the Bite", educating it in how to avoid being bitten and contribute to mosquito control measures.
Dr Michael Bunning, an epidemiologist at the CDC’s National Centre for Infectious Diseases in Fort Collins, Colorado, told the Washington Post that the current level of West Nile infection in birds and horses this year is "not a good sign".
"We haven’t seen any sign that things are on a downward cycle," he said. "It’s just a matter of time before we have the first human case."
Around one-fifth of those infected with the virus develop mild symptoms similar to flu. But at worst, the disease can cause encephalitis, a critical inflammation of the brain or the membrane that surrounds it.