- The first human case of West Nile Virus infection in
the US in 2004 has been reported in New Mexico. The mosquito-borne infection,
announced on Wednesday, has arrived earlier than 2003's first case, which
came at the start of July.
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- "It's the first probable case," says Christine
Pearson, a spokeswoman for the US Centers of Disease Control and Prevention
(CDC), in Atlanta. "We are working on some confirmatory testing now."
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- The man from San Juan County had only mild symptoms and
has since recovered, says the New Mexico health department.
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- "This means West Nile Virus season has started early
this year," says Patricia Montoya, secretary of health for New Mexico.
"But the virus is one of the few diseases where people can take a
few simple precautions, like using a repellent when outdoors, and be protected."
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- The West Nile Virus (WNV) outbreak in 2003 killed 264
people and caused nearly 10,000 cases - more than double the 2002 total.
However, many more of the 2003 cases were mild infections, with the number
of people seriously affected remaining roughly the same in both years.
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- Active mosquitoes
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- Nick Komar, a research biologist at CDC's arbovirus diseases
branch, Fort Collins, Colorado points out WNV can be spread whenever the
Culex mosquito species that transmits the virus are active. The virus had
already been detected in 2004 in birds and mosquitoes in many southern
and eastern states.
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- "In southern California, Culex tarsalis become active
in February. In Florida, Culex quinquefasciatus are active all year round.
None of the detected WNV activity in the US early in 2004 has been unexpected,"
he told New Scientist.
-
- CDC does not have an official prediction for the 2004
WNV season, but Komar warns: "We must always be prepared for an epidemic.
But one human case does not make an outbreak, so it is too early to say
what the significance of this case is for the US in 2004."
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- WNV was imported to New York in 1999, from where it spread
across the country. In the last two outbreaks, the disease hit the western
states harder.
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- "WNV has arrived in the western US relatively recently,
and will probably continue to spread there," says Komar. "As
it does, more people will be at risk, so it is possible that there will
be more western US cases in 2004 than in previous years."
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- Herd immunity
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- As part of its WNV research, CDC is studying the immunity
of house sparrows. These birds are not harmed by WNV but are an important
host for the virus, which is passed from infected birds to humans and horses
via mosquitoes.
-
- The virus is already endemic in some areas and when this
happens, birds develop immunity. If enough become immune - "herd immunity"
- the virus would die out, and transmission to humans might stop.
-
- Komar, who is running the study in Colorado, says that
a preliminary survey of house sparrows this spring in 23 towns has found
a "spectrum of immunity" ranging from seven to 54 per cent. This
means they are not close to the herd immunity level yet.
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- "It seems clear that the majority of sparrows will
be susceptible to infection and therefore available to contribute to the
transmission cycle in 2004," he says. However, it is likely that herd
immunity will develop eventually, as has happened in Africa where WNV has
been endemic for many years.
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