The first human case of West Nile Virus infection in the US in 2004 has been reported in New Mexico. The mosquito-borne infection was announced on Wednesday.
鈥淚t鈥檚 the first probable case,鈥� says Christine Pearson, a spokeswoman for the US Centers of Disease Control and Prevention (CDC), in Atlanta. 鈥淲e are working on some confirmatory testing now.鈥�
The man from San Juan County had only mild symptoms and has since recovered, says the New Mexico health department.
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鈥淭his means West Nile Virus season has started early [in New Mexico] this year,鈥� says Patricia Montoya, the state鈥檚 secretary of health. 鈥淏ut 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.鈥�
The West Nile Virus (WNV) outbreak in 2003 killed 264 people and caused nearly 10,000 cases 鈥� 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.
Active mosquitoes
The first human case of WNV in the US in 2003 was reported in July, but CDC says that information which surfaced later suggests the first human case was actually in April. So the WNV season is 鈥渆ssentially on-time鈥� in 2004 says Nick Komar, a research biologist at CDC鈥檚 arbovirus diseases branch, Fort Collins, Colorado. He 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.
鈥淚n 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,鈥� he told New 杏吧原创.
CDC does not have an official prediction the 2004 WNV season, but Komar warns: 鈥淲e 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.鈥�
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.
鈥淲NV has arrived in the western US relatively recently, and will probably continue to spread there,鈥� says Komar. 鈥淎s 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.鈥�
Herd immunity
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 鈥� 鈥渉erd immunity鈥� 鈥� 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 鈥渟pectrum of immunity鈥� ranging from seven to 54 per cent. This means they are not close to the herd immunity level yet.
鈥淚t seems clear that the majority of sparrows will be susceptible to infection and therefore available to contribute to the transmission cycle in 2004,鈥� 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.