What is West Nile Virus?
West Nile Virus (WNV) is a mosquito-borne virus that has been found in parts of Asia, Eastern Europe, Africa, and the Middle East. The virus was first detected in the US in 1999 in New York City. WNV has now been detected in 43 states in the US, including Washington State (raven in Pend Oreille County).

It should be noted that the US has other mosquito-transmitted encephalitis viruses that have been around for a long time. These include St. Louis Encephalitis virus and Eastern Equine Encephalitis virus, both of which can cause disease in humans.

What kind of disease does WNV cause?
The majority of people and animals that are exposed to the virus do not develop any disease. In some humans the virus may cause only a mild, flu-like illness. In rare cases, the virus can cause a more serious condition called encephalitis (inflammation of the brain). In humans, the elderly and the immunosuppressed are at higher risk for serious disease caused by WNV.

Which animals can get disease due to WNV?
Disease was first reported in birds, horses and humans. WNV has now been diagnosed in several mammalian species other than horses. Overwhelmingly, however, most non-human mammalian cases are in horses (over 9000 cases reported). To date cases have been reported in 3 canines, some captive mountain goats, a sheep, an alpaca, and some squirrels. The numbers reported in mammals other than horses are very few at this time.

Wild birds are the animals from which the mosquito vector acquires the virus. Infection has been reported in more than 70 bird species. Many birds that are infected with WNV do not develop disease. In some birds, however, serious illness and death occurs. Crows, ravens, and bluejays are particularly susceptible to disease caused by WNV.

How do people and animals get WNV?
WNV is transmitted to people and animals by the bite of an infected mosquito. Only certain species of mosquitoes carry the virus and only a few mosquitoes are actually infected. The mosquito first acquires the infection by feeding on a bird with the virus in its blood. The virus lives in the mosquito and is then transmitted to a new host (animal or human) in the mosquito’s saliva when it bites.

WNV has been diagnosed in humans that received organ transplants from an infected donor. It is unclear whether the donor became infected by a blood transfusion just prior to death, or was already infected. Transplant recipients take immunosuppressive drug to prevent organ rejection and this undoubtedly increased their susceptibility to infection with the WNV.

Humans and horses are accidental hosts for WNV. Only infected birds contain enough virus in their blood to pass the virus on to an uninfected mosquitoes. The virus cannot be transmitted human-to-human or horse-to-horse or horse-to-human, etc. It can only be transmitted by the bite of an infected mosquito.

What are the symptoms of WNV disease in humans?

• Remember that most people who become infected do not develop disease.
• Some people who do become ill develop a flu-like disease with some or all of the following symptoms: fever, headache, nausea, body aches, mild skin rash, and possibly swollen lymph nodes.
• In a few cases the disease will progress to encephalitis with severe headache and malise.
• The time between the bite of the mosquito and the onset of illness ranges from 5-15 days
• There is no specific treatment for the infection other than supportive care. Thus, prevention is very important.

What are the signs of disease in horses?

• Disease can occur in any breed or age of horse.
• Clinical signs consist of central nervous system abnormalities similar to those caused by infection with eastern equine encephalitis and western equine encephalitis viruses.
• A WNV vaccine for horses has recently become available under a conditional USDA license, but the degree to which the vaccine can prevent disease in horses has not been evaluated yet. The vaccine is definitely not 100% protective because some vaccinated horses have developed the disease.

How can exposure to WNV be prevented?
Control efforts are aimed at decreasing the numbers of mosquitoes and preventing mosquito bites by using repellents, protective clothing, and/or avoiding mosquito areas. There is no vaccine available for humans at this time.

What is the risk of getting WNV?

Humans: The overall risk of WNV disease in humans in the U.S. is very low. First, it is estimated that in areas known to have WNV that less than 1% of the mosquitoes are infected with the virus. Thus, most mosquito bites will not lead to infection. Second, most people who become infected do not get any disease. In the U.S. about 1 infected person in 150 has become seriously ill with central nervous system infection. However, some people do get serious disease due to this virus. No vaccine is available for humans. Therefore, prevention of exposure to infected mosquitoes is very important.

Horses: Horses are more likely than humans to become infected (more likely to be bitten by mosquitoes because of being outdoors) and are more likely than humans to get disease if infected with the virus. Fortunately, there is now a USDA-approved vaccine for horses. Although the vaccine is not 100% effective at preventing disease, it prevents disease in the majority of horses and it helps to prevents serious, life-threatening disease in vaccinated horses that do get disease.

For more information:

Fort Dodge Equine WNV vaccine

West Nile virus arrives in Washington state

http://environmentalrisk.cornell.edu/WNV/default.cfm

WSU News Story:  WSU Veterinary College Eyes West Nile Virus Advance; Urges Horse Vaccinations

http://www.cdc.gov/ncidod/dvbid/westnile/
http://www.aphis.usda.gov/lpa/issues/wnv/wnv.html
http://cindi.usgs.gov/hazard/event/west_nile/west_nile.html
http://www.doh.wa.gov/ehp/ts/Zoo/WNV/WNV.html