National Wildlife Health Center

...advancing wildlife and ecosystem health

West Nile Virus

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What is it?

West nile virus (WNV) was originally described from crows in Egypt in the mid 1950s. In 1999, WNV was introduced into New York by an unknown route. Since that time, the virus has spread through the 48 contiguous states through a combination of natural and man-made movements of animals. Alaska and Hawaii are currently the only states that have not yet documented the presence of WNV.

What does it do?

WNV is transmitted by various species of mosquitoes. When an infected mosquito bites an animal, the virus enters the animal and infects the central nervous system. In humans, WNV causes non-specific "flu-like" symptoms and can occasionally cause mortality. For an excellent reference on WNV, look at the web site by Centers for Disease Control. In Hawaii, all the pieces are in place for WNV to become established. We have the mosquitoes, the birds, and the warm climate. Thus, it behooves us to be vigilant and to prevent its introduction.

What does it affect?

In the US, WNV most commonly affects crows, jays, horses, and humans although the virus has caused illness in alligators, squirrels, seals, and other species. WNV is a classic example of a novel pathogen running amok in a naive ecosystem (in this case, the United States). If WNV got to Hawaii, it would be a disaster for a our native endangered birds and would probably severely crimp our tourist industry.

What are the field signs?

Birds affected typically are found dead or show nervous system disorders (inability to fly, incoordination). Horses show central nervous system disorders and humans manifest non-specific "flu-like" symptoms.

Where and when does it occur?

Depending on the area, WNV can occur seasonally or year-round and essentially wherever mosquitoes that are capable of transmitting the virus are present.

How do you detect it?

Because crows and jays are so susceptible to WNV, they are usually the first types of birds found dead and are a good indicator of virus activity in the 48 contiguous states. Unfortunately, in Hawaii, our only crows are critically endangered and in captivity. Jays don't exist in Hawaii, so depending on mortalities of crows or jays is not useful here to indicate presence of WNV. In order to help Hawaii detect WNV, the Honolulu Field Station has been collaborating with the US Department of Agriculture to sample wild birds for presence of the virus. Studies in California have shown that wild birds are the most sensitive indicator of WNV activity in an ecosystem, often preceding the first human case by several weeks.

How do you manage it?

Once WNV becomes established, it is virtually impossible to eradicate. So, the only option is to reduce mosquito populations thereby reducing transmission. In Hawaii, we are fortunate to have several thousand kilometers of ocean between us and WNV, so we have a real chance of preventing it or detecting it early and stamping it out. You can make a difference by making sure that containers that could hold standing water (potential breeding grounds for mosquitoes) are eliminated from your properties.

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Page Last Modified: Jun 20, 2018