Wildlife and West Nile Virus

Summer is upon us, and with the nice weather comes the downside of the season: mosquitoes. The West Nile Virus, a pathogen associated with mosquitoes that causes disease in humans, livestock and wildlife, has been part of life in the United States for the better part of the last two decades.

The virus originated in Eurasia and was first detected in the US in New York City in 1999. Since then, the presence of the virus on the landscape has moved westward, aided mostly by migrating birds. Like most novel diseases, WNV gained significant attention when it first arrived on the East Coast. Since then, as our knowledge about the virus and its impact has improved, it is accepted as a dangerous but not entirely foreign risk of being bitten by mosquitoes in the summer.

In humans, most infections are asymptomatic. 1 in 5 people infected with WNV will develop symptoms, and about 1 in 150 will develop very serious symptoms, including fever, and most dangerously, encephalitis.

The Cycle of West Nile Virus

But how does West Nile Virus move through animal populations? Like all pathogens that cause disease in wildlife, the answer lies in understanding the enzootic cycle of the virus in wildlife populations. There are approximately 62 species of mosquito that are suitable vectors for the virus, most notably mosquitoes of the genus Culex. Culex mosquitoes are generally "ornithophilic," or specialize on birds. 

The cycle of WNV transmission is primarily a bird-mosquito cycle but the virus also infects mammals and reptiles. Some bird species maintain high titers of the virus after being bitten by an infected mosquito. When a naïve mosquito bites the infected bird, it is now capable of being a vector for the virus as well, and can transmit the virus to another bird. Basically, the virus can live within a bird host without killing the bird, and serves as a source for mosquitoes to become infected with the virus and pass it to other animals.

WNV Effects on Wildlife

While many species of birds remain healthy while being infected with the West Nile virus, some birds are significantly affected by a viral outbreak. Birds in the corvid family, which includes jays, crows, and ravens, have a high mortality rate when infected with the virus. These dead birds can be a sentinel for an outbreak of the virus in a given area. Other effects of the virus on wild bird populations are seen from time to time.

 In 2013, an outbreak of WNV killed 10,000-20,000 eared grebes on the Great Salt Lake, Utah. At the same time, 85 bald eagles suffered mortality from the virus as well. While a single mortality event of this magnitude is notable, other species can suffer at the population level.

Ruffed grouse in the east have experienced population challenges due to habitat loss, but West Nile Virus infections kill grouse in high enough numbers to have population level effects. The magnitude of the outbreak varies with the severity of mosquitoes throughout the summer, which can be driven by environmental factors like temperature and rainfall. This issue has a direct effect on hunters due to the changes in historical grouse population levels regionally.

WNV Effects on Livestock

In addition to its effects on some species of wild birds, West Nile Virus is also known to have severe effects on horses. Infected horses have a 30% case fatality rate when infected with the virus, although a vaccine has been developed for use in horses. The virus has also been detected in other domestic livestock such as sheep, alpaca and reindeer, and even farmed alligators.

Prevention

While there is no West Nile Virus vaccine for humans, common-sense mosquito prevention is the best way to reduce exposure to the virus. Using insect repellent and eliminating standing water from your yard is a good way to start. People over the age of 50 are at a higher risk of experiencing the more severe symptoms associated with WNV infection. The virus is here to stay for now, and being informed about its natural cycle is always helpful when it comes to monitoring zoonotic diseases!

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