West Nile Virus & Horses

- A new and dangerous mosquito-borne virus -

Summary: West Nile virus is transmitted by the bite of certain mosquitoes. In humans, a rare but serious disease can result, while in horses serious disease is much more common. Protect yourself and horses with repellents and steps that reduce mosquito bites.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

How is West Nile virus spread?

Certain mosquitoes transmit the West Nile virus, an arbovirus (a virus transmitted by an arthropod), when they bite and feed on blood. Mosquitoes pick up the virus while feeding on infected birds and can then infect other animals, including us. West Nile virus was introduced into the US in 1999 and has rapidly spread throughout the country. Mosquito bites are the only way that this virus is spread.

West Nile virus naturally infects mosquitoes and birds (note: this is not the "bird flu"). West Nile virus can be fatal to birds but most survive to become natural virus reservoirs. If an infected mosquito bites an animal other than a bird the virus may be transferred but these animals are not good virus reservoirs.

 mosquitoes - line drawing

Certain mosquitoes can transmit West Nile virus when they bite.

In humans the virus normally causes only mild flu-like symptoms. However, in a small percentage of cases life-threatening encephalitis can develop. In horses the West Nile virus causes a very serious illness with up to 40% mortality (compared to less than 5% in humans), fortunately a horse vaccine and repellents are now available (see Insect Repellents For People & Horses).

Protect yourself from West Nile virus

West Nile virus is endemic to Africa, Western Asia, Middle East, but has recently expanded its range into Europe, Russia, Israel and the US. Culex mosquitoes are the most common vectors but other mosquitoes are competent vectors as well. Wild birds are believed to be the only natural virus reservoir.

Humans, horses, dogs and cats are secondary or "dead-end" hosts. This means that the virus can cause illness but does not multiply to levels that would "infect" a biting mosquito. There is no evidence that you can get the virus from handling live or dead birds. No person to person transmission occurs -- in other words the virus is not contagious.

Protect yourself from West Nile virus by reducing mosquito numbers in your yard and using mosquito repellents and mosquito traps when necessary.

Related Articles

Mosquito Control Around Homes

Do Mosquito Traps Work?

Fight The Bite! (a CDC site about West Nile virus)

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