What is West Nile virus?

board of animal health

Reportable Disease of the Month

West Nile Virus

What is it?

A viral, zoonotic, disease spread by mosquitoes, and thought to cycle between mosquitoes and birds. Mosquitoes contract the virus from birds and then spread it to mammals (and reptiles), most commonly humans and horses. Camelids can also be affected with alpacas being most susceptible. The virus can cause swelling of the brain and spinal cord. In 2002, the U.S. had a large West Nile virus outbreak in horses, totaling thousands of cases. Since then, the number of cases has steadily declined to average in the hundreds annually. This disease is reportable to the Board of Animal Health.

How is it transmitted?

The most common transmission route is from birds to mosquitoes to mammals. Mosquitoes acquire the virus from infected birds and then carry it to mammals, which can become infected through a mosquito “bite.” Other transmission methods are still being studied, and the bird to mosquito transmission cycle is the most common.

What are the clinical signs?

Infected horses can become anorexic, depressed or show neurologic signs and behavior changes. The incubation period in horses is between three and 15 days. Many infected horses are asymptomatic. Clinical signs are very similar to those seen with other neurologic diseases including rabies.

How is it diagnosed?

West Nile virus can be identified through serology tests, which means a blood sample is analyzed and diagnosticians look for antibodies. If antibodies are present, it means the animal is/was infected with the disease. It can also be diagnosed after an animal has died.

Is there a risk to people?

Yes, mosquitoes can transmit West Nile virus to people; potentially leading to serious illness. Information about the disease in people can be found at the Minnesota Department of Health.

What is the treatment?

No specific treatment is available and treatment is supportive, with the goal of making the animal more comfortable until it recovers. Horses able to recover usually begin to improve within one week of onset of clinical signs. Most horses return to full function, but a small percentage may suffer lasting neurologic deficits.

How can it be prevented?

Horse vaccines are available and have been used extensively, contributing to the decline in the number of affected horses. Additionally, disease risk can be reduced by limiting exposure to mosquitoes. This can be accomplished by changing horses’ drinking water regularly, mowing tall grass, draining stagnant water, maintaining screens and installing fans at horse stables, and using insect repellants on both horses and people. Also, mosquitoes primarily feed at dawn and dusk, and keeping horses indoors during these times can reduce risk.

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