What is it?

West Nile Virus (WNV) is a mosquito-borne virus that causes encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). Outbreaks of the disease caused by WNV have occurred in Egypt, Asia, Israel, South Africa, and some parts of Europe and Australia. The virus was first seen in the United States Fall of 1999 in New York City, and has since spread across the US to the Pacific Ocean, into several Canadian Provinces, and possibly into Mexico.

What types of animals does it affect?

WNV primarily infects and multiplies in birds, which serve as reservoirs for the virus. The severity of the disease in birds varies among species, with American Crows frequently dying from the infection. WNV can also infect and cause illness in horses and humans, although only a small percentage of infected animals or people become sick. The virus also has been shown to infect several other types of mammals (including bats, cats, chipmunks, squirrels, skunks, and domestic rabbits) usually without causing clinical signs. Very rarely, mammals other than horses and humans may become ill from WNV infection.

How do animals get the virus?

Animals must be bitten by a virus-carrying mosquito in order to become infected with WNV. The so-called "enzootic" cycle involves transmission from bird to bird. Mosquitoes become infected when they bite, or take a blood meal, from a bird carrying WNV; the infected mosquito then transmits the virus to another bird through a second bite. Migrating birds carrying WNV in their blood have had a significant impact on the spread of the virus across the United States. Some species of mosquitoes bite mammals as well as birds, and this is how the virus may be transmitted to a horse or human, or other mammal. There is no evidence that infected horses, people, or other animals are able to transmit the virus to other animals, people, or mosquitoes.

What are the clinical signs of WNV infection?

Birds that are infected with WNV may show signs such as the inability to fly, incoordination, and abnormal movements. Most crows are simply found dead. Clinical signs of horses which do become ill range from very mild signs to fatal illness. Typical signs include ataxia (incoordination, stumbling, limb weakness) that either appears suddenly or appears gradually and worsens, somnolence (sleepiness), dullness, listlessness, facial paralysis (droopy eyelids, lower lip), and inability to rise. Some horses may develop mild fevers, blindness, muscle trembling, seizures, and many other signs. Rabies, Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Equine Protozoal Encephalitis (EPM), and other neurologic diseases should be considered in a differential diagnosis for suspect horses.

How is WNV infection diagnosed in a horse showing clinical signs?

Diagnosis of WNV infection in horses involves testing the blood serum or cerebrospinal fluid (CSF) for antibodies against the virus. Testing can be done at the Michigan State University Diagnostic Center for Population and Animal Health (DCPAH). Horses that die with acute neurologic signs or are humanely euthanized should be tested for rabies, WNV infection, and other diseases. The head of the animal should be submitted to the MSU DCPAH. Michigan Department of Agriculture will provide transportation of the specimen for delivery to the lab and will also cover the laboratory expenses.

What is the incubation period of West Nile virus in horses?

The incubation period, or the time between the bite by an infected mosquito and the onset of clinical signs, ranges from 7 - 14 days.

Is there treatment for the disease?

Currently there is no specific treatment for the disease caused by West Nile Virus. Supportive therapy should be administered by the horse owner's veterinarian to reduce clinical signs and the possibility for secondary infections. Approximately 65% of affected horses will recover from the disease.

If my horse becomes ill and recovers, what are the long term effects of the disease?

When a horse becomes infected with WNV, with or without clinical disease, the horse develops an immune response to the infection. Immunity is believed to be long-lasting, such that the horse may be protected for several years from another WNV infection. Most horses with neurologic disease recover to be completely normal, although some may have residual nervous system damage.

What options are available to protect horses from WNV infection?

A vaccine is available as an aid in control of this disease in horses. The vaccine is a killed, adjuvented product similar to vaccines against Eastern Equine Encephalitis and Western Equine Encephalitis and is only available through licensed veterinarians. Horses vaccinated against Eastern, Western, and Venezuelan equine encephalitis are not protected against infection with West Nile Virus. Horse owners are advised to consult with their veterinarian concerning West Nile Virus vaccination of their horses.

In addition to the equine vaccine, simple insect control measures should be utilized. Consider the use of insect repellents, and place horses in barns/stables under fans during dusk, dawn, and other times when mosquitoes are present. Eliminate opportunities for mosquito breeding by draining wet areas of pasture, draining puddles, repairing eve troughs and gutters, clearing any containers that might hold even small pools of water, and draining water tanks one or twice weekly.