West Nile Virus Disease in Horses

Key Takeaways

West Nile Virus is acquired by mosquitos from infected birds and transmitted to mammals, including horses, when they bite them. 

• In many cases, the virus infects the brain, causing inflammation which can lead to long term damage or death

• Mortality rates range between 20-44%

• Symptoms in horses include limb incoordination, muscle twitching, stumbling and behavioral changes

• In severe cases, paralysis, inability to rise, coma and death may occur

• Horses showing symptoms of West Nile Virus require emergency veterinary care to improve their prognosis

• West Nile Virus infection is diagnosed using blood tests

• There is no treatment for West Nile Virus

• Supportive care including fluids and anti-inflammatories is standard practice for infected animals

• Prevention is possible through routine vaccination and vigilant mosquito control

A Closer Look: What is West Nile Virus Disease in Horses?

Outbreaks of West Nile Virus occur all over North America as well as in other parts of the world. In the northernmost parts of North America, the mosquito season lasts from July to October with the peak during August. In the south, mosquitoes remain active throughout the year. Many areas track bird deaths due to West Nile Virus in order to predict potential outbreaks.  Most cases of West Nile virus infect the brain or spinal column. The prognosis of these cases is highly variable, as some horses recover, some suffer long term damage, and some die or are euthanized. The estimated mortality rate ranges from 20-44%. Symptoms of neurological involvement are often sudden and progressive, getting worse within 7-10 days of first appearing. Horses experiencing symptoms of West Nile Virus require emergency veterinary care to improve their prognosis.

Risk Factors

Vaccinated horses or those protected from mosquitos are less likely to contract West Nile Virus and develop symptoms. Once infected, some horses remain asymptomatic or suffer only mild symptoms. 

Symptoms of mild cases of West Nile Virus include:

• Lethargy • Hyperactivity • Lack of appetite • Mild fever

In some cases, horses with West Nile Virus develop joint and tendon infections, sheath infections, diarrhea or pneumonia.

Possible Causes

West Nile Virus is a flavivirus that is transmitted from infected birds to mammals by mosquitoes. The virus causes damage to the brain, lining of the brain, and spinal column.

Main Symptoms

The primary symptoms of West Nile Virus disease include:

• Limb incoordination especially the hind quarters • Stumbling • Muscle twitching along the body, neck or face

• Periods of apprehension, drowsiness, or hyperexcitability • Difficulty eating • Head tilt • Colic • Inability to stand

• Facial paralysis with muzzle deviation or drooping lips • Weakness of the limbs • Paralysis of one or more limbs

Very young or very old horses show more severe symptoms.

Testing and Diagnosis

A physical examination, a neurologic exam, and a history of mosquito exposure provide an initial suspicion of West Nile Virus. Diagnosis is confirmed with bloodwork.

Steps to Recovery

No cure exists for West Nile Virus, so treatment is largely supportive. Treatments include:

• Adequate hydration (intravenously if necessary) • Adequate nutrition (intravenously if necessary)

• Anti-inflammatory drugs • Head and leg protection to prevent injury in cases of incoordination

• Slinging the horse into a standing position to prevent pressure point sores in horses that cannot rise

In mild or moderately severe cases, symptoms improve within several days of supportive care. In many cases, horses have long-term symptoms including incoordination or behavioral changes that last for up to 6 months. Some horses have residual symptoms that never resolve. Horses that are unable to stand on their own have a very poor prognosis, and are typically euthanized. Mortality rates for West Nile Virus range between 20-44%.

Prevention

West Nile Virus is not transmissible between horses, or from horse to person, or vice-versa. It is transmitted by mosquitoes that have bitten infected birds and then bite a mammal. Vaccination for West Nile Virus is recommended for all horses. Vaccines have proven effective in promoting immunity when properly administered. Horses require an initial series of two vaccinations four to six weeks apart. Following this, regular boosters are required. In northern areas where mosquitoes bite only during the summer months, a single annual booster is sufficient. In southern climates, two boosters a year are necessary to ensure protection.

Limiting exposure to mosquitos is also effective for preventing infection. Strategies include:

• Eliminating standing water • Housing horses indoors during dusk and dawn

• Avoiding lights in the stable during the evening and overnight

• Stabling horses away from birds, including chickens • Fogging the stable

• Applying vet-approved mosquito repellent directly to horses during peak season

Is West Nile Virus Disease Common in Horses?

Since its first appearance in the US in 1999, there have been more than 25 000 cases of West Nile Virus in horses.

Typical Treatment

• Hydration • Nutrition • Pain relief • Anti-inflammatory drugs • Slinging • Head and leg protection