What are West Nile Virus?
A pathogen seen throughout the world, West Nile virus is often found in Africa, Asia and the Middle East and was first noted in the U.S. in the late nineties. Now thought to be endemic to North America, the virus is maintained as a result of a transmission cycle among mosquitoes and wild birds. West Nile virus can impact horses of any age and can cause encephalitis and meningitis. The majority of horses that are bitten by a mosquito carrying the virus don’t develop the disease. One-third of the horses that do develop the disease will have such severe illness that it will result in death.
Infecting horses of any age throughout the world, West Nile virus is transmitted by mosquitoes and can lead to encephalitis or meningitis.
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Symptoms of West Nile Virus in Horses
When a horse is infected with the virus, the incubation period is between three and 15 days and in the U.S., clinical signs are seen in 10-39 percent of horses with the infection. Symptoms include:
- Lack of muscle coordination
- Leg weakness
- Partial paralysis
- Twitching muscles
- Unable to stand
- Altering of their usual mental state
- Decrease in appetite
- Grinding teeth
The majority of horses that are bitten by a mosquito that is carrying West Nile virus ultimately don’t develop the disease. Of those horses that do develop the disease, one-third experience such severe illness that it leads to death.
Causes of West Nile Virus in Horses
Birds are reservoir hosts of West Nile virus and the mosquitoes are used as vectors that move the virus to each host. On occasion, mosquitoes will spread the virus to horses and people. As a result, controlling mosquitoes around barns, stables and homes is imperative in order to minimize the chances of exposure to the West Nile virus. Only mosquitoes transmit the disease to horses and people; there is no proof of transmission from one horse to another or from a horse to a person. While the disease has been seen in foals as young as three weeks old, the risk of infection appears to increase with the age of the horse.
Diagnosis of West Nile Virus in Horses
Your veterinarian will require more than just clinical signs when diagnosing West Nile disease, as other diseases cause similar symptoms; for example: rabies, equine protozoal myeloencephalitis and equine herpes virus-1. These and other diseases will need to be ruled out in order for your horse to be diagnosed with West Nile virus. In addition to conducting a physical examination of your horse, your veterinarian will likely utilize serological testing in order to confirm the diagnosis of West Nile virus.
The most relied upon test is the IgM capture ELISA, which can confirm whether your horse was recently exposed to the virus, as the IgM antibody will increase quickly after the exposure (though this increased level does not last long). Should the test be positive, it will point to an infection having occurred within the last three months.
Treatment of West Nile Virus in Horses
Horses with severe symptoms as a result of West Nile virus may die due to the infection itself or may be euthanized as a result of secondary complications of the illness. Data shows that around 35% of horses that show clinical signs will ultimately pass away.
Should your horse be diagnosed with West Nile virus, your veterinarian will discuss treatment recommendations as there is no specific treatment for the disease. Your horse should be treated for the symptoms that he is presenting with a focus on reducing the disease’s severity. If your horse is weak or impaired, your veterinarian will want to assist in providing protection so that he does not harm himself. Fluid and nutritional support may be necessary, either intravenously or through a stomach tube.
Recovery of West Nile Virus in Horses
Some horses that recover from infection with the West Nile virus will show residual effects like walking abnormally or a change in behavior. It is important to communicate regularly with your veterinarian and attend follow up appointments as recommended to ensure the best outcome for your horse. Particularly when your horse has exhibited neurological symptoms, it is important that your veterinarian continues to be involved, as some of the effects of the condition may lead to safety issues for the person handling the horse.
A vaccine is available for West Nile virus and this is the main method used for decreasing the risk of infection of the virus in horses. The vaccine requires two initial doses three to six weeks apart, followed by an annual or semi-annual booster. As it is not clear whether once acquiring the infection a horse can be re-infected, it is recommended that a horse that has recovered should receive the vaccine one year after their infection.