West Nile Virus: What You Need To Know
Author: Maggie McCaslin, CVBL, Practice Manager
Attention Horse Owners in the Star Valley, WY Area
West Nile Virus has been confirmed in two equine patients in Star Valley, WY. These patients have cleared the virus and are doing well at the time of this blog post.
West Nile Virus has been around the United States for roughly 25 years. And up until recently, it has not been in our Star Valley, WY area. At Bridger Animal Hospital, it’s very important to us that our equine owners are educated about potential risk to the health of their equine friends. Now that the virus is present in our area, we want to make sure you have all of the information you need in order to keep your horses safe and protected.
What is West Nile Virus?
West Nile Virus is a virus that humans, birds, and horses can contract from mosquitos. The virus is most prevalent in the Summer months in our area, when we notice mosquitos the most. This is especially true during the late summer months when temperatures are consistently warmer than any other time of the year.
Virus transmission occurs when a mosquito bites an infected host, and then bites another animal. For example, a mosquito would bite a bird in your area, then bite your horse. The virus is transmitted during that bite. Your horse can then be infected with West Nile Virus.
There is usually a 7-10 day incubation period, where symptoms would not yet be present. The infected horse would then begin to exhibit some or all of the following symptoms:
- Depression
- Lethargy
- Anorexia
- Inappetence
- Neurologic Signs: cranial facial nerve paralysis, inability to eat, head tilt, muzzle droop, complete paralysis of one or more limbs, recumbency, hyperexcitability, apprehension, and drowsiness.
- Colic
- Fine tremors and fasciculations of the face and neck muscles.
- Mild, low-grade fever of 101.5F-103.5F in only 25% of horses. Most horses do not exhibit a fever, especially during the initial phase of the virus.
It’s important to note that these symptoms also mirror those of Rabies, Eastern Equine Encephalitis, Western Equine Encephalitis, Venezuelan Equine Encephalitis, and the neurologic form of Equine Herpes Virus. It is imperative that a licensed veterinarian perform blood tests to determine the cause of the symptoms.
Who is at risk?
Horses that have not been vaccinated against West Nile Virus are at the greatest risk of acquiring this virus. It’s important that each horse receives their first vaccine 90 days prior to the first mosquito hatch of the year. In our area, we suggest vaccinating your horse as early as March. If your horse has never received a West Nile Virus vaccine, they will receive two injections, 4 weeks apart. The vaccine will then need to be updated annually thereafter. If your horse skips a year, they will need to follow the initial protocol as if they have not received the vaccine. It’s imperative that this vaccine is given in the Spring to ensure that antibodies in the blood are at effective levels PRIOR to mosquito season.
The virus is most often acquired in Summer months when the temperatures stay warm for a longer period of time, and mosquitos are most active. The current risk in our area during the time of the blog post (September, 2025) is subsiding as temperatures begin to drop below freezing.
Can my horse give West Nile Virus to another horse?
No, West Nile Virus is not directly contagious from horse to horse. It is not air born, or passed in bodily fluids like some other viruses. They do not shed the virus, nor develop a sufficient viral load to serve as a source of virus to insects. There are no isolation protocols required for a horse that is infected with the West Nile Virus.
Is West Nile Virus Fatal?
The average case fatality rate for West Nile Virus in horses is 30-40%. Some patients have residual neurologic deficits, and some horses heal completely.
What can you do to help prevent West Nile Virus?
Vaccinate your horses! And don’t forget to vaccinate them every year. Also, take steps to mitigate mosquito infestations in your area. Use mosquito repellent sprays. Don’t forget to reapply after rain storms or bathing.
We are here for you and your horses. Please don’t hesitate to reach out to us with any of your questions or concerns.
References:
https://pubs.nmsu.edu/_b/B719/?index.html
https://ceh.vetmed.ucdavis.edu/health-topics/west-nile-virus