West Nile Virus
West Nile is a disease acquired by horses from mosquito bites. The mosquitoes pick up the disease from infected birds and can then transfer the disease to horses, humans and other mammals. Horses are considered dead-end hosts which means they can not transmit the disease via mosquito bite. This disease is ubiquitous to the United States.
This disease causes an encephalitis, or inflammation of the brain and spinal cord. This leads to neurologic signs. Symptoms could be any combination of a fever, weakness, paralysis, muscle fasciculations, convulsions, inability to swallow, circling and hyper-excitability. These signs are not unique to West Nile Virus infection so a thorough veterinary exam should be performed.
Treatment is completely supportive care. Diagnostic tests can be performed to confirm infection. Around 30% of horses infected die and 40% of those that survive retain some neurological deficits at 6 months post-diagnosis.
The AAEP recommends all horses be vaccinated against West Nile Virus. There are several vaccines to choose from and all have been tested and meet USDA standards.
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Timing |
Additional Information |
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Foals (vaccinated mare) |
1st dose: 4-6 months of age 2nd dose: 4-6 wks after 1st dose 3rd dose: 10-12 months of age, prior to onset of next vector season. |
May start earlier for foals at increased risk (southeast USA). |
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Foals (unvaccinated mare) |
1st dose: 3-4 months of age 2nd dose: 4 wks after 1st dose 3rd dose: 8 weeks after 2nd dose |
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Boodmares |
Annual, 4-6 wks before foaling |
If previously vaccinated. Try and vaccinate naïve mares when open. |
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Vaccinated adults |
Annual |
Prior to vector season; spring. Consider 6 month booster for endemic areas, young, old or immunocompromised horses. |
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Unvaccinated adults |
2 doses: 2nd dose 4-6 weeks after first. Annual thereafter. |
See above. |