Younger equine enthusiasts may not remember horses ever becoming ill from West Nile Virus. This virus appeared in South Dakota in the summer of 2002. Hundreds of horses across the state succumbed to this disease, with at least a third of those affected dying or needing to be put down.

Shortly after that summer, clinical West Nile cases in horses began to subside greatly. Today a clinical case of West Nile virus infection in a horse is a relative rarity. A young person showing horses today probably wonders what the big deal is about this disease.

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West Nile is still a “big deal,” as evidenced by the human illnesses that still occur. South Dakota records West Nile Virus cases every year, usually beginning in June. In “good” years, South Dakota may see a dozen or so human cases of West Nile Virus; in “bad” years the toll can be in the hundreds.

This is all the evidence one needs to know that West Nile virus is still a potential threat to our horses. The virus has a reservoir in wild birds such as crows and blue jays. Mosquitoes that bite those infected birds and then bite a horse or person can transmit the disease. Once the virus is inside the body, it seeks out and attacks cells of the nervous system: nerves, the spinal cord, and the brain. The results for infected horses are signs such as muscle tremors, incoordination, weakness, behavior changes, and fever. The incoordination and weakness is often present only on one side of the animal, which can make initial signs look a lot like lameness.

It’s important to know that horses can’t transmit the virus to people, even if a mosquito bites an infected horse and then a person. The same holds true for a person transmitting West Nile Virus to a horse. Both are (sometimes ominously) referred to as “dead end” hosts.

What are the reasons for the decline in West Nile Virus cases in horses, while human case numbers remain high? One substantial reason is vaccine. Once a vaccine was approved for horses, it was widely adopted, and remains so today. Today there are several West Nile Virus vaccines available to horse owners, some of them in combination with other important disease agents such as sleeping sickness and tetanus. West Nile Virus vaccines should be a routine part of every horse’s preventive health program. For more information, view the SDSU Extension Vaccinating your horses for West Nile Virus publication.

Yet a handful of West Nile Virus cases still appear in our area every year, almost exclusively in horses that have not been vaccinated. This demonstrates the effectiveness of the vaccine, as well as the extent to which horse owners and veterinarians have embraced its use.

While vaccine has been a powerful tool to protect horses from West Nile Virus infections, horse owners should not forget about basic mosquito control. After all, no vaccine is 100 percent effective in every instance. In addition, there are other horse diseases spread by mosquitoes. Some of them, such as sleeping sickness, can also be prevented through vaccination. For others, vaccines do not exist – although these tend to be very rare occurrences. Probably more importantly, we need to control mosquitoes to keep us and our family members safe from West Nile and these other mosquito-borne germs.

We know that mosquitoes breed and thrive in standing water and surrounding muddy areas. In pasture situations, it’s impossible to remove all of these conditions. But one should do what’s possible to reduce their number. In areas frequented by people and horses, stale standing water in buckets and other containers should be removed. Some premise sprays may be useful to control mosquitoes around barns and small lots. When using these products, animals should stay out of those areas until the spray is dry. On some nights, it may be best to simply bring horses inside.

The duration of effect of insect repellants for use on the horses themselves sometimes leave much to be desired. Some products have better residual activity than others. It’s always safest to use products specifically labelled for horses. The mosquito repellants containing DEET, which are recommended for people use, are not labelled for horses. The research on DEET in horses is old, but indicates that skin problems can result from its use – so use of those products is best avoided.

West Nile Virus problems in horses have been reduced almost to the point of irrelevance through widespread adoption of vaccination and other control methods. In some ways, this is a dangerous point in time. If we assume this disease is no longer one we need to be concerned with and let our guard down, it will come back to afflict more horses in years to come.

By Russ Daly, DVM, DACVPM,