West Nile Virus: Still a threat to horses

Farm Forum

Time has a way of getting away from a guy.

I was just realizing that many of our younger equine enthusiasts probably don’t remember horses ever becoming ill from West Nile Virus. It was late summer of 2002, practicing in southeast South Dakota, when I made countless farm visits to diagnose, treat – and often euthanize – horses affected by this once-exotic virus.

Thankfully, clinical West Nile cases in horses dropped greatly after that terrible summer. Today a clinical case of West Nile Virus infection in a horse is a rarity. A young person showing horses today probably wonders what the big deal is about this disease.

West Nile is still a big deal. Just ask our colleagues in human medicine. South Dakota recorded its first human West Nile case of the year in late June. By the end of the summer, if we’re lucky, we’ll have recorded a couple dozen human cases in our state. If we’re not, we’ll have seen hundreds.

This is all the evidence we need to know that West Nile virus is still a potential threat to our horses. The virus resides in wild birds such as crows and blue jays. Mosquitoes bite those infected birds and then a horse or person, transmitting the disease. Once inside the body, the virus seeks out and attacks cells of the nervous system: nerves, spinal cord, and brain. In horses this damage shows up as muscle tremors, incoordination, weakness, and behavior changes. Incoordination and weakness may be present only on one side of the animal, which can make initial signs look a lot like lameness.

Why don’t we see as many West Nile Virus cases in horses anymore? One word: vaccine. Once a vaccine was approved for horses, it was widely adopted. And it 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.

Yet a handful of West Nile Virus cases still occur 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, mosquitoes can transmit diseases other than West Nile Virus infection. 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 usually not possible to remove all of these conditions. But do what you can. In areas that you and your horses frequent, get rid of stale standing water in buckets and other containers. 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. For some especially nasty nights, it may be best just to bring horses inside.

Those of you who have tried them know that insect repellants for use on the horses themselves sometimes leave a lot 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 we recommend for people use, are not labelled for horses. The research on DEET in horses is fairly 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 by forgetting vaccination and other preventive practices, it will come back to afflict more horses in years to come.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University. He can be reached via e-mail at or at 605-688-5171.