Having spent most of my career in a mixed animal veterinary practice, I had a front row seat to all the bad things that could happen to a horse. Colic, sprained fetlocks, and injuries from trying to run through a barbed-wire fence were calls we’d get on a regular basis. There are some other things you don’t want to happen to your horse, either – and these are preventable.

I’m talking about certain virus-caused horse diseases – particularly those spread by mosquitoes. West Nile Virus infections and sleeping sickness are examples of these. They are debilitating, hard to treat, and very often end up being fatal.

These diseases are possible every year, but 2019 is shaping up as a potentially bad one. This year is different because of the moisture left here by this past winter’s snowy weather: moisture that’s making its way down our waterways, but also pooling in our ditches, pastures, and yards.

This moisture is just what mosquitoes need to reproduce and thrive. Coupled with the inevitable warmer temperatures, this extra moisture could be a boon for all the different types of mosquitoes.

Each of the mosquito-borne horse diseases features a slightly different mosquito species as its main carrier, but each of those species requires water for their reproduction. With Western Equine Encephalomyelitis and Eastern Equine Encephalomyelitis (the two versions of sleeping sickness we can encounter here) and West Nile Virus infections, the main culprit is the Culex species of mosquito. These mosquitoes lay their eggs on the surface of still, standing water; those warmed by sunlight are particularly attractive. This includes the ponds of standing water on pastures, in lots, or in our yards (as well as receptacles like tires and empty cow lick tubs). Our nuisance mosquitoes (of the Aedes species) are a bit different in that they lay their eggs in moist soil outside the water. Temporary pools or water bodies with fluctuating levels are attractive to these bugs. For either version of mosquito, the moisture left over from this winter helps them reproduce and thrive.

The more mosquitoes that are out there, the more likely your horse is to encounter one that carries one of these viruses. It takes more than just the mosquitoes, though. The real reservoirs (dependable sources in nature) of sleeping sickness and West Nile viruses are certain bird species. The birds don’t usually succumb to these diseases themselves; rather, they incubate and multiply the viruses within their body, making it easy for a mosquito to pick it up from them, then fly over to a horse to deposit the virus in her bloodstream. The horse is considered a “dead end” host, meaning that she’s not involved in further transmission of the disease once infected.

Fortunately, for these viruses, good vaccines exist. Now is the time – before the mosquito populations expand – for you to get your horse current. These vaccines require annual boosters, so horses need a dose this spring regardless of their past vaccine history. If your horse has never been vaccinated, this is a very good year to do so! Unvaccinated horses usually need two doses their first year, then annually after that. For young foals, the age at which they’re vaccinated sometimes depends upon their mother’s vaccination status; your veterinarian can guide you through those decisions.

Sleeping sickness and West Nile Virus aren’t the only vaccines your horses should get this, and every, spring. Horses are a species extremely susceptible to tetanus – every horse should get an annual booster. Rabies is a perpetual risk here in the Dakotas, and horses should be protected with an annual vaccine for this as well. For horses that travel and come in contact with others, influenza and “rhino” should be on the shot list, too.

Mosquitoes cause us enough trouble even without the infectious diseases they expose our horses to. While they can’t prevent us from being annoyed or bitten by them, the effective vaccines we have at our disposal can prevent some of their deadliest effects.

Russ Daly, DVM, is the Extension veterinarian at South Dakota State University. He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.

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