Mosquitoes are more than just a pain in the butt for us humans, and a means for transporting deadly heartworm disease that can infect our feline and canine friends (read our blog post on heartworm disease for more information!). They are also the carriers for a slew of infectious diseases in horses as well. While many people think of summer as “mosquito season,” over the years more and more studies have shown that year round protection against mosquito borne diseases are the only way to make sure your horse is safe.

Between milder winters, and longer summers, mosquitoes have a way of hanging around longer than you may think. When you review the list of diseases that mosquitoes can infect your equine with, you may want to reconsider your fall vaccinations to make sure your horse is really covered.

So what can mosquitoes really carry anyway? Here’s a list of the most common mosquito borne diseases in horses and the most important things to know about them.

West Nile Virus

One of the most common and well known of the mosquito borne diseases in horses, West Nile Virus is also a disease that affects humans. Horses are not good carries of the disease, however, as not enough of the disease circulates in their blood to make it easily transferrable from an infected horse via mosquito bites to non-infected horses or people. As per the American Association of Equine Practitioners regarding WNV, horses represent 96.9% of all reported non-human mammalian cases of WNV disease.

With an over 33% fatality rate, and surviving horses having a high instance of lifelong side effects, vaccination is absolutely recommended. Vaccinations should be done for West Nile Virus in the spring with annual revaccination. First time vaccination protocols can vary depending on which specific vaccine is used.


Equine encephalomyelitis comes in two common and one rare form: Easter, Western, and Venezuelan, respectively. Like WNV, equine encephalomyelitis is transmitted when a mosquito bites an infected host such as a bird or rodent, and then bites a horse. Also like West Nile Virus, there is a vaccine available and it is more cost-effective than treatment. VEE is the rarest form of the disease, but Eastern and Western variations can occur throughout the United Staes with the Eastern variation more prevalent on the East Coast and the Western variation less common but possible out West.

Vaccinations are recommended in the spring with annual revaccination, and can be related at 6 month intervals in areas where horses are considered to be at high risk of exposure.

Potomac Horse Fever

Unlike the prior two diseases, it’s been more recently discovered that PHF is not transmitted by mosquitoes, but rather by bugs such as mayflies and dragonflies. Infected bugs then die, and contaminate hay, pasture and feed which puts horses at risk.

Although different in that it’s not a mosquito borne disease, it is often grouped in the same category because it is spread by insects that are prevalent in areas of standing and stagnant water. In addition to vaccination, prevention is also similar as the other mosquito borne disease in eliminating standing water on your property, reducing use of lights at dusk and dawn and at night when these types of bugs may be drawn to areas where your horse is stabled, and of course, remaining diligent in the event your horse shows any symptoms.

Vaccination can be done spring and fall, and in endemic areas, as frequently as every 3 months during high risk season (spring, summer, fall) to keep your horse protected. While the vaccine may not prevent contraction of the disease, horses who are vaccinated frequently have much milder symptoms and a higher chance of recovery if they do contract the disease.

Additional Reading

Want to learn more about vaccination guidelines for these and other diseases common to horses in the United States? The American Association of Equine Practitioners’ Vaccination Guidelines area a great resource!

And when it comes to mosquito borne diseases in dogs and cats, heartworm is one of the greatest risks. Read our full guide to heartworm prevention for your pet in our blog now!