These Are the Risks Mosquitoes Pose to Your Horse

This article is part of our Fight the Flies Awareness Campaign, brought to you by UltraShield.

Flies are a common nuisance in the summer. We’ve been covering the basics of fly control throughout the month of April, but what about other winged pests? Mosquitoes can leave itchy bumps on you and your horse, but the risk extends beyond discomfort. In fact, mosquitos are responsible for spreading the deadliest diseases of all the annoying insects that horse owners cope with.

Here you’ll learn about three frightening mosquito-borne diseases that threaten your horse’s health. You’ll discover how they’re spread, what symptoms they can cause, and how to prevent them. As a part of your prevention strategy, I’ll also give you tips for controlling mosquito populations at your farm. Arm yourself with this information before mosquito season sets in so you’re ready to fend them off and keep your horse healthy.

A mosquito bites an animal. A mosquito sits on a horse’s nose.

The Big Three


Three main viral diseases can be transmitted to your horse by a mosquito: eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), and West Nile virus (WNV). These may not be diseases you think about—you may not even realize they’re serious threats to your horse’s life. Yet they’re three of only five diseases that the American Association of Equine Practitioners recommends targeting with an annual vaccination for your horse.

These three diseases have many similar features, as well as some distinct differences. First, let’s take a look at their common features.

Similarity #1

All three viruses are carried by birds. A mosquito must first feed on an infected bird, then bite your horse to spread the virus. Horses are dead-end hosts for all three of these viruses, meaning they don’t develop a high enough viremia (amount of virus in their bloodstream) to allow transmission. If a horse is diagnosed with EEE, WEE, or WNV in your barn, your horse won’t be at risk of contracting the virus from the sick horse. Instead, your horse’s risk is from a potentially shared source. If mosquitos can infect one horse, they’re likely to infect another. It’s interesting to consider that if a horse in your barn or neighborhood is diagnosed with one of these diseases, quarantine isn’t really necessary, so don’t waste your time worrying about avoiding contact with other horses. Instead spend your time ramping up mosquito control and updating your horse’s vaccinations.

Similarity #2

All three of these viruses are neurotropic, meaning they impact your horse’s nervous system. As with most viral infections, the first sign of all three diseases is usually a fever, demonstrated when your horse shows vague signs of not feeling well. He’ll be off his feed and may be generally stiff or sore. After this initial infection there’ll be a period of time (usually one to three weeks) before the virus settles in your horse’s blood (he becomes “viremic”), and nervous-system signs appear.

Initial neurological signs include depression combined with excitability when your horse is stimulated. He may become aggressive and might walk or pace aimlessly around the stall. As signs progress, your horse could develop a head-tilt and start pressing his head against a wall or repetitively circling. He may go down and be unable to get up. It could lead to death.

Your vet can identify the specific virus causing your horse’s symptoms by testing blood or cerebrospinal fluid (the fluid that surrounds his brain and spinal cord) for antibodies. A high antibody level or “titer” can confirm the virus as a cause of the problem if your horse has not been vaccinated. If he has been vaccinated, he’ll have developed antibodies in response to the vaccination. Hence antibody levels must be measured when he first gets sick (acute) and again two to four weeks later (convalescent) and show four-fold increase during that time. An additional diagnostic test called a polymerase chain reaction (PCR) that detects genetic material from the virus can also be helpful for making a specific diagnosis.

The bottom line? If your horse develops a fever and neurological symptoms, any of these three viral infections could be to blame. Distinguishing between the three diseases will require more than just a clinical examination.

Similarity #3

There’s no specific treatment for any of these diseases. Supportive care consists of non-steroidal anti-inflammatory medications and IV fluid therapy plus dedicated nursing care to tend to injuries your horse might experience as he staggers around his stall. If your horse does become recumbent (goes down and is unable to rise), his care will require rolling or changing his position regularly to manage bedsores. And once this happens, his chances for survival are slim.

Similarity #4

All three of these diseases can impact humans—and can be deadly. Although you can’t contract any of these viruses directly from your horse, if any one of them is diagnosed in a horse in your area, you’d be wise to pay attention. Unlike horses, humans aren’t vaccinated against these viruses, so mosquito control is the best protection you’ve got.

What’s the Difference?


While these three diseases share a lot of similarities, there are also some key differences.

Difference #1: EEE is the most deadly. Death can occur within two to three days of when your horse shows symptoms, and 75 to 95 percent of horses diagnosed with EEE will die. Fatality rates for both WEE and WNV are closer to 20 to 40 percent.

Difference #2: WEE is the least concerning. In fact, if your horse were to contract WEE (which is highly unlikely), it’s possible the disease won’t progress beyond the fever stage.

Difference #3: The diseases appear in different locations throughout the U.S. As a general rule, EEE stays east of South Dakota and Texas, while WEE stays west of Mississippi, and WNV can be seen throughout the United States.

Difference #4: EEE and WEE have both been recognized since the 1930s (they were called “brain fever” by the old-time horsemen), while WNV is a relatively new disease that was introduced to the Eastern United States in 1999, and rapidly spread west.

Difference #5: Both EEE and WNV are still clinically relevant and are diagnosed in horses every year in the U.S. WEE is not diagnosed often, and is very rare, although the virus has still been found circulating in bird populations in Western regions—meaning the risk is still out there.

Find a fly spray that also repels other pesky critters. Photo courtesy of UltraShield.

Keep the Bugs at Bay

Vaccination against these deadly diseases is quite effective, but no vaccine is perfect. Controlling mosquitos is an important part of keeping your horse protected. (And has the added benefit of eliminating irritation caused by bites!) Here are some steps to control mosquito populations on your farm.

Eliminate standing water. Mosquitoes can breed in any puddle that lasts longer than four days. This can include water that accumulates in roadways or drainage areas, as well as ponds or ditches on your farm. Trash piles with old tires, buckets, or other vessels that hold water can become mosquito breeding farms. Even hoof prints can cause small pools of standing water in areas with poor drainage. Remove trash, maintain roads and walkways, and make sure all of your drainage systems work properly.

Clean troughs and buckets regularly. Even a pasture trough can be a mosquito breeding ground if left too long without a thorough cleaning. For best results, completely dump and replace water sources at least twice a week.

Introduce Deterrents

Invite predators. If you have a pond on your property, mosquitos will move in. Consider stocking that pond with mosquito-eating fish, and encourage dragonflies to populate the area. Dragonflies are attracted to ponds with either submerged vegetation or floating plants like water lilies. A single dragonfly will consume its own body weight in bugs every 30 minutes. Consult with your local nursery for the best plant options to attract dragonflies in your area. They also like flat rocks where they can sun themselves.

Mow weeds around ponds or streams. Clear banks allow natural mosquito predators to hunt more effectively.

Install and run barn fans as well as screens on doors and windows. 

Choose lighting carefully. Mosquitos are less attracted to yellow incandescent or fluorescent lights than basic white light.

Apply repellents. Be sure to watch for skin irritation, and check with your vet before using a higher concentration.

Try to avoid turn out or riding at dawn or dusk. Mosquitos are most likely to feed during these times of day.

[Find 5 Simple Ways to Keep Your Barn Fly Free]

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