Everything You Should Know About Mosquito-Transmitted Diseases

EEE, WEE and WNV…..What do these letters mean and why you should be aware? Summer is here and with it comes the dreaded mosquitoes. Those that live in more tropical climates have certainly been fighting these pests already this season and for those in more northern climates they are just beginning.  In recent years the mosquito has become more than just a biting pest. They carry diseases that can infect your horse and at best put him out of commission. The most common diseases to be concerned about are Eastern and Western Equine Encephalomyelitis as well as West Nile Virus. Keep reading to find out the symptoms of each and how to help prevent your horse from being infected.

Western Equine Encephalomyelitis

WEE has been recognized in bird hosts in the eastern USA, but clinical disease in horses is not as common. This is the least concerning of the three major mosquito borne diseases. Horses cannot develop high enough levels of the virus in their blood to be contagious to other animals or humans. Symptoms include a mild to severe fever that lasts about 24 hours, poor appetite and stiffness. Most cases of WEE do not progress beyond this point. Despite the relatively mild effect of these symptoms you should still contact your vet right away if you suspect your horse has WEE. The mortality rate ranges from 20 to 50%, this disease is not to be taken lightly.

Eastern Equine Encephalomyelitis

While EEE is very similar to WEE, it is much more common in the United States and it has more severe clinical signs. Acute (sudden) symptoms begin the same as WEE with a mild to severe 24 hour fever, poor appetite and stiffness. Unlike WEE, progression in EEE is nearly certain. The disease progresses to include depression, aggression and excitability. Some horses may become frenzied after any kind of stimulation. Later signs show the severity of the neurologic decline and includes head pressing, propulsive walking, blindness, circling, head tilt, facial and limb trembling or twitching. Paralysis of the throat and tongue are also common. Defecation and urination will become difficult. Recumbency (laying down and unable to get up) will happen 1 to 7 days prior to complete paralysis and death.
EEE should be taken seriously and your vet should be contacted at the first sign of infection. The mortality rate for EEE is 75-100% and treatment is supportive care. Typically, EEE will not respond to Banamine or steroids, leaving only IV fluids to help make your horse more comfortable. 

West Nile Virus

WNV is a potentially deadly disease in horses and appeared more recently in 1999. The virus will typically appear in the blood 3 to 8 days after a horse is infected, with symptoms beginning shortly after. In a very small percentage of cases, WNV will overwhelm the immune system and result in inflammation of the brain and spinal cord. Typical symptoms include loss of appetite, depression, a fever lasting two or more day, hind end weakness or paralysis, twitching or trembling muscles, impaired vision, incoordination, head pressing or head tilt, aimless wandering, convulsions, tongue or muzzle paralysis, droopy ear, vertigo, drowsiness, narcolepsy, inability to swallow, circling, hyperexcitablility or coma. Horses that have laid down and are reluctant to rise are the least likely to survive.
When these symptoms appear you should contact your vet immediately, unlike with WEE and EEE there are more treatment options. Your vet can give anti-inflammatory medications which may be able to minimize neurologic deficits and improve the chances of a full recovery.  The mortality rate for WNV is 30-40%.

How do we prevent infection?

The two main routes of prevention are vaccination and minimizing mosquito bites. Your horse should be vaccinated against all three diseases at least once a year in northern climates and twice a year in warmer climates. The vaccine is proven to last 6 to 8 months.  If you head south in the winter months, be sure to revaccinate.

The second route is to prevent mosquito bites. A great way to do this is to cover your horse with a full coverage fly sheet, such as our Oasis Plus Summer Turnout, that has a fine enough mesh to keep mosquitoes from biting through it. Other options are to use insect repellents, avoid turnout and riding outdoors at dawn or dusk and use fans in your barn, as mosquitoes have a hard time flying in the wind. Preventing the breeding of mosquitos is important and all standing water should be removed from your farm if possible, or treated with an insecticide. Also avoiding turning lights on in or near the barn after dusk, as this will lure mosquitoes in.

While all of this sounds scary and makes you want to put our horse in a bubble, the best thing you can do is stay on top of prevention. By vaccinating your horse, keeping them covered during turn out and minimizing areas that mosquitoes breed in, you are significantly reducing the risk of contracting WNV, WEE or EEE. So don’t let the mosquitoes ruin your summer, get out there, enjoy the sun and grass, and be safe.

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