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Tri-State  Blog

What You Need to Know About Horse “Sleeping Sickness”


When you hear “silent but deadly,” your horse’s health may not be the first thing that comes to mind. But when it comes the virus equine encephalitis (also referred to as “sleeping sickness”), we want horses owners and stable owners to be aware that this virus goes symptomless for days and fatality rates between 50% and 90% in horses. 

What is Equine Encephalitis?

Equine encephalitis is a zoonotic virus that affects horses and people by attacking the central nervous system. Mosquitoes play a vital role in the transfer of equine encephalitis between animals. The virus enters the bloodstream of its victims and gets to work ravaging the brain and spinal cord by causing severe inflammation. This ultimately renders the infected person or horse unable to move, eat, and breathe. 

Many people that witness the death of a horse infected with equine encephalitis often describe it as “spontaneous” because the deterioration happens so rapidly.

How  Is  Eastern  Equine  Encephalitis Transmitted?

Sleeping sickness is a virus that thrives by traveling from host to host. It can survive in rodents, birds, and horses. A mosquito bites the infected animal and then bites a horse or person. The bite recipient then becomes infected with the virus. 

Eastern vs. Western  Equine  Encephalitis

Eastern Equine Encephalitis Virus or EEEV occurs east of the Mississippi River throughout the United States and Canada. And Western Equine Encephalitis (WEEV) occurs west of the Mississippi River. Both affect the brain and spinal cord.

Seasons of Heightened  Risk 

While equine encephalitis cases pop up throughout the year, they most frequently occur when mosquitoes are more active. This means horses and people are at greater risk in spring, summer, and fall or May through November.

Signs and Symptoms of Equine  Encephalitis

One of the most frightening aspects of equine encephalitis is that infected animals go symptomless for four to ten days. Once this virus has incubated long enough, its effects on the central nervous system begin to show. 

The apparent first signs of equine encephalitis are fever and depression. Then more symptoms occur. These include

  • Diarrhea

  • Drowsiness

  • Lethargy and inability to move

  • Lack of appetite or refusal to eat

  • Refusal to drink

Symptoms continue to escalate to 

  • Paralysis

  • Convulsions

  • Coma

Some cases show no symptoms before death occurs. 

How Deadly is Equine Encephalitis?

While EEE and WEE are both dangerous and potentially deadly, EEE shows a higher mortality rate. The rates of survival range between 0% and 25%, which means fatality rates are between 75% and 100%.


Most horses die within two days of showing symptoms. And while there is no treatment for WEE or EEE, there are steps and support in recovery that can help a horse regain her strength and health.

When people become infected with EEEV, death can occur within one to two days of symptoms. And many people that survive are left with lifelong physical and mental disabilities.

Equine  Encephalitis Prevention

Fortunately, prevention of equine encephalitis is possible. A simple vaccine protects horses from becoming victims of sleeping sickness. 

The equine encephalitis is administered in a two-part series and last six months. The first vaccine can be given at any time of year, the end of fall or beginning of spring make excellent options since they often align with other equine veterinary care. The second vaccine should be given two to four weeks after the first.

Additional Precautions

Controlling mosquitoes will help control the risk of an Equine Encephalitis outbreak. 

You can eliminate opportunities for mosquitoes to breed by ridding your stable and pasture of unnecessary standing water. Empty buckets of water, add plants to areas that tend to stay swampy, and close off areas of pasture with stagnant water. Look into adding drainage features to areas where water collects. This will assist in stopping the life of mosquitoes by eliminating areas for larvae to develop into adults and areas where adults can lay eggs.

Introduce natural predators like fish into ponds and increase the ability of natural predators to hunt mosquitoes. Trimming weeds around ponds and ditches can help bats and birds get their fill of mosquitoes. 

Keep troughs filled with clean, freshwater and check to see that your tanks are well sealed. 

There are chemical options to kill and reduce mosquitoes, but these can have the opposite effect in the long run as mosquitoes build up a resistance to the poisons. If you choose a chemical control approach, rotate which chemical you use to avoid this.

We  Want  to  Help  Keep You  and  Your  Horses Safe  from  Equine  Encephalitis

We can answer questions, schedule vaccinations, and check your horse if you suspect she’s presenting any questionable symptoms. Just give us a call to make an appointment or set up a consultation. 



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