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Veterinary Division - Animal Health Programs

Eastern Equine Encephalomyelitis (EEE)


Eastern Equine Encephalomyelitis Fact Sheet

History
Equine encephalomyelitis is an inflammation of the brain and spinal cord. This disease not only affects horses, but can also be deadly for humans. The virus was isolated, characterized, and equine vaccines were developed in the 1930s. The two forms currently active in the United States are Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE). The viruses responsible for causing these diseases are members of a family of viruses called the alphavirus.

WEE is most active in Western United States and parts of the Midwest. EEE has an Eastern United States distribution. Historically, there have been isolated cases in Michigan and Wisconsin as well as outbreaks in the Caribbean, Dominican Republic, Haiti, Panama, and Central and South America.

Transmission
Wild birds serve as reservoirs for the virus. Mosquitoes bite the birds and then can transmit the virus to humans and equine. The warm, humid weather of the summer is good for mosquito breeding and this is when outbreaks are more common. Humans and horses are dead-end hosts and do not spread the disease. Ratites (emu and ostriches) have also been shown to be susceptible to the disease.

Clinical Signs and Diagnosis in Equine
Approximately two days after equine infection with encephalomyelitis, there is an infection and low-grade fever, which are not readily apparent. The first visible signs are at four to five days. At that time, the animal generally has a fever and rapid heart rate, is showing signs of anorexia, depression, and variable other neurological signs.

As the illness progresses the brain stem and spinal cord are affected. Muscle weakness becomes apparent and there are behavioral changes and dementia. Notable symptoms aggression, head pressing, wall leaning, compulsive circling, and blindness. Other signs might include uncontrolled twitching of the eyeball, and facial muscle paralysis.

As the disease progresses, a semi-comatose and convulsive state occurs. Death usually follows two or three days later. If the animal survives, residual nervous system problems result. The reported mortality rate for EEE in equine is 75-90 percent.

Vaccination and other prevention methods
There is no specific treatment for equine affected with EEE, making vaccinations essential to the prevention of the disease. The vaccination protocol consists of two vaccine injections 30 days apart, followed by booster shots every six months, preferably in early spring and late summer or early fall.

Equine owners should contact their veterinarians about the vaccine, which is readily available. In addition, owners should limit mosquito exposure through the use of repellents and protective stabling practices, particularly during the late afternoon and evening hours when mosquitoes are especially active. They should also eliminate sources of standing water on their premises, which serve as breeding areas for mosquitoes.

Zoonotic Risk
North Carolina averages one human case each year. The disease is fatal in about 50 percent of human cases, with young children and the elderly most at risk. Questions about the human health risk should be directed to the N.C. Department of Health and Human Services Public Health Division at (919) 733-7081.

Information compiled from AnimalForum.com, Merck Veterinary Manual, and others.


Facts About EEE
By Jennifer Kunz, AnimalForum.com staff

Equine encephalomyelitis is an inflammation of the brain and spinal cord. This disease not only affects horses, but is also deadly for humans.

The virus was isolated, characterized, and vaccines were produced in the 1930s. The two forms currently active in the United States are Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE). The viruses responsible for causing these diseases are members of a family of viruses called the alphavirus.

WEE is most active in the western United States and parts of the Midwest. WEE was responsible for the death of 180,000 horses between 1931 and 1938 in the San Joaquin Valley, California. There have been isolated cases in every state west of the Appalachians, parts of western Canada, and in central and South America.

EEE has an eastern United States distribution. There have been isolated cases in Michigan and Wisconsin as well as outbreaks in the Caribbean, Dominican Republic, Haiti, Panama, and central and South America.

The mosquito transmits the virus from small infected animals such as birds and rodents to horses. The warm, humid weather of the summer is good for mosquito breeding and this is when outbreaks are more common. Transmission of EEE and WEE is not horse to human, but bird or rodent to human via the mosquito.

In contrast, Venezuelan Equine Encephalomyelitis can be transmitted directly from an infected horse to a human via a mosquito. In countries where Venezuelan Equine Encephalomyelitis occurs, the epidemics are considered explosive. A 1969 outbreak near Ecuador caused 31,000 human cases. The last known outbreak of Venezuelan Equine Encephalomyelitis in the United States occurred in Texas in 1971.

Approximately two days after equine infection with encephalomyelitis, there is an inapparent infection and low-grade fever. The first apparent signs are at four to five days. At that time, the animal generally has a fever and rapid heart rate, is showing signs of anorexia, depression, and variable other neurological signs.

As the illness progresses the brain stem and spinal cord are affected. Muscle weakness becomes apparent and there are behavioral changes and dementia. Notable symptoms aggression, head pressing, wall leaning, compulsive circling, and blindness. Other signs might include uncontrolled twitching of the eyeball, and facial muscle paralysis.

As the disease progresses, a semi-comatose and convulsive state occurs. Death usually follows two or three days later. If the animal survives, residual nervous system problems result. The reported mortality rate for EEE is 75-90 percent, and it's 20-50 percent for WEE.

Encephalomyelitis vaccines are available for horses from several different companies. They are packaged as single or combination vaccines. The combination commonly referred to as a "three-way shot" includes a vaccination for EEE, WEE, and tetanus. "Four-way shots" include EEE, WEE, tetanus, and influenza.

All unimmunized horses are at risk regarding this disease. Speak to your vet today if you are unsure about your horse's vaccination history.


Precautions to take to protect Equine

Reduce Mosquito Breeding Sites

You can decrease the chance of your animals’ being exposed to the virus by limiting their exposure to mosquitoes. The best way to do this is to reduce mosquito breeding sites.

Mosquitoes can breed in any puddle that lasts more than 4 days. The best way to reduce your risk is to remove any potential sources of standing water in which breeding can take place.

  • Dispose of water-holding containers such as old tires.
  • Drill holes in the bottom of containers that are left outside.
  • Thoroughly clean watering troughs, bird baths, etc., every few days.
  • Clean clogged roof gutters every year.
  • Turn over wading pools or wheelbarrows when not in use, and do not let water stagnate in bird baths.
  • Aerate ornamental pools.
  • Clean and chlorinate swimming pools that are not in use and do not let water collect on pool covers.
  • Use landscaping to eliminate low spots where standing water can collect.

Screened Housing
Well-maintained insect screening can be useful to reduce exposure to adult mosquitoes if precautions are taken to first eliminate mosquitoes from inside the structure. Fans may reduce the potential ability of mosquitoes to feed on horses.

Insect Repellant
Using insect repellants may help decrease exposure of horses to adult mosquitoes. Because under certain conditions (e.g., perspiration) some products have a limited duration of effectiveness, it is not wise to rely solely on repellants to prevent mosquito exposure. Use repellants according to label instructions. Products containing a synthetic pyrethroid compound (such as permethrin) as the active ingredient serve two purposes: (1) they offer superior safety and repellent efficacy and (2) they are contact pesticides that kill mosquitoes.

Outdoor Exposure
Mosquito species vary in their feeding habits, making transmission possible at any time of day or night. However, a recent epidemiologic study of WNV suggests that keeping horses in stalls at night may be helpful in reducing their risk of infection.

Information from USDA-AHPIS web site

 

 

NCDA&CS Veterinary Division, Dr. Steven Wells - Interim State Veterinarian
Mailing Address:1030 Mail Service Center, Raleigh, NC 27699-1030
Physical Address: 2 W. Edenton Street, Raleigh, NC 27601
Phone: (919) 733-7601; FAX: (919) 733-2277