August Newsletter

Eastern Equine Encephalomyelitis
By Charlene B. Cook DVM

It’s summertime and our office has been flooded by alerts for Eastern Equine Encephalomyelitis (EEE) recently. From Massachusetts to Florida the case reports are coming in almost daily. The first reported cases in Georgia this year came in July from Brooks County where 2 horses died. In August we diagnosed a case in Peach County.
So what is EEE and what can you do about it?
EEE is the inflammation or swelling of the brain caused by the Eastern Encephalomyelitis Virus. Commonly called sleeping sickness, the EEE virus is a zoonotic (infects both animals and humans) alphavirus and arbovirus present in North, Central and South America and the Caribbean. EEE affects the central nervous system of many species of animals but is most often detected in birds and horses. EEE was first recognized in Massachusetts in 1831 when 75 horses died of encephalitic illness. Outbreaks in horses have continued to occur regularly in the United States. EEE is most commonly found today in the eastern and southern states in the US.
The EEE virus was first isolated from infected horse brains in 1933. In 1938, the first confirmed human cases were identified when thirty children died of encephalitis in the northeastern United States. These cases coincided with outbreaks in horses in the same regions. The fatality rate in humans is 35% and there is currently no cure for human infections.
EEE is capable of infecting a wide range of animals including mammals, birds, reptiles and amphibians. The disease is transmitted to horses, and sometimes humans, by mosquitoes that have become infected after feeding on birds, which are circulating the virus. This is known as the mosquito/bird transmission cycle. It is important to remember that the virus is not transmitted directly from an infected horse to other horses or people.
The EEE virus normally circulates between birds and mosquitoes in swamp areas.  There are two mosquito species primarily involved in this portion of the cycle, they are Culiseta melanura and Cs. morsitans. These mosquitoes feed on the blood of birds. The amount of virus found in nature increases throughout the summer as more birds and more mosquitoes become infected. Transmission of EEE to mammals occurs via other mosquitoes. These other mosquitoes are called bridge vectors because they bring the virus from avian populations to mammalian populations. They include Coquiletidia perturbans, Aedes vexans, Ochlerotatus sollicitans and Oc. canadensis. All these mosquitoes are primarily mammalian feeders. Generally, people only become sick through the bite of an infected mosquito and the illness is rare in humans. Humans, horses and other infected mammals do not circulate enough virus in their blood to infect additional mosquitoes.
After inoculation by the vector, the virus travels via lymphatics to lymph nodes and replicates in macrophages and neutrophils (white blood cells), resulting in lymphopenia, leukopenia and fever. Subsequent replication occur in other organs leading to viremia. Symptoms in horses occur 1–3 weeks after infection and begin with a fever that may reach as high as 106 °F (41 °C). The fever usually lasts for 24–48 hours. Nervous signs appear during the fever that include sensitivity to sound, periods of excitement, and restlessness. Brain lesions appear causing drowsiness, drooping ears, circling, aimless wandering, head pressing, inability to swallow, and abnormal gait. Paralysis follows causing the horse to have difficulty raising its head. The horse usually suffers complete paralysis and death 2–4 days after symptoms appear. Once a horse becomes infected with the EEE virus and develops neurologic signs, the disease is fatal in roughly 90% of cases. Survivors are left with severe sequelae.
 In humans symptoms include high fever, muscle pain, altered mental status, headache, meningeal irritation, photophobia, and seizures, which occur 3–10 days after the bite of an infected mosquito.
The disease can be prevented in horses with the use of vaccinations. These vaccinations are usually given together with vaccinations for other diseases, most commonly WEE  and tetanus.
There is no cure for EEE. Treatment consists of corticosteroids, anticonvulsants, and supportive measures such as intravenous fluids, antiinflammatory medication and antipyretics] Some 30-35% of infected humans die, and half of survivors are left with neurologic sequalae.
The USDA's new National Surveillance Unit (NSU) has established a web site on which the public can see where equine diseases such as Equine Infectious Anemia (EIA) and Vesicular Stomatitis (VS) have been detected. Before the launch of this web site, finding the disease status of a state might have meant contacting several sources or playing phone tag with a handful of agencies. In addition, each state's health requirements for transported horses are available on the site. The site appears at
In 2009 there were 301 of Eastern Equine Encephalomyelitis reported from 18 states. As you would expect the highest number of cases are located in the southeastern US.

Alabama -23 cases
Arkansas – 1 case
Connecticut – 1 case
Florida – 75 cases
Georgia – 44 cases
Louisiana – 28 cases
Maine - 15 cases
Maryland – 1 case
Massachusetts – 1 case
Mississippi – 43 cases
New Hampshire – 3 cases
New Jersey – 6 cases
New York – 7 cases
North Carolina – 23 cases
Rhode Island – 2 cases
South Carolina – 14 cases
Texas – 5 cases
Virginia – 9 cases

In most reported cases, the infected horses are four years or younger and are not current on vaccination against EEE. The initial series of two vaccinations should be given four weeks apart followed by semiannual boosters. Foals should begin their vaccine series at 4 months of age or earlier if the dam was unvaccinated. A series of two doses of vaccine is absolutely essential to protect foals.
The use of vaccine to protect horses against EEE and other encephalitic disease, such as West Nile, is only a tool and should be used in conjunction with good mosquito reduction and avoidance measures implemented to protect yourself and your horse. The key is mosquito control to stop the mosquito/bird infection cycle. The most effective method is to destroy the mosquito larval habitat by removing all potential sources of stagnant water in which mosquitoes might breed. Mosquitoes can breed in any puddle that lasts more than four days. Water buckets, water troughs, wading pools, birdbaths, wheelbarrows, clogged roof gutters, discarded tires, plastic containers or any water-holding container should be cleaned or emptied on a weekly basis. Drill holes in the bottom of containers that are left out-of-doors, turn over wheelbarrows, aerate ornamental pools and stock them with fish or chlorinate them.
In addition to reducing mosquito populations, preventing animals from being exposed to adult mosquitoes is important. Horses should be stabled inside during peak mosquito feeding times, which are dawn and dusk. Use of mosquito-resistant structures such as well-maintained insect screening and fans may reduce potential access of mosquitoes to equine and other livestock hosts. Insect repellants can be effective for humans and have some limited value for horses as well.
Steps you can take to help protect yourself
  • Use an EPA-registered repellant anytime you are outdoors for any length of time.
  • Wear protective clothing such as long sleeves and pants.
  • Avoid prolonged exposure during mosquito biting times. The hours of dusk to dawn are peak biting times for many species of mosquitoes.
  • Remove containers that collect water or empty them daily.
  • Check windows or screen doors for holes to keep mosquitoes from entering your home.
Horse owners should use repellents approved for use on horses, but the effectiveness of some formulations under certain conditions (e.g., rain, perspiration) may be limited. Always follow label instructions. Horse owners are encouraged to contact their veterinarian immediately should they notice any signs or symptoms of EEE infection in their horses, especially those exhibiting neurologic signs.

The single most important thing for you to know is that the most deadly of these viruses can be prevented. Vaccines for EEE, WEE, West Nile and Rabies are inexpensive and provide excellent protection when given correctly.

Well that's all for now, thank you for reading.