EASTERN EQUINE ENCEPHALITIS
Eastern Equine Encephalitis is a disease of concern for the Florida horse community. The state averages over 70 reported cases of Eastern Equine Encephalitis (EEE) each year. In years when conditions favor the spread of the EEE, the number of reported cases can exceed 200, with over 90% of affected horses dying.
EEE is a viral disease that affects the central nervous system of many species of animals but is most often detected in birds and horses. 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.
Signs in infected horses can be varied but usually begin with fever, depression and listlessness, which then progress to more serious neurologic signs such as in-coordination, stumbling, circling, head pressing, coma and usually death. Once a horse becomes infected with the EEE virus and develops neurologic signs, the disease is fatal in roughly 90% of 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. In years with an above-average incidence of EEE, vaccination boosters given three times a year are recommended. Ask your veterinarian at what age to begin and the frequency of foal vaccination, as it differs from adult horses.
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. The Florida Department of Health recommends the use of repellents containing DEET (N,N-diethyl-meta-toluamide,or N,N-diethyl-3-methylbenzamide) or Picaridin and oil of lemon eucalyptus for preventing mosquito bites in humans.
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 neurological signs.
The Eastern Equine Encephalitis Virus belongs to a family of viruses called the Arboviruses. Arboviral diseases are those that are caused by an arthropod (insects such as mosquitoes, ticks, mites, and midges). These include West Nile virus, Eastern Equine Encephalitis virus, and St. Louis Encephalitis virus, which are all mosquito-borne diseases found in Florida. Because the Arboviruses poses a constant threat to humans and animals, in Florida, an extensive surveillance program has been implemented. This program involves the monitoring of mosquito pools, sentinel chickens and equine cases. Based on experience from previous years, one of the first indications that EEE might be present in an area is the unusually high death rate of birds, particularly crows or blue jays. Dead birds should be reported by contacting the local county Department of health office or by entering information on the Florida Fish and Wildlife Conservation Commission’s dead bird reporting web site at: http://www.myfwc.com/bird
The Florida Department of Health (DOH), in conjunction with the FDACS, the Florida Fish and Wildlife Conservation Commission (FWCC), local health and mosquito control agencies have developed a WNV Plan which provides a tiered prevention, control and response program for WNV depending on the location and type of evidence indicating an outbreak has occurred. For complete updates and information on the WNV Surveillance and Response Plan you may access the Florida Department of Health web page at: http://www.doh.state.fl.us/environment/community/arboviral/index.html, or you may contact: The Florida Department of Agriculture and Consumer Services, Division of Animal Industry, 407 South Calhoun Street, Mayo Building, Tallahassee, FL, 32399-0800. The division personnel can be contacted at (850) 410-0900.
Last Updated: October 9, 2008
