Eastern Equine Encephalomyelitis Confirmed in Two Maryland Horses

The first positive cases of Eastern equine encephalomyelitis (EEE) this year in Maryland have been confirmed in two Lower Eastern Shore horses. Tissues from two horses, both from the Pocomoke City area in Worcester County, were submitted to the Maryland Department of Health and Mental Hygiene (DHMH) on July 21 and July 23 and were confirmed positive and reported on July 28, 2003.

“Mosquito populations have been high all spring and mosquito-borne diseases like West Nile virus and EEE are present earlier than ever in Maryland and on the East Coast this year,” said Maryland State Veterinarian, Dr. Phyllis Cassano. “There are effective equine vaccines to prevent both diseases and I encourage all horse owners to contact their veterinarian and to have their animals vaccinated. Prevention is key because there is no specific treatment or cure for EEE or West Nile viral encephalitis.”

EEE can also cause serious illness in ratites (flightless birds such as ostriches and emus). Clinical signs of EEE in horses and ratites include apprehension, depression, listlessness, paralysis, lack of coordination, weakness, head-pressing, circling, and stumbling. These signs are not exclusive to EEE, so it is important to seek professional veterinary diagnosis immediately if any of the signs are present.

“Mosquito-borne infections may be a serious public health problem for people of all ages, but especially for young children, those over the age of 50, and those with already compromised immune systems,” said DHMH Secretary Nelson J. Sabatini. “That is why we encourage people to eliminate standing water in and around their property to help decrease the mosquito population.”

Although EEE tends to occur in humans less frequently than West Nile virus (WNV), it can be far more devastating. Up to 50 percent of EEE infected persons who develop neurological signs might die compared to fewer than 10 percent who die following WNV neurological illness. Many EEE survivors have long-term neurological damage.

Typical symptoms of EEE in humans include fever, headache, mental confusion, vomiting, extreme tiredness, muscle aches, and sometimes seizures and coma. Individuals reporting these symptoms should be referred to their health care provider. Symptoms usually occur four to 10 days after exposure to a mosquito carrying the virus. There is neither a specific treatment nor a vaccine for use in people.

Eastern equine encephalomyelitis is spread by the bite of an infected mosquito and can cause a swelling of the brain (encephalitis). The disease in humans and other animals is rare but can occur when an infected mosquito takes a blood meal. Human-to-human or horse-to-human transmission has not been documented to date. However, health officials should be aware of the theoretical possibility of transfusion- or transplantation-associated EEE transmission. EEE disease occurs primarily in rural areas close to swamps and marshes with high mosquito populations.

“Mosquito control remains especially important to decrease the risk of infection with EEE,” said Maryland Agriculture Secretary Lewis R. Riley. “The Maryland Department of Agriculture will increase mosquito control spraying in the areas where the infected horses were kept to reduce the population of older mosquitoes likely to be carrying the EEE virus.” Aerial spraying might occur to treat areas inaccessible to truck sprayers.

People are encouraged to take the following personal protection measures to avoid mosquito bites:

  • Wear insect repellents according to product labels, especially if you will be outside between the hours of dusk and dawn when mosquitoes are most active. Wear long sleeves and long pants to help avoid mosquito bites. Avoid mosquito-infested areas.
  • Remove standing water around homes, businesses, and stables (bird baths, flower pots, gutters, old tires, tin cans, plastic containers, or similar water holding containers, etc.). Water buckets and troughs for animals should be flushed and refilled frequently.
  • Drain standing water from tarps or other coverings used on boats or swimming pools. Arrange the tarps or covers to allow water to drain.
  • Install and inspect window and door screens in homes and stables and repair any holes found.
  • Check ornamental ponds, tree holes, and water holding areas for mosquito larvae. Drain or flush the water if larvae are found.

The last reported human case of EEE in Maryland was in 1989. The state’s last reported case of equine EEE was in 1996. The human and equine cases occurred on the Lower Eastern Shore. The federal Centers for Disease Control and Prevention is indicating that 2003 might become an unusually active year for EEE along the eastern seaboard, so vaccinating equines and taking preventive actions against mosquito bites are especially important. Already this year, a Georgia man has died of EEE, and there have been numerous EEE cases in unvaccinated horses in the southeast states of Florida, Georgia, South Carolina, and North Carolina. EEE has also been reported in two birds from West Virginia.

For more information about mosquito-borne diseases, contact your local health department.

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