Maryland officials said Friday that a case of Eastern equine encephalitis has been confirmed in a horse in Worcester County and urged horse owners to consult with their veterinarians about vaccinations.

The horse, which had not been vaccinated, tested positive for the virus, which, like West Nile virus, is spread by mosquitoes. The disease can cause a swelling of the brain. Although it is rare in humans, it can occur when an infected mosquito bites a person. The last confirmed human case in Maryland was in 1989. The last confirmed case in a horse in the state was in 2009 in Wicomico County.

Although EEE occurs in humans less frequently than West Nile virus, it can be more serious. Typical symptoms of EEE in humans include fever, headache, mental confusion, nausea, vomiting, muscle aches, joint pain and sometimes seizure and coma. Symptoms usually occur four to 10 days after exposure to a mosquito carrying the virus. There is neither a specific treatment nor a vaccine available for use in humans infected with EEE virus.

In horses, EEE is a serious disease that can be fatal; however, well-vaccinated horses are generally safe from the disease. Infected horses show a range of clinical signs that often progress over two to three days, including depression, altered mental status, circling, problems with balance, weakness, aimless wandering, impaired vision, walking abnormalities, head pressing, paralysis, convulsions and death. Horses that survive serious disease often have permanent nervous system deficits.

The Maryland agriculture department is working with Worcester County officials and will conduct air spraying over 6,000 acres in the Whaleyville area. The spraying begins Friday at 5 p.m.