Maryland horse owners are being urged by the state Department of Agriculture to vaccinate their animals against equine encephalitis--horse sleeping sickness--which they say is often a serious problem during warm weather when mosquito populations increase.

The disease is spread by mosquitoes. Late last summer, several horses in Maryland got sick and died, but a potential outbreak of the disease was blunted by last minute vaccinations and a killing frost that stopped mosquito activity, Agriculture Department officials said.

Scientists anticipate a problem this summer from the viral disease that could lead to an outbreak of encephalitis in horses. The disease usually kills its victims, and survivors often show incoordination and impaired vision, officials said. A specific treatment for the virus is not available.

Effective vaccines are available for horses and other equine species, and animals must be vaccinated annually to be adequately protected, officials said.

State veterinarian Dr. John C. Shook advises horse owners to contact their private veterinarian to have their horses vaccinated now.

"If vaccination is delayed until sickness occurs, veterinarians are often too bogged down to carry out vaccinations trying to save sick horses. Supportive treatment can be expensive, yet not save the horses from death," he said.