Two Frederick County horses have been euthanized and a Howard County emu chick died after all three developed symptoms that have since been linked to the West Nile virus.

The cases are the first West Nile viral infections in horses reported in Maryland this year. Last year, eight horses tested positive for West Nile, all but two in Baltimore County. One died; the rest recovered.

"We're expecting we probably will see more," said Sue duPont, spokeswoman for the Maryland Department of Agriculture.

The state is urging horse owners to complete the vaccination of their animals against both the West Nile virus and Eastern equine encephalitis. Both diseases are transmitted by the bite of infected mosquitoes.

The horse vaccine first became available last year, and is said to be widely available this year.

Nationally, 644 cases of equine West Nile encephalitis had been reported in 26 states by mid-August this year. That compares with 738 cases in all of last year.