PORTLAND (AP) - Three horses in Eastern Oregon have tested positive for West Nile virus, public health officials said Thursday.

The test results are being confirmed at an Iowa lab, according to the Oregon Department of Agriculture. Preliminary test results are pending on three other horses.

The virus was discovered last week in a crow in Malheur County, the first case of West Nile to be detected in the state.

No humans have been infected in Oregon. Twenty-four Oregonians have been tested for the disease this year; 21 have been cleared and three test results are pending, said Dr. Emilio DeBess, the state's public-health veterinarian

The horses that came down with the disease were in Malheur County and had fever and trouble walking. One of the horses died and two are improving, DeBess said.

Mosquitoes spread the disease, so infected horses can't pass it to other horses, or to humans.

"There's nothing to worry about," DeBess said. "We just need to keep reminding people to take precautions."

Health officials say people can protect themselves by wearing long-sleeved clothing during peak mosquito-biting times in the morning and evening. People should also empty standing water from any containers such as flower pots, tires, and gutters.

West Nile rarely kills, but those with previous medical conditions can be at higher risk of complications. This season, 27 states have reported 689 West Nile cases, including 20 deaths, the Centers for Disease Control and Prevention said Thursday.

The disease first hit the nation in New York in 1999, and has been progressing west ever since. Oregon was one of the last states to get hit.

News of the dead crow has led to expanded concern about the disease. DeBess said 20 dead birds have been sent for tests in the last three days.

There are no drugs or vaccines to treat West Nile in humans, but horses can be vaccinated and DeBess urged horse owners to take that step.