A horse in Crook County that became ill in September has been diagnosed with West Nile virus, the first time a mammal has been infected with the disease in the county, according to Crook County Health Department officials.

The only other positive test of West Nile virus in Crook County was an infected bird in 2004, according to Karen Yeargain, communicable disease coordinator for the health department.

West Nile virus is transmitted through the bite of an infected mosquito. The virus can infect the central nervous system of horses and can cause weakness or paralysis of hind limbs, hyperexcitability and convulsions. The virus can be fatal for horses.

While it’s late in the season for mosquitoes, residents of Prineville and other parts of the county are advised to take extra steps to prevent mosquito bites.

This includes covering up in the evening with long sleeves and pants and using mosquito repellent that contains DEET, oil of lemon eucalyptus or Picaridin.

Yeargain also advises residents and ranchers to eliminate mosquito breeding grounds on their properties, such as birdbaths and discarded tires that can collect water.

“We know it’s here now, so people need to take their control measures seriously. We need to ramp up people’s awareness,” Yeargain said. “We have to get used to the fact that mosquitoes carry disease and cause health problems.”

Dr. Rene Villagrana, a Prineville veterinarian, encourages horse owners to get their animals vaccinated.

“If you horse is not vaccinated, you are encouraged to vaccinate before mosquito season next spring. If your horse is displaying signs of illness, call your veterinarian immediately,” Villagrana said.

Horses are considered “dead-end hosts” which means they cannot pass on the virus to other species. Only birds are known to pass the virus to mosquitoes, which can then transmit West Nile virus to other birds, animals or humans.

One in five people infected with West Nile virus will show symptoms of illness. The most vulnerable groups are people 50 years or older, as well as people with immune-compromising conditions such as diabetes and high blood pressure.

In humans, symptoms of West Nile include a fever over 100 degrees, a severe headache, stiff neck, mental confusion, muscle weakness, shaking, paralysis or rash. No residents of Crook County have been diagnosed with the illness.

This year Oregon has reported eight human cases of West Nile virus originated in three separate counties, including three in Harney County, three in Malheur County and two in Baker County. In addition, there was one case in Deschutes County that was acquired out of state. The virus has also been found in seven Oregon horses this year.

West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the U.S. in New York in summer 1999.

— Reporter: 541-617-7818, mkohn@bendbulletin.com