Chris Law, manager of the Union County Vector Control District, in July 2015 collects a mosquito trap in Ladd Marsh where West Nile virus had been detected. 

LA GRANDE — West Nile disease may have claimed the lives of three domestic animals in Union County.

The Union County Vector Control District is reporting that two horses and one mule were put down during the week of Sept. 3-9 after being diagnosed with what appeared to be the West Nile virus.

Chris Law, manager of the Union County Vector Control District, said it is not known for sure if the two horses and the mule had the West Nile virus because they weren’t tested. However, Law said a Union County veterinary told him the animals were showing clinical signs of the virus.

“They had symptoms of those like animals with West Nile virus,” Law said.

The two horses and the mule were from the outskirts of the Elgin and Summerville area and did not have the same owner.

The animals were among four from Union County that were reported with what was believed to be West Nile virus the first week of September. A fourth animal, a horse, with the same symptoms is expected to survive, Law said. The horse is from the Union area.

“The owner took the horse to a specialty clinic in Idaho and the last I heard it was recovering well,” Law said.

No other horses in Union County are suspected of contracting West Nile virus in Union County this year, Law said.

Law said that over the past five years one or two horses have contracted the virus each year in Union County. One possible reason that the number is higher this year is that fewer people are vaccinating their animals for the virus, he said, referring to what veterinarians have told him.

“This may be contributing to it,” Law said, referring to the higher than normal number of cases.

Law said the vaccine is effective when a horse is fully immunized. Horses should be vaccinated annually, and those that have not received a West Nile virus shot in several years may need two shots to become fully immunized.

Three animals, all horses, have tested positive for West Nile disease in Oregon this year, Law said. One horse was in Klamath County and two were in Harney County.

Four people have tested positive for the West Nile virus in Oregon this year, and there are six other suspected cases involving people in Oregon that are waiting for confirmation.

Law said the West Nile virus has been higher than normal this year in the western United States. He said the likelihood of West Nile being spread in the area will drop significantly as the weather continues to cool. There will be no mosquitoes to transmit it because they will be hibernating until the spring.

Earlier this summer, a pair of mosquito pool sites in the Ladd Marsh Wildlife Area, just outside La Grande, and in the north end of Elgin tested positive for the virus this summer.

In 2022, mosquitoes with West Nile virus were detected in testing pools around Union County in nine instances, but no humans or animals were reported to have contracted it, according to the Oregon Health Authority database.

In August, Emilio DeBess, public health veterinarian at Oregon Health Authority’s Public Health Division, said high heat combined with sporadic precipitation has created conditions for mosquito growth. As a result, eastern parts of the state are seeing more mosquitoes and a corresponding increase in traps containing West Nile-positive insects.

Mosquitoes can transmit the virus to people through bites.

There has been one confirmed human case this year in Baker County, a Huntington man diagnosed in August. Huntington is outside the Baker County Vector Control District, which covers 200,000 acres in Baker, Keating and Bowen valleys.

According to the USDA Animal and Plant Health Inspection Service website, West Nile virus, which was introduced to the United States in 1999, can infect horses, humans and many different species of birds. Clinical signs of the disease usually present within about 15 days after a bite from an infected mosquito.

Clinical signs of West Nile virus in animals:

• Fever

• Incoordination

• Hind-end weakness

• Depression

• Anorexia

• Muscle tremors

• Teeth grinding

• Inability to swallow

• Head pressing

• Excessive sweating

• Behavior changes

• Down and unable to rise