West Nile virus, a disease spread by mosquitoes, has been detected in a horse in Union County, according to Oregon Public Health Division officials.

The horse in Cove is the first to test positive for the disease in Oregon in 2019.

Health officials are advising people in Union County to take precautions against mosquitoes to avoid the risk of infection. West Nile virus is transmitted through the bite of infected mosquitoes. Horses become infected from mosquitoes that have previously fed on infected birds.

“It is critical that all horses be vaccinated,” recommends Emilio DeBess, DVM, veterinarian for the public health veterinarian at the Oregon Health Authority’s Public Health Division. “If your horse has not been vaccinated, you are encouraged to vaccinate now. If your horse is displaying signs of illness, call your veterinarian immediately.”

The virus can infect the central nervous system of horses and cause symptoms of encephalitis that include weakness or paralysis of hind limbs, hyper-excitability and convulsions. Not all horses with clinical signs of encephalitis have West Nile.

Horses are considered “dead-end” hosts, which means they don’t develop enough virus in the bloodstream to infect mosquitoes. Only birds are known to pass the virus through mosquitoes to other birds, animals or humans.

About one in five people infected with West Nile virus may show symptoms. People at risk of serious illness include individuals 50 and older, and people with immune-compromising conditions, such as diabetes and high blood pressure.

West Nile symptoms may include fever above 100 degrees and severe headache, stiff neck, mental confusion, muscle weakness, shaking, paralysis or rash. People should contact their health care provider if experiencing any of these symptoms.

DeBess offers the following tips to prevent mosquito bites:

  • Eliminate sources of standing water that are a breeding ground for mosquitoes. This includes watering troughs, bird baths, clogged gutters and old tires.
  • When engaged in outdoor activities at dusk and dawn when mosquitoes are most active, protect yourself by using mosquito repellants containing DEET, oil of lemon eucalyptus or Picard in, and follow the directions on the container.
  • Wear long-sleeved shirts and long pants in mosquito-infested areas.
  • Make sure screen doors and windows are in good repair and fit tightly.

In 2018, there were two human cases of West Nile virus in Harney and Clackamas counties. The virus was found in one bird, 58 mosquito pools (samples of about 50 mosquitoes each) and two horses. In 2017, seven humans, 92 mosquito pools, five horses and one bird tested positive for West Nile. The virus also can be found in chickens, squirrels and dogs.

Climate change effects such as increased temperature and changes in rainfall have led to longer mosquito seasons and are contributing to the spread of West Nile virus, health officials say. They agree these, and other climate change indicators must be considered to help people better prepare for future transmission of the disease.