Eleven horses in Douglas County have contracted West Nile virus, and in Carson City, six horses have died.
Of the 68 confirmed cases in Nevada, only one horse was properly vaccinated, said State Veterinarian Dr. David Thain. He did not know if the animal survived.
On Thursday, officials confirmed that the first Virginia Range mustang had died of West Nile, on Aug. 20.
Sherry O'Mohony, executive director of the Virginia Range Wildlife Protection Association, said she has known the mustang mare, named "Big Momma," for many years and that the horse hasn't foaled in nine years.
The mare gave birth about five days before she was found dead. The foal died 24 hours later.
The foal was not tested for West Nile, said O'Mohony.
"But the foal wasn't right either," she said. "She couldn't nurse. She tried, but the mare didn't have anything to give her. I stayed up with her all night long, but the foal just couldn't get enough milk."
"When she walked up with her foal in Friday, we just about fainted," O'Mohony said. "No one thought she was strong enough to carry a foal full-term."
"Because of her age, 20 years, just being pregnant was enough to stress her," she said.
From 2-3 percent of the wild-horse population is expected to be affected, and the average herd growth averages 15 percent annually, said Thain.
Feed is sparse in the Virginia Range, but most of the 350 to 400 horses monitored in the Virginia Highlands appear to be in pretty good shape, O'Mohony said.
As temperatures drop in the next few weeks, the number of equine West Nile cases in Northern Nevada should drop significantly, Thain said.
"We could still see a few more cases in the third week of September, primarily due to the incubation period," he said.
Not every horse exposed to West Nile will develop clinical signs of the disease, which include stumbling, a wobbly gait and lack of coordination. The horses can also experience hind- limb weakness or the inability to stand.
Only one of four affected horses develops a fever, according to information from the California Department of Food and Agriculture.
Between 20 and 40 percent of the animals either die or are euthanized, the report said.
