While horse owners can vaccinate their equines against West Nile virus for around $20, about 18,000 wild horses are roaming free in Nevada that will have to face the virus with no medical support.
An estimated 30 percent of unvaccinated horses that develop West Nile symptoms die. And that 30 percent includes horses with access to post-infection medical care and with owners who bring them food and water when they lose the strength to fend for themselves.
"It would be my guess that in the wild population, there will be a higher mortality rate," said Nevada State Veterinarian David Thain.
Still, Thain said he doesn't expect a West Nile epidemic to dramatically thin the state's wild-horse herds.
"It's probably not going to be much of an issue statewide," Thain said. "In all likelihood, (West Nile-related deaths) probably won't be worse than 1 percent" of Nevada's wild-horse population.
West Nile harshly impacts areas with large mosquito populations, such as marshes, swamps and lakes.
"We don't have a lot of those in Nevada," Thain said.
Also, horses can't spread the virus to other horses or even to other mosquitoes.
Only certain birds have the right body temperature to incubate and spread West Nile. Most infected animals never show symptoms of the virus, and once they've had it, they are immune for life.
The worst-case scenario, Thain said, a herd could lose 5 percent of its population if it inhabits a mosquito-infested area.
So far, the U.S. Bureau of Land Management, which is charged with managing wild horses on public lands, hasn't recorded a case of West Nile virus in those horses.
When the BLM gathers horses from the range for population control, they are tested for West Nile, said BLM spokesperson Maxine Shane.
"We've been on the lookout for anything unusual on the range, gut we haven't seen anything yet," she said.
Every horse removed from federal land is also vaccinated against the disease before being put up for adoption or placed in a long-term holding facility.
Cory McConnell can be contacted at email@example.com