While horse owners can vaccinate their livestock against West Nile Virus for around $20, the state's 18,000 wild horses are on their own.
"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 mustang herds. He forecast that possibly 1 percent would die from the disease.
An estimated 30 percent of unvaccinated horses that develop West Nile symptoms end up dying. 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.
In the worst case, Thain said, a herd could lose 5 percent of its population if it inhabits a mosquito-infested area.
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.
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 Nevada, which is home to half the nation's wild horses and burros.
When the BLM gathers horses from the range for population control, they are tested for West Nile, BLM spokeswoman Maxine Shane told the Lahontan Valley News and Fallon Eagle Standard.
"We've been on the lookout for anything unusual on the range, but 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 being placed in a long-term holding facility.