MESA, AZ (3TV/CBS 5) - Affecting the babies and the older horses the most, a bacterial infection known as "Strangles" has been found in 12 Salt River wild horses, just five percent of the herd.
It’s a strain of Streptococcus bacteria. People can’t contract it, but it is very contagious among horses.
Symptoms include lesions under the neck by their cheeks, fever, and even trouble swallowing and breathing, which is why it's called strangles.
In rare cases it can be deadly.
"Maybe an asymptomatic domestic horse came here and people didn’t know it, so it got introduced to the herd,” said Simone Netherlands with the Salt River Wild Horse Management Group.
Of those 12 horses, five have already recovered. The other 95 percent of the heard all appear healthy and unaffected.
Thursday the Salt River Wild Horse Management Group posted signs in the area, warning people who bring their own horses there to ride.
"But it does have the potential to turn into an outbreak, so that's why we want to be very careful. And we really want to warn horseback riders that, 'Hey if you want to be on the safe side it's probably better to go ride somewhere other than at the lower salt river right now," said Netherlands.
People can also carry it with them on their clothes and shoes, potentially passing it on to their horses at home.
“Of course, we are very worried, so we are keeping a really close eye on it,” said Netherlands.
The signs will stay up until they can confirm all the horses have recovered from the illness.
Netherlands says they are continuing with their birth control program, which began earlier this year.
In January, we tagged along with the SRWHMG as they used dart guns to dose female horses with birth control.
Netherlands says they’ve now successfully treated nearly all of the mares. She says in the next ten years, the program would have helped humanely reduce herd size from 400 to 200.