U.S. government officials say an unknown and fast-spreading disease has killed 85 wild horses in less than one week in the western state of Colorado.
Federal officials made the announcement Wednesday.
The number of deaths at a federal corral, or holding area for horses, increased from an earlier announcement of 67.
Steven Hall of the U.S. Bureau of Land Management, BLM, told Reuters the facility is under quarantine.
That means animals are not permitted to enter or leave the area.
More than 2,500 animals are held at the facility in Canon City, 190 kilometers southwest of Denver.
Animals that are sick or have been exposed to infected horses are being kept away from the healthy ones.
A BLM statement earlier this week said, "Wild horses at the Wild Horse facility in Canon City have been infected with an unknown yet highly contagious and sometimes fatal disease."
Fatal is a term that means deadly.
Most of the sickened horses had been transported from Rio Blanco County, near the neighboring state of Utah.
Officials collected the animals last autumn, the BLM said.
Hall said the dead animals are being examined.
Blood and tissue samples are being carefully studied at two university laboratories.
"The main symptoms seem to be respiratory issues and chest congestion," Hall said, adding that federal, state and independent veterinarians are investigating the issue.
Horses are not native to the United States.
Their ancestors were animals freed or left by people during the settlement of the American West.
The BLM said the horses must be controlled because the animals damage water supplies and eat food that native species eat.
There are an estimated 80,000 wild horses in the U.S.
The agency collects some of the wild animals in events called roundups.
BLM transports the animals to holding areas where some are sold.
Activists say the roundups and living conditions for the animals are inhumane.
Suzanne Roy of the American Wild Horse Campaign said in a statement posted on the group's website, "Disease outbreaks and deaths are the direct result of the BLM's mass roundups."
Roy added, "We can expect to see more suffering and death if BLM continues down this dangerous and destructive path."
I'm John Russell.