Consumption of a highly toxic plant is believed to be behind the deaths of 19 horses early this month at the Canon City Wild Horse Inmate Program facility in Colorado.
The Bureau of Land Management says preliminary results from the Veterinary Diagnostic Laboratory at Colorado State University point to whorled milkweed as being responsible.
While final lab results are still pending, veterinarians have ruled out any infectious diseases as a possible cause of death.
Tests for rabies, the equine herpes virus and the West Nile virus came back negative.
All of the deaths occurred in one pen, despite close contact between the horses and those in neighboring pens.
Currently, nine of the horses also in the same pen with similar symptoms are either fully recovered or recovering quickly.
As a precaution, animals from pens immediately adjacent to the affected pen will remain at the facility until the final lab results are received, or for an additional three weeks.
With the approval of state animal health authorities, the bureau horse-training facility will resume operations early next week, when it will begin to ship wild horses and burros that were adopted.
These animals were geographically isolated from the affected pen and have been examined by a veterinarian and deemed healthy.
The horses at the Canon City facility are fed about 25 tons of hay daily.
The hay arrives in 1000-2000 pound bales.
The incident suggests that, in some cases, only small amounts of milkweed need to be consumed to severely affect a group of horses.
In order to help prevent a similar occurrence in the future, samples of the whorled milkweed will be kept on hand to educate both staff and feed crews, the bureau said.
Hay vendors will also be advised that hay will not be accepted from suspect areas, such as the edges of fields, along roads and continually wet areas.