The ingestion of whorled milkweed, a highly toxic plant, is suspected to have caused the deaths of 19 horses early in the week of Dec. 9 at the Bureau of Land Management’s Wild Horse Inmate Program facility in Cañon City.
The Veterinary Diagnostic Lab at Colorado State University issued preliminary lab results Monday.
While final lab results are pending, veterinarians have ruled out any infectious diseases as a possible cause of death. Tests for rabies, the equine herpes virus and West Nile virus came back negative.
All the deaths occurred in one pen, despite close contact between the horses and those in neighboring pens.
Nine horses in the same pen with similar symptoms are either fully recovered or recovering quickly, BLM officials said.
As a precaution, animals from pens immediately adjacent to the affected pen will remain at the facility until final lab results are received or for an additional 3 weeks.
With the approval of state animal health authorities, the inmate program will resume operations early next week when they begin to ship wild horses and burros that were adopted.
Those animals were geographically isolated from the affected pen and have been examined by a veterinarian and deemed healthy, officials said.
The horses at the Cañon City facility are fed approximately 25 tons of hay daily. The hay arrives in 1,000- 2,000-pound bales.
BLM officials said the incident suggests that in some cases only small amounts of milkweed need to be consumed to severely affect a group of horses.
To help prevent a similar occurrence in the future, samples of the whorled milkweed will be kept on hand for educational use.
The BLM will also advise hay vendors that hay will not be accepted from suspect areas, such as the edges of fields, along roads and continually wet areas.