It’s been a bad year for wild horses in my home state of Colorado. In October I wrote about the strong likelihood that mustangs were being illegally sold for slaughter, and now 19 horses taking part in the Colorado Wild Horse Inmate Program (a great service that goes by the unfortunate acronym W.H.I.P.) have likely died from eating hay contaminated with a toxic weed.

According to the Bureau of Land Management (BLM), the ingestion of whorled milkweed, a highly-toxic plant, is suspected to have caused the deaths of 19 horses in December at the BLM’s Wild Horse Inmate Program (WHIP) facility in Canon City, according to preliminary lab results issued from the Veterinary Diagnostic Lab at Colorado State University.

While the 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.

Nine of the horses that were 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 BLM WHIP facility planned to resume normal operations and begin to ship wild horses and burros that have been 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 approximately 25 tons of hay daily. The hay arrives in 1,000 – 2,000 pound bales. This 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. 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.

I must admit that when I first saw this report, I had to look up whorled milkweed (Asclepias verticillata) in my copy of A Field Guide to Common Animal Poisons. Milkweed contains cardiac glycosides, the natural substances from which heart medications like digoxin are derived. When ingested in toxic doses, the cardiac glycosides in milkweed stop the heart from beating normally. Very often, poisoned animals are simply found dead, but symptoms like diarrhea, loss of appetite, depression, weakness, unsteadiness when walking, collapse, and seizures may be noted in more mildly affected horses (or other species, for that matter).

As I am writing this, I remember that monarch butterfly caterpillars feed on milkweed, thereby ingesting cardiac glycosides and making themselves toxic to their predators. Just goes to show that nothing in nature is wholly good or bad. We have milkweed to thank for monarch butterflies and powerful heart medications while still mourning the loss of 19 grand symbols of the American West.

Dr. Jennifer Coates

Image: Horse Eating Hay by dagnyg / via Flickr

Ed. Note: Headline altered to more accurately represent that milkweed has not been confirmed as the cause of death for the horses.