The ingestion of the highly toxic plant whorled milkweed (shown below; image courtesy of Colorado State University) is suspected to have caused the death of 19 horses at the Bureau of Land Management’s (BLM) Wild Horse Inmate Program (WHIP) facility in Canon City, Colo., according to preliminary laboratory results released by the Veterinary Diagnostic Lab at Colorado State University. University pathologists are assisting the BLM, along with local, state and federal veterinarians, to determine the reason for the deaths.
The BLM reports that in early December, 19 horses died and nine other horses displayed clinical signs of neurological disease, including weakness, incoordination and seizures. Since then, several horses have exhibited similar symptoms but are showing signs of improvement and are expected to make a full recovery. It is believed that the illness was confined to 110 horses housed in one pen, although horses in adjacent pens are being monitored until final laboratory results are available.
Tests for infectious diseases, including equine herpesvirus, rabies and West Nile virus, have yielded negative results, so animal health officials no longer consider contagious disease a primary cause of the deaths. However, investigations continue and will include sample analyses from necropsy tissue of the affected animals.
The WHIP facility houses approximately 2,000 horses and 400 burros, all of which consume approximately 25 tons of hay daily. The hay is delivered in 1,000- to 2,000-lb bales, and the BLM reports that even a small amount of milkweed in the bales can cause serious illness in horses, as suggested by this incident. As such, the facility is keeping samples of the toxic plant to educate staff and feed crews and will no longer accept hay harvested from suspicious areas, such as the edges of fields, roadsides and consistently wet lands.