Federal officials in Colorado are investigating the mysterious deaths of 19 wild horses housed at an inmate program facility.
Authorities do not believe the deaths resulted from an infectious agent, although that cannot yet to be ruled out.
The Bureau of Land Management in Colorado said that, to date, 19 horses had died at its Cañon City Wild Horse Inmate Program facility and nine other horses had shown some signs of a neurologic problem causing weakness, incoordination or seizures.
No animals have died since December 4 and no new cases have been reported since December 5.
The bureau said several horses that showed symptoms earlier in the week have shown marked improvement and now appear to be recovering.
With just over 2000 horses and 400 burros at the facility, the illness remained confined to just one pen containing 110 horses.
Although the exact cause of the problem remained unknown, the bureau was still working with local, state and federal veterinarians as well as pathologists at Colorado State University to diagnose the illness.
Necropsies have been performed on six animals to date but, so far, the general results of those examinations have been inconclusive.
Samples taken from animals at the facility and from those animals autopsied are still being examined.
The pattern of the cases so far suggests it is not an infectious or contagious disease, but that possibility, along with the possibility that weeds or other toxins may have been accidently ingested along with hay, are being investigated.
Laboratory testing is expected to continue through next week when the bureau and animal health officials will revisit the situation and decide the best course of action to protect the health and wellbeing of the wild horses and burros.