Equine influenza virus likely cause of 94 wild horse deaths

Nearly 100 wild horses in captivity in Colorado have died to date in a virus outbreak.
Nearly 100 wild horses in captivity in Colorado have died to date in a virus outbreak. © BLM

Positive polymerase chain reaction (PCR) testing of nasal swabs and lung tissue from several horses has indicated that an equine influenza virus is the likely cause of death of 94 wild horses in captivity in Colorado.

The outbreak at the Bureau of Land Management (BLM) Wild Horse and Burro Corrals in Cañon City, Colorado began on April 23, with 56 horses dead by April 25. The toll continues to rise.

Horses were found dead in pens or with “severe acute pulmonary edema and mild hemorrhage” or a respiratory condition characterized by mild/moderate fever, coughing or nasal discharge, depression, and labored breathing. The first death was of a foal on April 18, and five days later nine horses were found dead in the morning.

The affected horses are from a group of about 450 who were gathered from the West Douglas area in the autumn of 2021.

BLM officials said this strain of equine influenza (subtype H3N8) is not related to the current outbreak of highly pathogenic avian influenza (subtype H5N1) that is currently impacting wild birds and poultry across the United States.

The PCR testing has also identified two equine herpes viruses (EHV-2 and EHV-5) but these commonly occur in normal, healthy horses, and it is not clear to what extent these may be also be contributing to the severity of the clinical signs observed in the more severely affected group of horses at the facility. EHV-1 had been ruled out by PCR testing.

Additional, more-typical mild clinical signs of influenza are also being observed in 10 to 20% of the other 2150 horses at the facility. No mortality has occurred in the larger groups of horses.

BLM continues to work with veterinarians on the scene as well as diagnostic laboratories, veterinarians and epidemiologists from the US Department of Agriculture and the Colorado State Veterinarian’s Office. Attempts are being made to mitigate the factors that may be contributing to the most severe cases and prevent further spread of the disease.

BLM Colorado Acting Associate State Director Ben Gruber said operations at the Cañon City facility are being reviewed to prevent future outbreaks.

“This tragic outcome was influenced by a population of horses that may have been particularly vulnerable given their time in the West Douglas area and their exposure to last year’s wildfire that prompted their emergency gather,” he said.

The facility remains under a voluntary quarantine with no horses allowed to leave the premises at this time and for the foreseeable future until it has been determined that the animals are gain healthy and pose no risk to the domestic equine population in the community.


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