Bureau of Land Management report details factors in 146 wild horse deaths

An equine influenza outbreak that killed 146 wild horses may have caused more severe disease because of vaccination delays, concurrent bacterial infections, and previous lung damage.

The deaths occurred in April and May at the Bureau of Land Management’s Wild Horse Off-Range Corral at the Canon City Correctional Facility in Colorado. A joint incident review team from the BLM, Colorado Department of Corrections, Colorado Correctional Industries, and Colorado State Veterinarian’s Office since have written a report released in July on how the deaths occurred and how to prevent future high-mortality events.

The Canon City facility housed about 2,550 horses, but the severe influenza and deaths primarily occurred among about 450 horses that had been removed in summer 2021 from the West Douglas range in western Colorado, where fire had damaged their habitat, according to reports from the BLM and Colorado State Veterinarian’s Office. The horses arrived in the facility in July and August 2021.

Workers at the Canon City facility found nine of the West Douglas horses dead on April 23, and another 46 would die or be euthanized in the next two days. Results of diagnostic tests revealed the horses had been infected with an H3N8 equine influenza virus, which the tests identified in nasal swabs and lung tissue.

Investigators have found that the influenza infections were complicated by coinfections with Streptococcus equi subspecies zooepidemicus. The horses’ lungs also may have been compromised by wildfire smoke and a dust storm, “making the horses particularly susceptible to complications arising from equine influenza,” the joint report states.

“Lastly, the affected group of horses have a consistent and uniform phenotype and may be genetically very similar,” the report states. “This and the specific range on which these horses occupy, i.e. fairly isolated from potential contact with domestic equines, may play a role in the increased susceptibility of this group and higher than normal mortality for such an outbreak.”

Though most horses in the facility had been vaccinated against influenza in the prior six months, the West Douglas horses were all unvaccinated, partly vaccinated, or given booster shots within 10 days of the outbreak, BLM reports state.

The joint report indicates vaccination usually begins within 30 days of arrival. A refrigerator malfunction earlier in the summer had compromised some of the facility’s vaccines, and the facility manager—in consultation with the attending veterinarian—decided to hold available vaccines to administer boosters for other horses because the West Douglas horses would be housed in corrals at least 180 yards away.

Horses that arrived later from a herd with a passionate public following were bumped ahead of the West Douglas horses in priority for vaccination and other preparations for adoption, and another group of horses shipped from Wyoming received priority attention upon arrival because of an agreement between BLM and the Colorado State Veterinarian’s Office that the horses be tested immediately for equine infectious anemia because they had been shipped across state lines without being tested.

The review team recommended that the BLM implement a policy on how quickly to vaccinate newly gathered wild horses and burros and that agency officials consider a facility’s current capabilities when planning to gather and move animals. The team’s report also echoes previous findings from BLM officials that the Canon City facility was understaffed—affecting animal care, facility maintenance, and record-keeping—and the report recommends that refrigerators used to store vaccines have temperature monitoring devices that signal when cooling systems fail.