Chokecherries suspected in horses' deaths

Experts are searching for what killed two well-fed, watered horses in a pasture on the property of the Northern Agricultural Research Center earlier this week.

The two horses were found dead on Monday in a pasture east of the research center headquarters, according to a press release from the Montana State University News Service.

Four other horses in the same pasture were alive and unharmed when the horses were discovered, it said.

The animals, owned by two research center employees, were "healthy and well cared for," had plenty of food and water and were checked daily by center staff, said Gregg Carlson, superintendent of Northern Agricultural Research Center.

Chokecherry foliage wilted by last week's frost and poisoned oats designed to kill gophers were in the pasture and may have caused the deaths, Carlson said. The horses had been vaccinated for West Nile virus for the past two years, and the mosquito-borne disease was probably not the culprit, he said.

Plant and animal tissue samples taken shortly after the horses were discovered are still being analyzed at laboratories at MSU-Bozeman and Michigan State University to determine the cause of death, Carlson said. It will not be known whether the horses died from poisoning until results come back from Michigan next week, he said.

Jeff Jacobsen, interim director of the Montana Agricultural Experiment Station and interim dean College of Agriculture at MSU-Bozeman, said Thursday that chokecherry bushes are one of the likely reasons for the deaths.

Chokecherry leaves, blossoms, shoots and bark contain a chemical compound called prunasin which, when ingested, releases cyanide in the animal's stomach and causes cyanide poisoning, said Bob Gough, a professor of horticulture and extension horticulture specialist at MSU-Bozeman.

The prunasin level in the trees is typically highest in the spring, and when the leaves become wilted like they did after last week's freeze, the prunasin becomes more concentrated and more toxic, said Gough, who added that people should not jump to conclusions about what killed the horses until the lab results come back.

An animal must eat about a quarter of a percent of its own body weight in chokecherry foliage within an hour to be killed. For an adult cow that means about 1 pounds of material, he said.

Chokecherry bushes were present in the pasture where the horses were found, and they showed evidence of being browsed, Carlson said.

"There's been substantial browsing on some of the bushes in the pasture," he said.

There were also three dispensers of poisoned oats in the pasture, Carlson said. The dispensers, made from PVC pipes in an inverted T shape, are designed so larger animals can't get the oats. If a horse kicked the dispenser a few oats might come out, Carlson said, but it would take at least 2 pounds of the oats to kill a horse, much more than the amount that had been placed in the pasture.

Even so, Carlson said he has not ruled out the possibility that poisoned oats in combination with some other factor may have contributed to the deaths.

He said he is not aware of any other horses in the area that have died recently.

The pasture the horses were found in, known as the "H-field" was not intended for grazing, the press release said, and center protocol would have dictated removing the poisoned-oat dispensers.

Carlson said a group of six horses had been grazing in an adjacent pasture and apparently moved into the pasture through an open gate some time over the weekend. The gate was found propped open, apparently intentionally, he said. The gate normally was kept permanently closed and its handle was tied with twine so the horses couldn't open it he said.

"So that gate should have been closed and no one from here would have been going through there for any purpose," he said. "As far as we know none of the staff has any knowledge why that gate was open or who opened it."

Carlson said it was possible that somebody from outside the staff opened it, and that most likely the person did not know it would cause a problem.

"This is a large public facility with a lot of people coming and going that we don't have an opportunity to see," he said.

The death of the horses, which were used to monitor cattle feeding patterns in the surrounding area, is very different than the deaths of several experiment station horses at Fort Ellis two years ago, Jacobsen said.

"At this point with the two horses in Havre we don't know the causes of death. What we do know is that they had water and food," Jacobsen said. "That is in contrast to the tragic deaths that happened in Fort Ellis a couple years ago, where the animals were not checked routinely and because of poor communications and mismanagement, water was not available to them for multiple days."

Gough said horse and livestock owners should keep chokecherry trees away from livestock and prune chokecherry branches hanging over pasture fence lines. Animals don't prefer chokecherry branches, and typically only eat them in dry conditions where grass is scarce.

Symptoms of cyanide poisoning include rapid breathing and gasping, salivation, slow pulse, dilated pupils, staggering, convulsions, protruding tongue, rolling eyes, blue-tinged skin from a lack of oxygen, cherry-red blood, and loss of consciousness.

Death from respiratory failure often follows within an hour, Gough said, but because cyanide is not produced until an animal drinks water, they may remain without symptoms for some time after eating the foliage.

Gough urged area residents not to jump to conclusions about what caused the deaths.

"Don't go nuts about pulling all the chokecherries out of Hill County," he said, adding that the threat of cyanide poisoning diminishes as summer progresses.