For many horse owners in the West and Southwest, locoweed poisoning is a problem. With no viable antidote to reverse the neurological and systemic damage it wreaks, prevention through land management is key.
BY KATIE FRANK
For Britt Stubblefield, DVM, the tragedy of losing a horse to locoweed hits too close to home. About 10 years ago, he turned out his sorrel Quarter Horse, “Deuce,” along with a handful of other horses, on a mountain pasture in Colorado for the summer. From a distance, the horses seemed fine, and when Stubblefield saw them from afar while gathering cattle, there didn’t seem to be any red flags.
It wasn’t until he and his friends went to catch the horses that fall that they saw the telltale signs of locoism, the poisoning associated with consumption of the locoweed plant.
“One of [the horses] was a little skinny. And the horse I lost, and the other one [that wasn’t mine], didn’t act right. They couldn’t handle being haltered, caught or anything,” recalls Stubblefield, of Rocky Top Veterinary Service in Guffey, Colorado.
Locoweed, Astragalus spp. or Oxytropis spp., is a leafy perennial plant that is highly toxic even when dead and dried. It has compound leaves usually covered in silvery hairs, and white or purple flower petals. In North America alone there are 22 species, 21 of which have been associated with locoism. Most species, such as spotted locoweed, woolly locoweed and lambert locoweed, grow throughout the West and Southwest, often in dry, sandy soil.
The plant contains the toxin swainsonine, an alkaloid that inhibits enzyme production involving sugar and carbohydrate metabolism. As a result, protein synthesis and function are disrupted, and the partially metabolized sugars build up in the brain.
“Once that accumulation of the toxin happens and the change in the carbohydrate-sugar process [begins], then it impairs the function of that cell or that organ,” Stubblefield says. “But it also can have an effect on liver function and can cause [gastrointestinal] changes, as well, which will cause some changes in the absorption of the GI tract and decrease appetite.”
Stubblefield witnessed the same effects in Deuce and the other horses. In addition to looking “withered” from their dwindling appetites, he says the horses were ataxic and easily frightened, a result of the brain damage.
Outward signs of locoism develop after several weeks of consumption, in part because poisoning is typically dependent on duration rather than dose. Other symptoms can include stumbling, tremors, head bobbing or high, exaggerated steps. Deuce was turned out on the mountain pasture for several months, and there was no telling how long he had been nibbling on the palatable plant.
What makes locoweed so dangerous is the control it can have on livestock’s appetite. though traditionally thought to be addictive, the plant is actually habitually eaten. In other words, consumption of the plant is a learned behavior, and it’s extremely difficult to stop animals from eating the plant even if other forage is provided. Still, some horses will never touch the poisonous plant even if it’s growing in their pasture, while others can’t seem to stop eating it.
There is no treatment for locoweed poisoning, and the only thing that can be done is pulling the horse off the infested land. Even testing for the disease is nearly impossible.
“One of the problems with locoweed is [even though] we suspect it because of the prevalence of the plant in the pasture, or for in my case, in my area, the testing is really hard to do because the swainsonine has a very short half-life. When I talk to toxicologists they say that by the time you pull a sample [ from the affected animal], ship it overnight to them, and they get to it, the levels of swainsonine are so low you’d be wasting your money,” Stubblefield says.
The half-life of swainsonine is a mere 24 hours, which gives veterinarians and toxicologists an extremely small window of opportunity to test the sample.
The lack of treatment, combined with the irreversible effects of the disease, render affected horses unsafe for riding. Their unpredictable behavior and inability to cope with the stresses of being under saddle should not be pressed, and many infected horses are either put out to pasture or euthanized, the veterinarian says.
Stubblefield was aware Deuce had low odds of a full recovery, but he still wanted to give his gelding a fighting chance.
“I tried to rehab him for a year and a half, and thought I had him to where he’d be safe enough for me to ride again,” he says. “The first time I tried to ride him [after eating locoweed], he was not safe at all. When we went to put the halter and headstall on, he reared up. He was really frightened easily, and couldn’t deal with any sudden changes. He had nervous signs of ataxia. His brain couldn’t handle the stress.”
Because Deuce was a working horse and was happiest when he had a job to do, Stubblefield made the difficult decision to euthanize his gelding.
“If you know you have locoweed-infested pasture, do your best to keep your livestock from grazing on that locoweed or to mitigate the weed,” Stubblefield stresses.
George Beck, PhD, a professor of weed science at Colorado State University, says that most horse owners, especially ranchers who own thousands of acres, will turn out horses only on land they know does not contain locoweed.
“What most people do is they’ll manage around it,” Beck says. “They’ll have areas on their ranch, maybe it’s pastures, divided up by fence, and they’ll have some areas with the problem and some areas they don’t.”
Beck says that there are many commercial herbicides that can be sprayed on the plant in the early growth and flowering stage with 100 percent control, meaning it will completely kill the plant. However, locoweed has hardy “seed banks” underground, which germinate after rainy seasons.
“The problem is you have a very long, slow seed reserve. That’s the issue; it’s about 100 years,” Beck says, which is why spraying every year is essential for complete control.
Herbicide application costs only a couple of dollars an acre, but if spraying is not in a rancher’s budget, Beck recommends breaking the task into several smaller jobs and spraying a couple of acres every year.
“Figure out what you can afford to spend based on potential loss and realized loss, and try to deal with that as the justification,” he says. “So if you have 10,000 acres, you don’t go and spend $1,000 or so when you don’t have it to spend, because that’s just the herbicide cost. With 10,000 acres, you’re not going to apply it yourself. You’re going to use an airplane and that’s getting into big bucks. Unless it’s [extremely] widespread, it’s probably just patchy, and typically that’s the case, so just spray the patches.”
Biological control through insects has been used to help eliminate locoweed infestation, but it’s hard to keep the insect alive unless it is native to the environment. Root-eating weevils have been used to control locoweed, but unless the insects are already established, introducing them to a new habitat is usually met with disappointment.
“It’s possible, but almost all of those [attempts] have been met with failure because you rear these insects and then put them out into the wild to work on locoweed, and then that insect’s pathogens and predators find it. So its populations can’t build up high enough to impact the locoweed,” Beck explains.
Rainy seasons trigger bursts of Locoweed growth. Educating yourself about prevention and identification greatly reduces your horse’s chance of being affected.
Locoism can be transferred through a dam’s milk and have irreversible effects on her suckling foal.
“It’s concentrated in the milk and sent to the offspring. If mom’s eating the locoweed and baby isn’t, whether cow or horse, the baby is getting it through the milk,” says Britt Stubblefield. “And in the cows’ side of things, because I deal a lot with it in cattle, I see increases in congestive heart failure, increases in abortions, late-term pregnancies, and decreased pregnancy rates.”
Cases of deformities have also been reported in horses, sheep and cattle. Mares found eating locoweed need to be removed from the infected pastures immediately. With no viable treatment to counteract harmful effects of the toxin, swainsonine, prevention is key to avoiding locoism.