Investigating Horse Poisoning

When an owner suspects that a horse might have eaten something poisonous, the veterinarian is generally the first person he or she calls. In addition to treating any health problems, that veterinarian is uniquely suited to investigate the cause of the problem as well, said Bob Wright, BSc (Agr), DVM, of the Ontario Ministry of Agriculture, Food and Rural Affairs, during the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla.

"Private practitioners often lack confidence in their abilities to investigate these cases, but we're ideally suited because we're on farms all the time, we have extensive knowledge of equine behavior, agricultural practices, what's normal in the horse world, feeding practices, etc.," he explained. "Often the primary problem is a change in husbandry that has encouraged horses to seek anything edible.

"Most horses won't touch poisonous plants unless they're forced to," he stated. "Thin pasture sets them up for that. Even poor-quality hay might cause a horse to look for other things to eat. And limited feeding because of weight or insulin resistance worries can make the horse go looking for more to eat."

Pasture plants aren't the only culprit, of course; toxicity from stored feed or supplements, or a combination thereof, can also cause clinical signs.

What signs suggest poisoning? Wright noted that clinical signs are often vague and varied, and they can include laminitis, colic, hair loss, skin lesions, photosensitivity (a skin reaction that can look like sunburn), or, at worst, death.

Poison Investigation

Ferreting out the cause requires a systematic approach, he said. "I start with body condition scoring and determine whether the horse might be looking for something else to eat, or if there might be a point source of toxin," he said. "And I'll record feeds and feeding practices, including the presence or absence of pasture, supplemental feeding, every scoop and pinch of anything, and the feeding schedule. Management is also important; horses housed individually or limit fed are often at greater risk. Bedding should be evaluated, too."

Yew is poisonous to horses
Photosensitization-type lesions

Top: Ornamental yew shrubs are a common source of toxicity. Bottom: Photosensitization-type lesions with damage and peeling of the skin are sometimes a clinical signs of poisoning.

Feeds are sampled and pastures are carefully walked, but Wright warns that pasture walks can turn up a lot more than you might think. "Most pastures and fencerows contain numerous poisonous plants," he noted. "You need to be able to recognize the most common toxic plants and trees in your area, and know under what circumstances they are toxic (i.e., when they are fresh or frozen). Sometimes horses can eat certain weeds and be okay, and other times it will really hurt them.

"Don't jump to a conclusion about the first toxic plant you find," he advised. "Often the primary cause of the problem is poor-quality feed or underfeeding. Look at those things first, because the toxic weeds will often be there, but the horse might not be eating them. In my experience, improper or poor husbandry practices were a primary contributing factor to the disease problem in more than 50% of recent poison investigations. Too commonly, a single plant in the pasture is blamed when an underlying problem, such as insufficient high-quality feed, is the primary cause."

Feed and Weed Sampling

Wright tests all feeds, bedding, hay, vitamins, minerals, whole grains, nutraceuticals, pasture, water--anything the horse might consume. Visual examination is the start, identifying things such as whether there are a lot of noncultivated plants (weeds) in hay and what species of grasses/legumes are in the hay. Then he sends carefully collected samples to laboratories for further investigation.

When walking the pasture, he prefers to work alone--without owners or assistants asking questions--so he doesn't miss anything. He'll look for places where trees can be reached by horses near fences and see if any toxic plants appear to have been equine snacks, especially along fencelines.

"All plant material is suspect," he said. "Look for potential contamination from roadways or neighbors' lawn clippings."

He noted the following common toxin sources:

Botulism, which often occurs from haylage contamination;

  • Clippings from ornamental yew shrubs;
  • Poison hemlock, which is common in swampy areas;
  • Blister beetles in alfalfa hay, which can cause severe toxicity;
  • Oleander leaves;
  • Red maple leaves.

Public health units can help test the water for contamination, he said. Streams and ponds carry a higher risk of contamination and blue-green algae poisoning than deep wells or city water sources.

"The take-home message is that you've got to know the common acute toxins in your area," he said. "Take charge of the situation and don't rely on the lab to give you a diagnosis. Complete a thorough farm investigation, keep an open mind, and use a common-sense approach to find the inciting cause.

Recommended reading:

Burrows GE, Tyril RJ. Toxic Plants of North America. Ames, Iowa: Iowa State Press, 2001.

Knight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Jackson, Wyo.: Teton, NewMedia, 2001.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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