Is Selenium Deficiency Deadly to Horses?

Good-quality hay might not be enough, on its own, to meet a horse's selenium needs. As such, it's important to work with a veterinarian or nutritionist to ensure your horse's dietary needs are being met.

Photo: iStock

The warnings are nothing new to owners: Too much selenium in a horse’s diet—even as little as 5 mg per day—can cause signs of toxicity or even death. But did you know too little selenium can also be life-threatening?

Andrew Allen, DVM, an assistant professor at Washington State University’s (WSU) College of Veterinary Medicine, in Pullman, reviewed several recent cases of death due to selenium deficiency in adult horses at the 2016 American College of Veterinary Internal Medicine Forum, held June 8-11 in Denver, Colorado.

In equids, Allen said, the muscle disease nutritional myopathy (also referred to as white muscle disease), resulting from a selenium deficiency, generally presents itself in young, rapidly growing foals. Affected foals can show a variety of clinical signs depending on disease severity, including painful hind limb, back, or neck muscles with increasing weakness, stiffness, trembling, and recumbency (the inability to rise); difficulty swallowing; an irregular heartbeat; generalized weakness; and sudden death. This condition, however, is rare in adult horses.

However, in 2014, the bodies of two Paint horses from the same farm were brought to the Washington Animal Disease Diagnostic Lab at WSU for necropsy. The examiners found that both horses had myonecrosis (muscle cell death) of the head and skeletal muscles, subcutaneous edema (fluid accumulation beneath the skin), pleural effusion (fluid accumulation in the chest cavity), and ulcerative glossitis (an ulcerated and inflamed tongue).

“These findings are suggestive of a full body muscular involvement, instead of selected muscles such as the masseter muscles,” which play a role in chewing, Allen explained. “We speculated that the glossitis was due to severe pica (eating nonfood items—in this case, mostly trees) and feeding methods (the owner would leave hay bales intact with strings still wrapped). Severe pica may be due to mineral deficiency.

“We opted to perform the field investigation due to the difficulty in diagnosing these types of problems from just laboratory findings, and it helped us rule out more likely causes of this type of problem—ionophore toxicity and atypical myopathy,” he added.

As part of the Field Disease Investigation Unit at WSU, Allen and fourth-year veterinary students subsequently visited the horses’ home farm to investigate further, where they learned that 13 horses—including the two recently deceased ones—exhibiting similar clinical signs had died on the farm within the past year.

Twelve horses remained on the farm. The team performed physical exams on all 12 and drew blood from four to carry out complete blood counts and serum chemistry panels.

“The herd had an overall low body score, three horses had elevated heart rates, and one horse had pitting ventral edema (fluid swelling under the horse’s abdomen in which, if pressed, a depression remains in the skin),” Allen said.

He also noted that the four blood-tested horses had increased creatinine kinase activity and aspartate aminotransferase levels, both indicators of muscle damage or degeneration.

The team did not find any box elder trees (seeds from the tree are linked to seasonal pasture myopathy, an often-fatal muscle disease) on the farm and or any ionophores in the horses’ feed or supplements.

They did, however, find that all tested horses had extremely low blood selenium levels, Allen said. The team recommended selenium injections and oral supplementation and reevaluated the horses two to three months later.

Six months after diagnosis, the veterinarians returned to perform an echocardiographic study on three of the most severely affected horses. Allen said the findings from all three horses were within normal limits.

All the surviving horses returned to good health, he added.

Not long after, in 2015, Allen and colleagues conducted a second investigation on a different farm after two more deceased horses were presented for necropsy. He said the investigation “revealed a very similar scenario and herd-wide myodegeneration secondary to selenium deficiency.

“Although unusual, selenium deficiency should be considered a rule-out with adult horses showing signs of ventral edema, weakness, and abrupt death,” he added.

Allen said that during the team’s investigation, the horses’ owners revealed that advice from online blogs led them to believe that selenium was dangerous for their horses and, as such, they should not provide supplementation. “They also thought by purchasing expensive hay that they would be covered,” he said.

As a result, Allen and colleagues stressed to the clients the importance of working with their veterinarians to devise nutritional plans for their horses.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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