Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 Kentucky Equine Research Conference, held May 17-18 in Lexington, Ky.

For years, researchers, scientists, and veterinarians have advised horse owners to be cautious and aware of mycotoxins in horse feeds, forages, and pastures. And while these tiny toxins can cause serious illnesses in horses, they can also be difficult for owners to understand.

At the 2012 Kentucky Equine Research Conference, held May 17-18 in Lexington, Ky., Ramesh C. Gupta, DVM, PhD, Dipl. ABT, professor and head of the toxicology department at Murray State University's Breathitt Veterinary Center, in Hopkinsville, Ky., gave an overview of mycotoxins that affect horses and the clinical signs, diagnostics, and treatments associated with mycotoxicosis.

Gupta explained that mycotoxins--dangerous toxins exuded from molds--can form or develop during any stage of the food production chain, from when the host plants are germinated all the way through when the finished product is consumed. There are more than 100 species of mycotoxin-producing fungi worldwide, he said, noting that the species can produce one or multiple types of toxins.

There are five major mycotoxin groups that affect horses, Gupta noted, and he described each in detail.

Aflatoxins

Gupta explained there are four major aflatoxins that affect horses: aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1), and aflatoxin G2 (AFG2). Aflatoxins, he said, are produced mainly by the fungi Aspergillus flavus, A. parasiticus, and A. nomius, and vary widely in toxicity.

"AFB1 occurs with the greatest frequency in feedstuffs and is found to be the most toxic in all laboratory, companion, and livestock animals," Gupta said.

He noted that in most cases toxicity is subacute or chronic; however acute cases have been reported. Clinical signs include reduced growth rate, weight loss, immune suppression, jaundice, hemorrhagic enteritis, decrease in performance, and death.

In other species, researchers have showed that aflatoxins have detrimental effects on the reproductive tract and the developmental system. The liver is the most common organ targeted by aflatoxins, he said. At necropsy, Gupta said, horses with aflatoxin poisoning often have:

  • Histopathologic lesions in the kidney and liver;
  • A firm and pale liver;
  • Clear yellow ascites (fluid in the peritoneal cavity); and
  • Pleural fluid accumulation.
ELEM

ELEM causes damage to horses brains (seen here at necropsy) and can cause death within two to three days after clinical signs appear.

Gupta relayed that there is no specific treatment protocol for aflatoxin exposure except for eliminating contaminated feed from the horse's diet and avoiding any future exposure. L-methionine and sodium thiosulfate administration every eight hours can help return affected animals to health, he said. In some cases, increased supplementation of protein, vitamins, and antioxidants are also helpful, he added.

Fumonisins

Fumonisins are typically found in corn and other feeds contaminated by the fungus Fusarium verticillioides or F. proliferatum, Gupta said. It takes only a small amount of fumonisin to cause irreversible damage to horses, as horses are the most sensitive species of animal to fumonisin exposure, he noted.

One of the most common effects of fumonisin exposure in horses is equine leukoencephalomalacia (ELEM, often called moldy corn poisoning, see image at left). Clinical signs include decreased appetite; behavioral changes such as depression, anxiety, or hyper-excitability; and neurologic signs such as circling, blindness, difficulty chewing or swallowing, muscle tremors, ataxia, recumbency, and death. Moldy corn poisoning takes seven to 35 days before the horse demonstrates clinical signs; once signs are observed, death can occur within two to three days, Gupta said.

Gupta relayed that toxin exposure can also prompt a hepatotoxic syndrome that causes jaundice, elevated serum bilirubin and liver enzyme concentrations, and head and face swelling. One particular fumonisin, FB1, is known to cause cardiotoxicity in horses, as well, he said.

As with aflatoxins, there is no specific treatment for fumonisin exposure except removing the contaminated feed from the diet.

Slaframine

slaframine

Produced by the fungus Rhizoctonia leguminicola, slaframine causes excessive salivation or slobbering after horses consume affected red, white, and alsike clover or other legumes.

Even if they don't realize it, many horse owners are familiar with slaframine exposure in horses. Produced by the fungus Rhizoctonia leguminicola, slaframine causes excessive salivation or slobbering after horses consume affected red, white, and alsike clover or other legumes (see image at right). The most common time for slobber outbreaks are spring and fall, he said.

Gupta said that aside from excessive salivation, slaframine exposure can cause lacrimation, colic, and diarrhea; one abortion resulting from slaframine consumption has also been reported, he noted.

In mild cases, slobbering resolves within two to five days of removal from contaminated forage, Gupta relayed. In more serious cases, atropine sulfate has been proven effective for treating clinical signs associated with parasympathomimetic system, but clinical signs rarely resolve completely, he said.

Trichothecenes

Trichothecenes are found worldwide and are toxic to all animal species on which they've been tested, Gupta said. There are three trichothecenes of concern: deoxynivalenol (DON or vomitoxin, the most common of the three), diacetoxyscirpenol (DAS), and the T-2 toxin.

Clinical signs of trichothecene exposure depend on how long the horse has been exposed, and to how much they've been exposed, he said. Common signs include reduced feed consumption or anorexia, decreased weight gain, skin irritation, decreased immune function, bloody diarrhea, emaciation, and death.

There are no specific treatment options for exposure, aside from eliminated the contaminated feed, Gupta said. Clinical signs can be treated medically and mild signs typically resolve within a week of feed removal, he noted.

Zearalenone

Known as the F-2 toxin, zearalenone is commonly found in barley, oats, wheat, corn, silage, rice sorghum, and some forages, Gupta said. Often produced by the fungus F. graminearum, zearalenone typically forms if the fungus is stressed by cool temperatures for several weeks, he said.

Generally speaking, zearalenone isn't overly toxic in most animal species, Gupta said; however zearalenone poisoning was implicated in at least one instance of equine deaths. Researchers reported vulvar edema, vaginal prolapsed, oversized uteri, and internal hemorrhage in mares, and severe genital flaccidity in male horses, he noted; all sick horses collapsed due to "respiratory paralysis and sudden blindness, and died quickly."

Conversely, he noted, another research team found that daily administration of purified zearalenone had no adverse effects on mares' reproductive efficiency.

Treatment of clinical signs is recommended for horses suffering from zearalenone poisoning, Gupta said, and removal of contaminated feed is crucial to prevent recurrence.

Take-Home Message

Mycotoxin exposure in horses can be dangerous, or even fatal, if not caught and corrected quickly. If poisoning is suspected, contact your veterinarian to obtain a diagnosis and treat clinical signs to help the horse on his road to recovery. Replacement of contaminated feed with clean feed appears to be the best solution.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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