Wet-weather conditions also are associated with another potentially deadly feed-associated condition in horses. Mycotoxins
are poisons produced by molds in living and stored plant materials such as grains and hay. These toxins are heat- and chemical-stable,
so once feed materials are contaminated, it's nearly impossible to remove this toxic threat. Adding to the problem: Most molds
in equine feeds don't affect palatability, so horses readily consume the contaminated feeds.
Three mycotoxins—aflatoxin, vomitoxin (also referred to as deoxynivalenol or DON) and fumonisin—have been recognized by the U.S. Food and Drug Administration as being so potentially damaging to horses that
there are regulatory and advisory limits for them in horse feeds. While all these mycotoxins can potentially produce medical
problems, fumonisin is of particular importance because it's the cause of equine leukoencephalomalacia (ELEM), or moldy corn
ELEM is caused by Fusarium moniliforme, a fungus that tends to grow best on corn that's stressed during the growing season. Thus, excessively wet or dry conditions
favor ELEM development. Just as the case with ionophore poisoning, horses are especially sensitive to fumonisin toxin and
can show signs of poisoning after exposure to concentrations as low as five parts per million.
Clinical signs of ELEM are incoordination and dullness or reduced response to stimuli. Affected horses can then progress to
aimless wandering, circling, head pressing, blindness, recumbency and paralysis. Extensive deterioration of the cerebral cortex
produces this array of neurologic signs, and most horses with ELEM die, usually in the first 24 to 48 hours.
Some horses affected by ELEM develop liver disease rather than neurologic issues. These horses show weight loss, edema, unthriftiness
and hemorrhage. Blood work will show increased bilirubin concentrations and increased serum hepatic enzyme activities such
as gamma-glutamyltransferase and bile acids. If this liver damage becomes chronic, the result is a horse in poor condition
that loses weight, even though it's generally fed more than an adequate diet.
Treatment for ELEM is supportive, with some horses being maintained in slings throughout the recumbency period. If these horses
are not permitted to lie down, they seem to respond better, and numerous support systems (e.g., The Becker Sling—Häst, hast.net/rescue-equipment.htm; UC Davis Large Animal Lift—Large Animal Life Enterprises,
http://largeanimallift.com/) have been developed to help with these cases. Still, many horses that survive ELEM develop long-term neurologic problems.
Avoiding mycotoxin seems to be the best plan, but it's also the most difficult approach. Rapid and accurate detection tests
are available and can be done (in fact, many companies already use these systems) to help identify and avoid loads of grain
that are contaminated with mycotoxins. These tests are expensive and represent a "major investment," according to Jeff Katelan
of Pennfield Feeds. "Our No. 1 priority is to have a top-notch quality-control process."
Pennfield Feeds relies on these highly effective, rapid-detection chemical tests to identify and eliminate tainted ingredients
before they enter the various processing plants. Various companies also use buffered mold inhibitors. These products are sprayed
onto the corn kernels to reduce the possibility of mold growth. Companies committed to purchasing high-quality, safely produced
grains tend to produce high-quality feeds that have been checked for molds and other contaminants before they arrive on a
farm. That said, there is no substitute for owner vigilance. Each bag of grain and flake of hay should be inspected—both visually
and by smell.
What veterinarians can do
Veterinarians can help educate clients about potential problems with contaminated or toxic feeds. Good knowledge of clinical
signs of these conditions is also important because cases of unthrifty, weak or unusually thin horses (despite normal to increased
feeding) should be investigated, and it's wise to keep chronic poisoning cases high in your index of suspicion.
Dr. Marcella is an equine practitioner in Canton, Ga.
1. Diagnostic update: equine. Ariz Vet Diagnostic Lab Newsletter 1997;2(4):2.
2. Ionophore poisoning in horses. Ariz Vet Diagnostic Lab Newsletter 1997;2(4):1-2.