by Bill and Mary Weaver
Dr. Robert Van Saun, a Penn State University Extension veterinarian and professor, was called to a farm to investigate why the horses became colicky after eating. The hay in their diet appeared to be quality orchard grass hay. Van Saun’s suspicions were aroused, however, when he noticed a musty smell. An analysis showed that the hay contained over 25 percent moisture — a dead giveaway to the problem.
“To prevent mold growth,” Van Saun explained, “hay must be dried to below 15 percent moisture. Thirteen percent is safer.” He took a mold culture, and found more than a million colony forming units (CFUs) per gram of penicillium mold — a potential mycotoxin-producing mold — on the hay.
“Penicillium can also produce neurotoxins,” he added. In essence, the horse owners were “fortunate” that the horses’ symptoms were limited to colic. “If there are fewer than 500,000 CFU’s per gram, the risk to the animals (from the mold itself, without regard to  mycotoxin content), would be considered low. Between 1 and 2 million CFUs, feed with caution.”
The type of mold also matters. With mixed mold types, there is usually less of a problem. But if you have a large amount of a single penicillium, fusarium or aspergillus mold, the risks to your horses are higher. Beware of musty-smelling hay!
Some potential hazards to look for when purchasing hay
“With large round bales, in the east, we can’t get the moisture low enough in our environment. The large bales were developed for the hot, dry conditions in the west. Closely scrutinize these large bales,” said Van Saun.
Mold growth is more likely if: the hay is less acidic, with a pH above 5; if the moisture level is above 13 percent; if the temperatures are above 55 degrees; and if the humidity is high. Also, molds need oxygen to grow. It helps if bales are wrapped tightly to exclude air. Molds also need an available substrate of sugars and starches to support growth.

Fungal blooms

Mold growth is dangerous to both horses and people. Don’t sniff closely to check for mold. To avoid having your horses inhale mold spores, Van Saun recommends feeding hay to horses outside and placing the hay at a low level, making it less likely that mold products will be inhaled. “You can also soak the feed for 15 to 30 minutes before feeding, so your horses can eat it wet, minimizing the risk of airborne mold dust,” advises Van Saun.
A partial list of potential consequences to your horses as a result of inhaling mold spores includes: not wanting to eat, performance losses of 5 to 10 percent, slowed growth rate, respiratory distress, heaves and Recurrent Obstruction Airway Disease and mycotic abortions.
Moldy grains
As with hay, in addition to molds, you may also have mycotoxins, poisons produced by molds to fight off bacteria. Horses are most sensitive to mycotoxins growing on grains.
Mycotoxins can be deadly. “We have identified 400 mycotoxins in corn and cereal grains, and that only scratches the surface,” continued Van Saun. Aflatoxin from aspergillis, for example, can cause cancer in humans. T-2 toxins from Fusarium cause digestive disease, and were responsible for killing 40 animals in a dairy herd in Europe. Mycotoxicosis can cause acute liver or kidney dysfunction, and chronic problems, like weight loss, reproductive dysfunction and a weakened immune system.
Depending on the mycotoxin, you can see colic, neurologic symptoms, hypersensitivity to sound and touch, altered growth rate, reproduction problems and even paralysis. The damage from mycotoxins can be temporary or permanent, but horses are more sensitive to mycotoxins than ruminant animals. Ruminants can break down the toxins a little. Horses can’t.
As you would expect, these symptoms are more likely to be seen in animals requiring higher grain diets, such as foals, lactating mares and working horses.
Moldy corn poisoning, or equine leukoencephalomalacia, is a “one way trip.” It liquefies the brain. Early symptoms of this problem may be an animal that is jaundiced, depressed, unresponsive and unable to eat or chew properly. Eventually the animal may be unable to walk and be blind, with head pressing and seizures. There’s no repairing this. But it can be prevented, if the owner is watchful.
Claviceps ergot reveals itself as black spheres is cereal seed heads that almost look like mouse droppings. This mycotoxin can cause gangrene in the extremities. “The horse may slough off hooves or tail, and may lose coordination,” explained Van Saun.
You can also see ergot-like poisoning in fescues. Horses are much more susceptible to this than other species. There are maximum feed levels of fescue for horses to avoid this problem.
What to do?
If you suspect your horses may have symptoms of mycotoxin poisoning, what next? First, take a good look at your feed history. Have you changed hay sources? What groups of animals are affected? Was their diet different? You may have a mycotoxin even if your feed doesn’t really look moldy. If your suspicions are aroused, the next step is to take a feed sample. But this sample will be quite unlike most feed samples, and will need to be taken in a very specific manner.
Because mycotoxins can have a very uneven, sometimes sporadic, distribution in feed, take a large sample from different areas in the feed. Van Saun recommends taking at least a 10 pound sample. Mix, and then grind the sample. Then either dry it thoroughly or freeze it.
If testing shows the feed is the culprit, there is one simple solution — get rid of the feed.
Give horses supportive care. Probiotics and yeasts can sometimes be helpful, along with fluids, antioxidants and a high-quality protein diet. To prevent mold and mycotoxin problems in the future, avoid moldy feed altogether, or dilute the mold with clean feed. Dry hay to minimum moisture. Use propionic acid as a mold inhibitor.