Problems for Horse Owners Caused by This Summer's Wet Weather

From Penn State University's Dairy & Animal Science News

The wet weather this summer has not only increased the price of quality hay, but creates the right conditions for increased mold, fungi and mycotoxins in hay and grains. The cool, wet growing season has delayed the harvest of hay and grains and has also set up the right conditions for mold and mycotoxin to be found in crops before harvest. The following are some concerns that horse owners have voiced.

During Aug. and Sept. 2003, Ann Swinker, Penn State University, Extension Horse Specialist has had several reports of horses that were eating red clover (on pasture or fed as hays) show signs of excessive salivation and increased water consumption. Excessive salivation and dulling "Slobbers" is observed in horses eating red clover or Lucerne infested with Rhizoctonia leguminicola. Slaframine are associated with the parasympathetic signs. The slaframine breaks down with time. One study reported that slafrmine fell from 100 mg/kg to 7 mg/kg after ten months of storage. So if you are planning on feeding infested red clover/grass hay, long term storage may help reduce the problem.

Coughing, is one of the most common signs of respiratory distress and occurs in response to irritation from bacteria, viruses, or inhaled environmental dusts and molds and other allergens. Over time, this irritation can lead to development of "heaves" formerly known as chronic obstructive pulmonary disease (COPD). This is one of the most common respiratory disorders of horses, particularly in temperate climates like those of North America and wet climates like we have been experiencing this year.

Chronic obstructive pulmonary disease in horses has been associated with poorly ventilated stables, exposure to dust and mold and as a sequel to bacterial and viral respiratory tract infections. The incidence and severity of COPD is dependent upon the quantity of antigenic material inhaled and the sensitivity of the individual horse to specific antigens. Be aware of the strong potential for problems, look for molds and mold spores, do some testing if you suspect problems, and consider ration changes where possible. If you throw a flake of hay on the stall floor and a slight puff of white dust emerges you may a mold problem.

Horses can ingest aflatoxin when they eat contaminated corn, cottonseed products, or other high-energy grains. Aflatoxins are primarily produced by a strain of Aspergillus flavus mold when conditions are warm and humid, so they are most commonly found in grain crops grown in southern states. Historically, problems with mycotoxins have been most severe when crops suffered drought stress through the end of the growing season. Infestation with insects, such as the corn ear worm, can make matters worse because the insects bore into the grains and allow the mold easy access.

Equine clinical signs of aflatoxicosis include feed refusal, diarrhea, colic, anemia, immune suppression, oral inflammation, and liver damage. Equine leukoencephalomalacia (ELEM), commonly known as moldy corn poisoning or "blind staggers," is the mycotoxin-related syndrome with which many horse owners are most familiar. This syndrome is the product of a fungus called Fusarium moniliforme, which often invades cornfields in the Midwest. The fungus looks gray or cottony white, with affected kernels of corn turning gray to brown. But F. moniliforme can be present with little or no outward signs in the corn.

F. moniliforme produces fumonisins, which increase in concentration when corn crops are stressed during the growing season. Outbreaks of ELEM are usually associated with drought or very wet conditions at harvest that favor fungal growth. When consumed daily in sufficient quantities over a week or more, fumonisins can trigger liver or neurological problems in horses after three to four weeks. Affected horses usually show in coordination and a reduced response to external stimuli at first, followed by circling or aimless wandering, hyper excitability, pressing the head against solid objects, blindness, partial paralysis, weight loss, unthriftiness, and jaundice. Eventually, they go down and are unable to get up. Any neurological signs should immediately be brought to the attention of a veterinarian.

These clinical signs are a result of extensive brain damage (necropsies reveal a rather shocking liquefying of portions of the cerebral cortex, the outer portion of the brain); and horses exhibiting these clinical signs usually die within 24-48 hours. Those which survive often suffer from life-long neurological defects.

Horses are more vulnerable to moldy corn poisoning than any other livestock species, sometimes showing symptoms after exposure to levels as low as five parts per million. The survival rate depends on how much of the toxin horses ingest and over what length of time, but the survival rate is estimated at less than 50%. Since corn kernels infected with F. moniliforme are more brittle than healthy kernels, they tend to crack and break during harvest and drying, and end up in the screenings, so do not feed these screenings. Fusarium toxins occur more often with cool wet growing conditions, while hot humid conditions favor the formation of aflatoxins in feeds and forages. Molds in general will increase as we wait for forage crops to dry down, again as a result of wet field conditions.

Always store your hay and grain in a dry environment to minimize the chances of mold growth, inspect it thoroughly before you feed it, and immediately discard any feed that is suspect. Pay attention to your horse's eating habits; his refusal of a feed that looks and smells perfectly normal to you might be an indicator of the presence of molds and/or mycotoxins. Molds will not always contain dangerous compounds, however they most often result in reduced feed intake, reduced digestibility of feeds and poor performance for horses.



About the Author

Ann Swinker, PhD

Ann Swinker, PhD, is an extension horse specialist and associate professor in equine science at Penn State University.

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