(Equine Leukoencephalomalacia)

Mark Russell, PhD, Department of Animal Sciences
Don Scott, PhD, Department of Botany and Plant Pathology
William Hope, DVM, Department of Veterinary Clinical Sciences

Cooperative Extension Service
Purdue University
West Lafayette, IN 47907

The purpose of this paper is to provide the horse owner or manager with accurate information so they can understand the problem, recognize the symptoms and causes, and make feeding decisions based on the best balance of risk and economic concerns in each situation.

Since 1891, diseases called moldy corn poisoning, leuko, circling disease, and leukoencephalomalacia have been reported, but horsemen have long known that moldy corn can cause severe neurological disorders and kill horses. Inasmuch as corn and oats are the most commonly fed grains for horses in the U.S., a corn related problem is not uncommon, particularly when corn is stressed by drought, insects, or wet harvest conditions. Poor storage conditions can cause moldy corn.

There are many potential problems associated with a wide variety of mold problems in feed grains. Equine Leukoencephalomalacia (ELEM) stems from a very common fungus in corn called Fusarium moniliforme. Each year the fungus is found to some extent in 80% to 90% of the corn fields in the Midwest, but the presence of the Fusarium moniliforme appears to be higher in stress-weakened corn. Although the exact conditions causing the fungus to produce fumonisin is not known, it appears that corn severely stressed during the growing season produces more mycotoxin.

The general term, mycotoxin, includes not only fumonisin, but also aflatoxin, vomitoxin, zearalenone, and others. The Fusarium moniliforme can produce secondary metabolites called fumonisins identified and isolated in South Africa in 1988. At this time, Fumonisin B1 (FB1) and Fumonisin B2 (FB2) are the most important members of a growing list of water soluble fumonisins. The mycotoxin impregnates the kernels of corn; and when eaten in sufficient concentrations, the fumonisin causes brain and liver problems (Leukoencephalomalacia/hepatosis). It appears that horses must be fed contaminated corn for over 1-2 weeks for clinical signs of ELEM to occur.

Early clinical symptoms may become apparent as soon as 10 days or as late as 90 days after being fed contaminated corn. The neurological form of the disease is manifested by reduced responsiveness to external stimuli, incoordination, aimless wandering, head pressing, circling, hyperexcitability, paresis, and eventually recumbency. Because the extensive brain damage is what causes these symptoms, horses with these problems usually die within 24 to 48 hours after the symptoms are present. Surviving horses often have life-long neurological defects, making them unproductive. Postmortem examination of the brain reveals areas of liquefactive necrosis of the cerebral cortex.

Another form of leukoencephalomalacia is seen as liver or hepatic disease. Hepatic failure causes icterus, edema, hemorrhage, weight loss, elevated serum hepatic enzyme activities and bilirubin levels, and an overall unthrifty appearance. The hepatic effects are believed to be more reversible than the neurological form of ELEM. However, once fifty percent of the liver becomes damaged, chronic liver disease may exist. These horses often loose weight regardless of adequate nutrition. It is unclear how long horses can tolerate low levels of toxicity, but it is reported that some horses have been on the same corn for 30 days before elevated hepatic enzymes indicated liver damage.

Horses seem to be the most susceptible to toxic levels of fumonisin. Combined levels of FB1 and FB2 as low as 5 parts per million have been associated with ELEM in horses, whereas swine are affected with pulmonary edema and occasional hemorrhaging at levels of 15 parts per million. Cattle have been fed corn with combined levels of 50 parts per million of FB1 and FB2 with decreased weight gains due to the reluctance to consume contaminated corn the only symptom. But elevated hepatic enzymes are detected. Feeding contaminated corn to poultry, especially ducklings, should be avoided. The symptoms found in horses, poultry, and hogs are of result of toxic levels fed over na extended period of time. The effect of a single massive dose has not been studied. The effect on humans is poorly known. Fumonisins have been associated with cancer of the esophagus in Africa and east coast of the U.S. Very low levels of fumonisins (parts per billion-ng/g) have been found in corn-based food products, but these levels seem to be safe.

No proven therapies for equine leukoencephalomalacia exist. Supportive therapies with IV fluids, dimethyl sulfoxide and/or B-vitamins may be tried to help stabilize horses with neurological symptoms, but there is no data to indicate much benefit. Animals with liver problems should be supported as a veterinarian would other forms of potentially reversible liver failure. Diagnosis is established with clinical signs, suggestive postmortem lesions, and feed grains contaminated with fumonisin.

The critical issue is clear. How can horsemen minimize risks when the horse is this susceptible? The answer is not simple. But in as much as the fumonisin is in all portions of contaminated kernels of corn, the cracking, rolling, steam flaking, and pelleting processes will not change the toxicity. All corn is at some risk because the fumonisin cannot be seen, and the degree of damage to the kernels varies greatly. Thus any feed, whether pelleted or mixed, containing corn may have fumonisin; however most are at levels below those which cause disease. The label should be checked to determine the presence or amount of corn in the mixture. Fumonisin is a mycotoxin that is present at harvest and does not increase or decrease with time or normal storage conditions. The black light test which detects the bright green effervescence of Aspergillus flavus is unable to detect F. moniliforme because the fungal enzymes that react with the corn enzymes to produce cogic acid are absent.

Fusarium moniliforme infected kernels are usually more brittle and break during harvesting and handling. Thus, fumonisin is usually higher in corn that falls through the screen with the finer, smaller particles (fines or screenings). So, although some risk is present in all stressed corn, the risk is much greater when feeding corn screenings, discolored, shriveled, dirty, and weed-seed infested corn. It becomes a question of what is good-quality grain. Good-quality grain is bright yellow in color. It has a shiny sheen and is full and plump; it is free of apparent discolored, small kernels. Corn with these characteristics is safer than the fines or screenings from that same corn. Horses should never be fed corn screenings

To minimize risk, all horsemen should feed as much good forage as possible. Horses should consume as much, or more, hay or pasture as they eat in concentrates. Many horses receiving concentrates probably could get along nicely on hay, minerals, and water. Secondly, anyone feeding concentrates including corn as an ingredient should call their feed source and discuss the quality assurance program used to provide only good corn. The quality assurance program should include sorting corn loads by quality, ingredient testing, random or spot-check fumonisin tests, and fanning or screening the corn to remove fines or damaged corn.

Elevators should be sure all residues from one batch is thoroughly removed from the mixers and the delivering legs. Owners should work with their elevator and where there is a question of quality, submit samples of feed, or the ingredients, for fumonisin testing. University and commercial laboratories are prepared to test feed samples for fumonisin. The test is a high-performance liquid chromatography (HPLC) test. Results are usually available in 7 to 10 days. There are no rapid tests (ELISA, minicolumns, etc.) currently available.

Reducing the proportion of the energy in the ration that comes from corn decreases the risk of disease from fumonisin. This can be done by decreasing the concentrate and increasing the hay, or by changing the concentrate diet. If changes are made in the ration, they should be done gradually, over a 7 to 10 day period to avoid colics and other digestive upsets. To maintain the desired condition of the horses, be sure to balance the ration to provide the same nutrient levels of crude protein, digestible energy, calcium, phosphorous, and trace minerals.

Other energy sources can be used such as barley, oats, rye, or wheat. It is very rare for F. moniliforme to infect barley, oats, and wheat. Fumonisin can be produced by the fungus on these grains in the laboratory, but it has not been encountered under natural conditions at toxic levels. Infected kernels are very light weight. Most are blown out the back of the combine.

When some of the corn is replaced with other grain, the composition of the ration changes. Barley is an excellent grain for horses; it is slightly higher in protein and lower in energy than is corn. Crimping or rolling significantly increases the utilization and may increase cost 20% more than corn. A bushel of barley should weigh at least 45 to 47 pounds. Oats are much higher in protein and lower in energy than is corn. Per unit of energy, oats may cost 30% more than corn; however, good heavy oats are hard to find. A bushel of oats should weigh at least 35 to 40 pounds. Rye is high in protein and lower in energy than is corn, but it is very expensive and not very palatable for horses. The ration should not contain more than 25% rye. Soft wheat is higher in protein and about the same as corn in energy. Wheat should be either steam rolled or flaked and can be included in up to 30% of the concentrate mixture.

Although a wide variety of other grains exist, most horsemen choose to feed oats. Good, heavy, plump oats that are bright in color are expensive and hard to find. Light oats with a lot of empty hulls provide no more energy than average grass hay. When good oats are used to replace some of the corn in the concentrate mixture, the protein level increases and the energy decreases. Hence, some of the soybean meal (or other protein source) can be removed. Increasing energy can be done by using vegetable oil. Replace half or all of the molasses with oil. Research has shown oils can be successfully utilized in diets (up to 15% oil). The important thing is to balance the ration, rather than replacing corn with oats

The only way to be guaranteed of no risk from fumonisin is to feed no corn. But the total ration will be more expensive. Fumonisin only has been reported in corn usage and not in other energy sources. Other energy sources may be more expensive, but they do not pose a ELEM threat.

Regardless of what the concentrate mixture is, be sure to individually feed horses to maintain the amounts consumed per horse. Group feeding is acceptable for hay or a chopped hay and grain mixture (total mixed ration), but it is not recommended for grains.

Horse owners must become good observers of their horses' normal behavior. Watch how each horse eats, its behavior towards other horses and people. Any sudden change in the horse's behavior when being fed corn should be an early warning signal.

The bottom line is to feed the best quality feeds available, insist on quality feed or take it back to the feed source, and determine your balance between economics and the level of risk you are willing to accept. No one other than the feeder is ultimately responsible for the feeding and well-being of the horses.