Q&A    Questions and Answers:

Hi Cowboy Bob.

My brother had a horse that was over 20 years old. One morning he went out to feed him and he was down and couldn't get up. He could get up on his front feet but couldn't get his back parts up. We thought he had the colic, but the vet we talked to said he had the syptoms of "possom disease." Could you please tell me what that is and if other horses can catch it from my brother's horse.(We lost Jake later on that night.)

Howdy, K.B.!

You had me stumped for a while for a couple of reasons:
1. We don't have opossums where I live, and
2. "Possum Disease" was just discovered in the 1960's and has only been identified with the 'possum in the last decade or so.

After a bit of digging, here's what I've learned:

Equine Protazoal Myeloencephalitis (EPM), or "possum disease" is a neurological disease that can sometimes be confused with West Nile disease, rabies, or equine encephalitis

Some of the symptoms of EPM are lack of coordination, weakness and spastic behavior, lameness, airway abnormalities such as snoring, stumbling, or an uneven gait in the hind legs. Other signs include muscle atrophy or loss of condition, locking up of the stifle, or back soreness. The disease tends to affect one side or part of the horse more than another. The only way to know for certain that a horse has EPM is a blood test.

EPM is caused by parasites in opossum droppings which contaminate hay, grass, or grain consumed by horses.

The organism is carried by certain birds or insects, which are then eaten by 'possums. The little critters grow in the 'possum's intestines and are deposited in the 'possum's droppings everywhere it goes -- including pastures and hay storage areas. When horses eat the infected feed, the EPM organism starts its migration to the spinal column, where it does most of its damage.

In the parts of the country occupied by opossums, anywhere from 50% to 80% of the older horses are infected with the EPM organism, but it is younger horses (one to five years old) that are most likely to show signs of the disease. The disease is not transmitted between infected horses.

Treatment is expensive and can last several months. Antiprotozoal, anti-inflammatory, anti-edema and antioxidant therapy is generally used, along with physical therapy. The antiprotozoal treatment is the base of the treatment programs, with the most common medicine being a combination of pyrimethamine and a sulfonamide, administered orally. The disease is frustrating, because it tends to keep coming back.

Fort Dodge Animal Health, a division of Wyeth, is working on a vaccine, but it is still being tested. In the meantime, the best prevention may be to invest in some good dogs and cats to keep birds and 'possums away from the horse feed!

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