While I have not heard of blindness being an issue, EPM is a serious and sometimes fatal disease spread to horses by their hay and feen being contaminated by the droppings of Oppossums.
Here is an article from Purdue University relating to EPM.
http://www.agriculture.purdue.edu/agans ... toryID=732Written Wednesday, October 22, 1997
Purdue's Extension service is alerting horse owners in Indiana and elsewhere about an alarming increase in the number of cases of a potentially fatal disease in horses.
Purdue's Large Animal Hospital recently has been treating eight times the typical number of cases of Equine Protozoal Myelitis (EPM), a debilitating neurological disease that while treatable, can be fatal when not diagnosed and treated early.
"Two years ago we might see one horse a month with neurological disease; now we are seeing about two a week with EPM," says Dr. Janice Sojka, Extension specialist in equine internal medicine.
People who own, board or work with horses should watch for the early warning signs of EPM. Symptoms may include lack of coordination, wobbling or lameness, according to Sojka. Horses also may lose muscle mass for no apparent reason.
Horses become infected with EPM through hay, pasture, grain or water that has been contaminated by possum feces, Sojka says. Feed and water contamination also may occur through birds and insects.
Horses stabled in urban and suburban areas are at the same risk for exposure as those kept in rural areas. "The possum population is throughout the state, and hay, particularly, is shipped throughout the country," Sojka says. "There have been outbreaks of EPM throughout Indiana and at the Chicago race tracks." Outbreaks also have occurred in Kentucky, Ohio, Michigan and Florida.
Like Purdue veterinarians, Dr. Justin Janssen is treating more horses with EPM at his Sheridan, Ind. practice. "We've had about a dozen cases so far this year," Janssen says.
Janssen, who almost exclusively treats horses, has seen symptoms ranging from subtle lameness and stumbling to nearly uncontrollable shaking of the head. "Horses may appear to be perfectly healthy except for a gait abnormality," he says. "Most of the symptoms are related to gait. We're not seeing the muscle atrophy that we were a year or two ago where the horse actually may fall down and not be able to get up."
While there is no vaccine or known cure for the disease, it is being treated with antibiotics. Treatment is expensive--about $200 per month--with no guarantee of success. While most horses require treatment for at least three to four months, veterinarians agree that the sooner treatment begins, the better the chances for recovery. However, Sojka cautions that if a horse is treated for six to eight weeks without improvement, treatment probably will not be successful.
"We're seeing about a 75 percent success rate in treating horses [at] Purdue's clinic," she says. "However, not all horses recover 100 percent. Once there has been nerve damage, it cant be repaired." Sometimes horses relapse, too.
EPM is caused by a parasite that passes between possums and birds. "Possums acquire the disease by eating infected birds. Up to 76 percent of the horses in Indiana may have been exposed to this parasite, according to a preliminary study of serum samples from horses throughout Indiana by Dr. Michel Levy, Purdue large animal internal medicine specialist.
It's believed about 75 percent of Indiana's horses have been exposed, but exposure doesn't always mean a horse will get the disease.
To reduce the chances of exposure, feed storage facilities should be kept as clean as possible. Sojka recommended putting grain in tight containers and keeping hay storage areas clean. "You should keep hay covered with a tarp if you have birds in the rafters of your hay mows," she says. Another preventive measure is keeping possums away from the area where horses, water and feed storage are located. "These things won't eliminate the threat, but they may reduce it," she says.
