"Two years ago we might see one horse a month with neurological disease; now we are seeing about two a week with EPM," said Dr. Janice Sojka, Extension specialist in equine internal medicine.
People who own, board or work with horses should be on the lookout for early warning signs of EPM, a debilitating disease that attacks the central nervous system of horses and can result in death if left untreated. Symptoms may include lack of coordination, wobbling or lameness not related to a specific cause, 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 said. Feed and water contamination also may occur indirectly 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 said. "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-based practice. "We've had about a dozen cases so far this year," Janssen said. "And we're hearing about similar increases from many of our colleagues."
Janssen, who almost exclusively treats horses, has seen symptoms ranging from subtle lameness and stumbling to an almost uncontrollable shaking of the head. "Horses may appear to be perfectly healthy except for a gait abnormality," he said. "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 -- upwards of $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 cautioned 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 brought to Purdue's clinic," she said. "However, not all horses return to 100 percent capacity. Once there has been nerve damage, it can't be repaired."
Janssen, too, treats horses for 90 to 120 days with about an 80 percent success rate, but he said some horses have shown signs of relapse once treatment was stopped.
EPM is caused by a parasite (Sarcocystis neurona) that passes between possums and birds. Horses get it by mistake, Sojka said. "Possums acquire the disease by eating infected birds. We see a lot of this disease in Indiana because of the possum population," she said.
Up to 76 percent of the horses in Indiana may be exposed to this parasite, according to a preliminary study of serum samples from horses throughout the state that was conducted by Dr. Michel Levy, Purdue large animal internal medicine specialist.
Levy tested serum samples drawn for Coggins tests, a test for Equine Infectious Anemia that is required for the sale, interstate transportation, and entry of a horse in shows and other competitions.
"We made an assumption that these horses were healthy," Levy said of the 86 horses in the sample. "We plan to continue the study by having vets take blood from horses that are known to be healthy." Levy said he also hopes that this follow-up work either will confirm or dispel early indications that EPM seems more prevalent in the northern half of the state than in the southern half.
Even though a large percentage of Indiana's equine population may be exposed, the number of horses that actually develop the disease is much lower. Blood tests determine only whether or not a horse has been exposed; it is not an effective test for diagnosis. "The only way to tell for sure if a horse is infected with the parasite is through a spinal tap," Sojka said.
To reduce the chances of exposure, grain and hay 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 said. 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 said.
Although the disease has been recognized since the 1970s, it wasn't until the early '90s that the parasite successfully was removed from a horse and identified.
"We don't know why one horse may develop the disease, when another horse that has been exposed isn't affected," Sojka said. "Right now there's a lot more we don't know about this disease than we do know. In five years what we know may be completely different."
Janssen agrees. "What we know now -- both how we're diagnosing and handling it -- is just the tip of the iceberg."
Source: Janice Sojka, (765) 494-8548; e-mail, firstname.lastname@example.org
Justin Janssen, (317) 873-5353
Michel Levy, (765) 494-8548; e-mail, email@example.com
Writer: Olivia Maddox, (765) 496-3207; e-mail, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com
To the Purdue News and Photos Page