A newer antiprotozoal drug, ponazuril, was approved by the FDA in 2001. It's highly effective and can be given for a shorter
period compared with pyrimethamine-sulfadiazine. The UC Davis Equine Health Center has been using it since 2001.
The drug has a stated success rate of 70 percent improvement or resolution of EPM clinical signs. It's administered once daily
for 28 days or more, depending on clinical signs and disease severity. Although not 100 percent effective, ponazuril shows
better outcomes when patients are treated earlier in the disease process. If the CNS is already affected, even though the
parasite is destroyed, the horse is left with a damaged nervous system. In short, the earlier the horse is treated, the better
Diclazuril, approved by the FDA in 2007, is about to be released. Studies done with diclazuril show it is as effective as
ponazuril. It's impregnated into an alfalfa pellet, so it can be easily top-dressed onto feed.
Both ponazuril and diclazuril don't have the concerns of pyrimethamine-sulfadiazine in regard to folate deficiency. "Those
concerns are mostly with pregnant mares," MacKay says. "Most other animals don't have a concern as to folate deficiency problems,
though it has that potential."
Treatment comparison and adjunct therapy
"I don't think there's any evidence that any of these treatments are better than the others," says MacKay. "The recommendation
for treatment with pyrimethamine-sulfadiazine is three to six months, so it has a longer treatment period. Both ponazuril
and diclazuril are given for 28 days. With either drug, at least 60 percent of horses improve. There's no information on how
many of the treated EPM horses completely recover, but I think that's a factor of how bad they were to start with."
Along with antiprotozoal drug treatment, anti-inflammatories and immunostimulants have been used.
Assisting horses to strengthen their immunity to help ward off EPM, and to limit those exposed from becoming infected, may
be accomplished by adding an antioxidant—specifically, supplementing horses with clinical EPM with natural-source vitamin
E at 10,000 IU/day to help build their immune response and help them recover from neurologic disease. This form of the vitamin
is able to cross the blood-brain barrier, and it is used as a routine treatment in horses with neurologic disease.
MacKay says it's rare for a horse that's severely affected by EPM to completely recover. "If it's treated very early when
the signs are mild, maybe 80 to 90 percent may completely recover," he says. "If it's severe, where the horse has difficulty
standing up, then 10 percent or less of those horses may completely recover. It definitely depends on the neurologic condition
of the horse when it begins treatment."
And when it comes to relapses, it's basically similar, MacKay continues. "The milder cases have less frequent relapses; the
bad ones relapse routinely. But we don't know why that is."
Pusterela cautions that many horse owners wait too long to have their affected animals examined. "There's a potential that
some of the antiprotozoal drugs, particularly the new formulas ponazuril and diclazuril, may be used as a prophylactic, especially
in stressed or immunocompromised patients," Pusterela says. "Diclazuril will be a very good drug to administer because it's
in a pelleted form. It will have great potential to be used more like a preventive drug."