The EPM vaccine is currently designated as "conditionally approved" by the USDA. However, the state veterinarian's office of Virginia in Richmond has not yet allowed for use of the vaccine in the state.
But studies continue and approval could be forthcoming this year. As "field test" results come in to the USDA, final approval by the Virginia state veterinarian is expected.
"We've been on the forefront of product development," said Jay Whalen, senior project leader in Fort Dodge's equine research. "We know there is a need in the equine industry."
Studies indicate the prevalence of sarcocystis neurona exposure to be as high as 50 percent for all horses, with the percentage increasing to 80 percent for older, adult horses as exposure increases with age. Stress appears to play an important role in clinical cases, and the incubation period is variable.
Traditional treatment for EPM is expensive and of questionable efficacy.
Rob Daily, director of equine products for Fort Dodge, said that EPM is "treatable, but it can cost thousands of dollars, for months at a time. This, coupled with the fact that the horse isn't performing, can be devastating for the owner."
EPM is a neurological disorder of the horse that was first recognized in the early 1970s, but the organism causing the disease was not identified until 1991.
The disease is caused by a protezoal parasite that invades the horse's nervous tissues, causing progressive damage and swelling to the spinal cord (myelitis) or the brain (encephalitis), or both. Cases of EPM have been reported in horses and ponies from all age groups and breeds throughout the U.S.
Though the definitive life-cycle of sarcocystis neurona is not altogether understood, the possum and scavenging birds serve as intermediate hosts for the organism in which the horse is a dead-end host.
First, a bird is infected with the organism, then a possum scavenges on a dead, infected bird. The possum does not become ill; instead, it sheds infective stages of the parasite in its feces, which can then contaminate food and water supplies and be ingested by other foraging animals, including horses.
Once consumed, the parasites develop and travel, through the bloodstream or via tissues, to the central nervous system. Clinical signs of EPM can develop in horses when the parasite crosses into the spinal cord or the brain and cause damaging lesions.
Some horses seem to develop an immune system response and rid themselves of the organism without showing clinical evidence of infection.
Other horses, however, can show widely varied clinical signs of asymetrical and progressive incoordination, a spastic gait, dragging of a hoof, knuckling over, muscle atrophy and paralysis.
It is quite clear that, in places where possums proliferate, many more horses have been exposed to the organism than show clinical signs.
But the clinical signs of EPM are not exclusive to the disease. Many of the same symptoms can occur secondary to other neurological damage, such as that induced by trauma, by viral infections such as rabies or equine rhinopheuminitis, by the narrowing of the spinal cord as in horses designated as "wobblers", or by other neurological disorders.
Previously, diagnosis was the only way to positively identify the condition, and even then EPM often could not be pinpointed with absolute certainty. A spinal tap, a test of cerebrospinal fluid, can show evidence of antibodies, but the limitation of this method is that a horse which has been exposed to the parasite and effectively fought it off will also test positive.
There is also grave risk, when taking a spinal fluid sample, of contamination by the slightest touch to any other blood within the horse's body.
The key to saving horses with EPM is early, aggressive treatment, though the appropriate length and method is not altogether standardized because of continuing research results.
Two anti-protezoal drugs, pyrimethamine with a sulfonamide anti-microbial, has proven efficacious, with or without trimethoprim, given over a period of at least three months.
The parasite can be eliminated with anti-microbial treatment, but the damage already caused to the nervous system may not be reparable, so, in some cases, the horse cannot return to normal use. It is for this reason that the Fort Dodge preventative vaccine appears to promising, especially in regions, such as Virginia, where the possum is common.
Closing in on an answer
The USDA gave "conditional approval" for use of the EPM vaccine in late January, according to Fort Dodge manager of professional services Rocky Bigbie.
"Horse owners experience a high level of anxiety with EPM," said Bigbie. "Fort Dodge is hopeful this new vaccine will help alleviate this anxiety.
"The vaccine is intended to aid in the prevention of new disease. EPM is of great concern to many horse owners. We hope our vaccine proves to be a useful tool for them."
The Fort Dodge EPM vaccine is now being distributed through licensed, practicing veterinarians only.
More information on the Fort Dodge vaccine, is available by calling the Kansas-based company at (913) 664-7034.
Details about Virginia's approval of the vaccine are available through the Virginia Department of Agriculture's animal health division in Warrenton at (540) 347-6385.