Published December, 2011

More than 130 equine practitioners and professionals from 10 states attended the Advances in Equine Neurological Diseases Symposium, a seven-hour program presented by the University of Kentucky Equine Initiative and Gluck Equine Research Center on Dec. 6. Featured topics included equine protozoal myeloencephalitis (EPM), wobbler syndrome, and the neurologic form of herpesvirus.

Steve Reed, DVM, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, was a moderator and organizer of the symposium, along with Dan Howe, PhD, of the Gluck Equine Research Center; Ed Squires, PhD, Dipl. ACT (Hon.), director of the Equine Initiative and executive director of the Gluck Equine Research Foundation; and Jenny Blandford, Gluck Foundation Coordinator at the Gluck Center.

EPM

Howe and Robert MacKay, BVSc, PhD, Dipl. ACVIM, professor of Large Animal Medicine at the University of Florida College of Veterinary Medicine, discussed EPM, an illness that strikes horses by way of the opossum. Opossums carry the parasiteSarcocystis neurona, the causative agent of EPM, and pass it to horses, usually by infecting feed (or water) with their parasite-laden scat. After infecting a horse, the parasite can invade and cause damage to the gray matter of the spinal cord.

Diagnosis of EPM is tricky, as many variables come into play. Onset can be quick and severe or slow and mild, and clinical signs can appear similar to those associated with wobbler syndrome.

The presence of antibodies against S. neurona does not equate to disease, as a high rate of horses test positive but do not develop EPM.

"Infection by Sarcocystis neurona is astonishingly common, but actual disease is very rare, at about 1%," McKay said.

It is not clear why so many horses are infected but so few develop the disease. It is also unclear if some affected horses carry latent forms of S. neurona and relapse, or if they encounter new exposure to the parasite.

Clinical signs of EPM include:

• Ataxia, or lack of muscle control, and weakness;
• Knuckling of hind legs;
• Asymmetry of stride length, with over-stepping in front and the hind legs taking short steps;
• Pelvic and back leg weakness; and 
• Lack of fever.

The low disease rate suggests that most infected horses mount a protective immune response to S. neurona. Failure to resist S. neurona is believed to be caused in some cases by stress, foaling, lactation, illness, long transport, immunodeficiency, and/or genetics. Geographic location also plays a part.

"In areas that see rainfall, like Kentucky, there is a seroprevalence (the frequency of individuals in a population that have a particular element in their blood serum) rate of about 50%," McKay said. "Cold, mountainous areas, like Canada and Montana, have a seroprevalence rate of about zero."

Howe said all strains of S. neurona are not the same, and the strain variation complicates our understanding of the disease and delays vaccine development.

He stated that sequencing projects like the ongoing S. neurona genome project have already lead to improved serologic testing for EPM, and should provide additional advances to control EPM in the future. ELISAs developed from S. neurona sequence information provide improved tests to aid EPM diagnosis and are offered commercially by Equine Diagnostic Solutions LLC, in Lexington, Ky.

Equine Herpesvirus (EHV)

Udeni Balasuriya, PhD, MS, BVSC, associate professor of virology at UK's Gluck Equine Research Center, and Lutz Goehring, DVM, MS, PhD, Dipl. ACVIM, assistant professor in equine medicine at Colorado State University's College of Veterinary Medicine and Biomedical Sciences, lectured on equine herpesvirus. Balasuriya began the presentation explaining that herpesviruses are widely found, usually species-specific, share common properties and morphology, and usually spread through mucosal contact.

Horses often test highly positive for EHVs, though in most animals it is a low-level presence that causes mild respiratory disease.

"The danger is EHV-1, the strain that can cross the placenta and cause abortion, neonatal death, neurological death, and respiratory illness," Balasuriya said.

Goehring pointed out that EHV outbreaks are almost always associated with an index case--a horse that travels for breeding, training, or showing. The upside to the index case is the population that can be infected is limited, especially in a closed herd, if proper measures are taken.

EHV shows specific age and breed predilections:

• Rare in yearlings and 2-year-olds;
• Not found in ponies of any age;
• Mules are viremic but do not get sick;
• Affects "tall" horses disproportionately; and
• Affects mares more than stallions.

Despite coverage in media, including the New York Times, stating that EHV cases have increased, Goehring believes the increase in cases is a result of greater detection, more reporting of the disease, and increased movement and longer stays at events, which increases the risk of infection.

During an outbreak Goehring recommends:

• Identifying and isolating febrile (feverish), shedding, neurologic horses;
• Increasing the distance between horses, closing doors and windows, etc.; and
• Treating horses with Valtrex or similar antivirals and NSAIDs.

Prophylaxis/therapeutic intervention for EHV-1 include vaccines, antiviral therapy, anti-inflammatory drugs to decrease the rate of endothelial cell infection, and management options.

Wobbler Syndrome

Jennifer Janes, DVM, PhD candidate in UK's Veterinary Diagnostic Laboratory, discussed the clinical and pathological aspects of wobbler syndrome. Brett Woodie, DVM, MS, Dipl. ACVS, of Rood & Riddle Equine Hospital, lectured on the intricacies of cervical stabilizing surgery for horses with wobbler syndrome. Barrie Grant, DVM, MS, Dipl. ACVS, MRCVS, a veterinarian based in Bonsall, Calif., also discussed surgical techniques as well as post-surgery recovery for horses.

Recognized since 1911, wobbler syndrome is a neurologic disease resulting in spinal cord compression in the cervical region leading to ataxia, Janes explained. It occurs more often in males than females and affects Thoroughbreds, Tennessee Walking Horses, Warmbloods, and Quarter Horse and Quarter Horse-types more than other breeds.

Wobbler syndrome also is a multifactorial disease, with the following believed to play a part:

• Genetics
• Rapid growth
• High-carb diets
• Decreased copper/increased zinc level
• Trauma

Clinical signs include ataxia, with the hind limbs affected more than the fore; signs that are both acute or progressive; and neurologic issues including toe dragging, circumduction of hind limbs, overreach of hind legs, and a base-wide stance.

Woodie discussed his surgical experience performing cervical stabilization on horses. He stressed the very specific challenges of this surgery, which requires a skilled team, exact positioning, and custom-built tables and cradles that hold the horse's neck far forward and very straight. This allows the surgical team to use K-wire placements as guides, indexing the surgical site, which is a critical aspect. Woodie noted that C6-7 is the site requiring the highest number of surgeries.

Grant was part of the team, along with human orthopedic surgeon George Bagby and veterinarian Pam Wagner, who helped develop the surgical procedure used in horses (and people) to treat cervical cord compression (wobblers). His many refinements of this specialized surgery include use of the "Seattle Slew" titanium implant, threads on implants to reduce fractures, and shortened surgery times.

His influence includes teaching his technique to many of the surgeons worldwide who practice cervical stabilization. Notably, in both of Triple Crown winner Seattle Slew's surgeries to stabilize his neck, Grant was lead surgeon.

The symposium was filmed by TheHorse.com. A DVD copy will be available for purchase in early 2012 at ExclusivelyEquine.com.

Karin Pekarchik is an editorial officer in UK's Agricultural Communications Services.