Every year equine veterinarians flock to the Kester News Hour session at the American Association of Equine Practitioners (AAEP) convention for reports on research that are too brief or new to be included in the scientific program. For the past two years, the fast-paced news broadcast format has been anchored by three renowned equine experts:

• Scott Palmer, VMD, Dipl. ABVP, hospital director/surgeon at the New Jersey Equine Clinic in Clarksburg and a past president of the AAEP, focuses on lameness and surgery topics.
  
• Margo Macpherson, DVM, MS, Dipl. ACT, associate professor at the University of Florida, discusses reproduction topics.
  
• Bonnie Rush, DVM, MS, Dipl. ACVIM, professor of equine internal medicine at Kansas State University, covers medicine topics.

Following are brief synopses of the studies discussed.

Medicine
Lawsonia intracellularis Rush reviewed two studies on this organism, which causes a proliferative ileitis (an intestinal disease) in horses and swine worldwide. The first study followed 57 affected horses between two and eight months of age. They showed ventral (lower belly) edema (fluid swelling, in 81% of horses) and hypoalbuminemia (low levels of albumin protein in the blood, 100% of horses), with other intestinal signs such as colic, fever, lethargy, and diarrhea to a lesser degree. The disease appeared seasonally, with these horses presenting for examination between August and January (half in November and December). A thickened small intestine (seen on ultrasound examination of the abdomen) was not present in all cases, but this finding is suggestive of the disease.

Only half of the horses tested positive for L. intracellularis on both PCR (polymerase chain reaction) and IPMA (immunoperoxidase monolayer assay) tests; the rest were positive on one or the other. Survival rate was good (93%) with antibiotic treatment (primarily oxytetracycline), but the disease did have some lasting effects; 14 affected Thoroughbred foals sold for 68% less than siblings by the same sire, presumably due to a negative impact on growth and development.

The second study found that the incidence of the disease is relatively low despite relatively high exposure. Of 102 healthy horses on farms with L. intracellularis cases, 32.3% had antibodies to the disease (seropositive, indicating exposure). None tested positive via PCR, and none had hypoproteinemia (low protein in the blood).

“Seropositivity can occur without clinical signs of infection and should not be used for definitive diagnosis by itself,” Rush commented. She said the author of the second study would also be presenting a study on L. intracellularis later in the meeting (coverage of that presentation on Lawsonia intracellularis).  

REFERENCES: Frazer M.L. Lawsonia intracellularis infection in horses: 2005-2007. J Vet Intern Med 2008; 22: 1243-1248.

Pusterla, N.; Higgins, J.C.; Smith, P.; Mapes, S.; Gebhart, C. Epidemiological survey on farms with documented occurrence of equine proliferative enteropathy due to Lawsonia intracellularis. The Veterinary Record, Aug. 2, 2008.

Polysaccharide storage myopathy (PSSM) “This disease (a form of tying-up) is an important cause of poor performance in many horses,” noted Palmer. Rush discussed two papers on this disease, starting with one that described the genetic mutation responsible for this form of tying-up.

In the early 1990s, PSSM was recognized as an abnormal accumulation of glycogen (a form of sugar that cells use for energy) in skeletal muscle. The authors of this study discovered that the disease results from a dominant mutation in the glycogen synthase enzyme (GYS1), which is unusual in that it causes overactivity of the enzyme, resulting in an accumulation of abnormal byproducts (most disease-causing mutations result in a loss or reduction of function). “That’s why it was hard to find this defect–and it is the feature that makes this genetic mutation unique of all muscle storage diseases,” Rush commented.

A second study described the prevalence of the mutation in 17 of 36 breeds of horses with PSSM, which affects up to 36% of draft horses and 10% of Quarter Horses today. The GYS1 mutation was found in nearly half of the affected horses, from 18% of affected Warmbloods to 72% of affected Quarter Horses and a whopping 87% of affected draft horses. “This mutation isn’t the only cause of PSSM, or the numbers would be 100%, but it appears to be the most important” said Rush. “There must be some other mechanism by which abnormal accumulation occurs. The bottom line is that we have a genetic test for this mutation to help make breeding decisions. There’s also a newly discovered genetic point mutation in Quarter Horses that causes the most severe form of tying-up we see when paired with the GYS1 mutation.”

Most owners of affected horses found an improvement in clinical signs with the recommended low-starch, fat-supplemented diet and regular daily exercise. If this disease is suspected, hair root testing and muscle biopsy can be used to confirm the disease (read more).

REFERENCES: McCue, M.E.; Valberg, S.J.; Lucio, M.: Mickelson, J.R. Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. J Vet Intern Med 2008; 22: 1228-1233.

McCue, M.E.; Valberg, S.J.; Miller, M.B.; Wade, C.; DiMauro, S.; Akman, H.O.; Mickelson, J.R. Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics 2008.