Genetic Conditions in Thoroughbred Horses

The Jockey Club closed the Stud Book for Thoroughbreds in the 1800s, thereby shutting out any new infusions into the gene pool. This raises the chances of sires with genetic conditions being bred to mares with genetic conditions. Not all Thoroughbreds have genetic conditions.

Eye Problems

There are many genetic eye conditions seen in Thoroughbreds, according to Dennis E. Brooks, DVM, PhD. These include birth defects such as entroption, where foals are born with eyelids turned inside out. Other genetic eye diseases seen in Thoroughbreds include congenital cataracts and progressive retinal atrophy or PRA which is similar to PRA seen in dogs. These often cause blindness, but horses can adapt to blindness. Some conditions like entroption can be treated surgically.

Hemophilia (Factor VII Deficiency)

This recessive trait is only seen in colts. Fillies become carriers. Colts inherit this potentially deadly disease from their dams. Colts with hemophilia lacks antihemophilic globulin, a chemical that helps form blood clots. Affected colts usually die of internal hemorrhaging, according to "Horse Breeding and Management" (Elsevier Health Services; 1992.)

Recurrent Exertional Rhabdomyolysis

This dominant trait occurs more in fillies than colts. It causes a muscular paralysis commonly known as "tying up." There is still some controversy as to whether genes are the major cause, but the general consensus since this mostly appears in Thoroughbreds and Standardbreds, genes cannot be ignored. This is a treatable condition with diet and exercise changes. It can be difficult to diagnose unless a horse has gone through several episodes of tying up.

Degenerative Suspensory Ligament Desmitis (DSLD)

This is an incurable genetic condition of the bodily connective tissues, especially those in the horse's legs. It causes pain and lameness. Treatment is multi-faceted, including corrective shoeing, extracorporeal shock wave therapy, medications and sometimes surgery.

Overo Lethal White Syndrome (OLWS)

Most often seen in breeds with pinto patterns, OLWS causes a foal to be born all white with blue eyes and a lack of vital sections of intestine. They die within a few days or are born dead. This is incurable. Thoroughbreds and also quarter horses carry the genes for OLWS. Any Thoroughbred about to be bred to American Paint Horses or similar breeds should be screened for OLWS.

Equine Neonatal Isoerythrolysis (NI)

This condition causes a foal to become allergic to the natural antibodies its dam's first milk or colostrum. It is treatable if caught in time. It's caused by both sire and dam, not just one parent. It is only prevalent in 0.05% of Thoroughbreds. But if a Thoroughbred is going to be bred, checking for NI in both parents is prudent.


References



Equus. "How Horses Inherit Genetic Diseases." Joanne Meszoly. http://www.equisearch.com/horses_care/health/breeding/geneticdiseases_011207/



BlindHorses.org. "Eye Diseases." Dennis E. Brooks, DVM, PhD. http://www.blindhorses.org/eye_disease.html



"Equine Genetic Diseases: Who's at Risk?" The Horse. Marcia King. October 1, 2005. http://www.thehorse.com/ViewArticle.aspx?ID=6169



"Equine Neonatal Isoerythrolysis." Howard P. Bouchelle, III, DVM, et al. 2003. http://www.vet.uga.edu/VPP/clerk/Bouchelle/index.php



"Horse Breeding and Management." James Warren Evans. Elsevier Health Services; 1992.