Fast Growth, A Risk Factor in Horses

Horse owners want their foals to grow into big strong horses. That's pretty normal for anyone caring for a growing animal. However, there are problems associated with fast growth that owners should understand. Developmental orthopedic disease, or DOD, has many possible causes, but the major factors are rapid growth rate, a high-calorie diet, nutritional imbalances, genetics, hormones, and trauma to growing bones.

"All breeds are prone to DOD, but Standardbreds, Thoroughbreds, warmbloods, draft horses, and quarter horses seem particularly prone to osteochondrosis dissecans (OCD), the best-known form of DOD," says Dr. David Freeman, equine surgeon at the University of Illinois Veterinary Teaching Hospital in Urbana. OCD is characterized by fragments of cartilage within the joint, and it most commonly affects the hock, fetlock, and stifle. Other DOD diseases include inflammation of the growth plate (physitis), angular limb deformities such as bowed legs or knocked knees, bone cysts, and cervical vertebral malformation or "wobblers."

The causes of typical DOD lesions are often related to how animals' bones grow. Since the typical foal will need to grow to the size of its mother in a short span of time, horses' bones are specially engineered to grow quickly. Bones can grow quickly in length and strength because of the cartilage -- or "growth plates" -- on each end.

Bones grow by a process in which the cartilage plates at each end calcify, or harden into bone. Little by little, this process adds length to the bones. In joints, cartilage in the growth plate forms bone that is eventually covered with a thin lining of cartilage. For unknown reasons, sometimes growth plate cartilage at the joint surface remains as an island of cartilage instead of converting to bone. If that occurs, OCD often results.

"The abnormally retained cartilage is weak and prone to mechanical damage, leading to defects such as cracks and cysts. A crack in the cartilage can form a flap that can become hardened to form a fragment. A bone and cartilage fragment that is formed this way is typical of OCD. A cyst forms as a hole in the bone beneath the joint cartilage," says Dr. Freeman.

A defect in the cartilage of the joint growth plate (the epiphyseal growth plate) may cause problems such as OCD and bone cysts. However, if the defect occurs within the bone at the metaphyseal growth plate (the growth plate for adding bone length), problems arise in alignment and weight-carrying ability, which can cause an angular limb deformity.

Since a high rate of growth is recognized as a factor in DOD, decreasing the grain intake of young horses may help decrease signs of DOD. "Excessive caloric intake, mainly from grain, results in rapid growth and above-average weight gain, which place greater stress on the skeleton. Trauma to the growth plate from normal play and activity can then cause damage," says Dr. Freeman. "However, prevention of OCD is not easy and genetic factors probably play a big part. In some cases, a tendency toward rapid growth could be an inherited trait."

OCD may cause lameness and swelling in an affected joint. "The majority of horses that have signs of OCD are athletes. The problems appear when the weakened cartilage is traumatized or stressed, as when a young horse starts training. Sedentary or noncompetitive horses may not show any signs of the problem," says Dr. Freeman.

Treatment for OCD depends on the joint affected, intended use of the horse, and economics. Conservative treatment involves rest, correction of nutritional problems, and hyaluronic acid or glycosaminoglycan injections into the joint. The treatment of choice is arthroscopy, surgical removal of the cartilage defect.

By Carrie Gustavson, Information Specialist
University of Illinois - College of Veterinary Medicine