Developmental Orthopaedic Disease: Protecting Growing Joints in Foals & Weanlings

Developmental Orthopaedic Disease

Developmental Orthopaedic Disease (DOD) is one of the most significant risks for foals and weanlings. The term 'Developmental Orthopaedic Disease' was first introduced at a symposium sponsored by the American Quarter Horse Association in 1986. It encompasses any orthopaedic issue involving tendons, joints, or bone tissue in growing horses. The diseases included under this definition are osteochondrosis (OCD), physitis and epiphysitis, angular limb deformities (ALD), flexural deformities (including contracted tendons), and wobbler disease. Despite their differences, these conditions were grouped under DOD because it was observed that studs with a high incidence of one condition often showed evidence of the others as well.

The causes of DOD remain unclear due to several factors:

• Serendipitous Nature: Lesions may appear and disappear on both radiographic and arthroscopic examinations.

• Self-Resolving or Treatable Forms: Many forms of DOD can resolve on their own or be treated effectively.

• Early Presence: The disease is often present long before clinical signs become apparent.

These complexities make it challenging to pinpoint the exact causes of DOD, but ongoing research continues to shed light on this important area of equine health.

Osteochondrosis (OCD)

The most common age for the development of osteochondrosis (OCD) is from 3-10 months, with stifle OCD being a major issue for Quarter Horses. Biomechanical influences (related to body weight, height, and conformation), excessive exercise, and nutritional imbalances account for 75% of the risk factors, while genetics make up the remaining 25%. Joint and cartilage development is a complex process, and any interruption can result in defective cartilage. Nutritionally, above-average gains in height and weight are associated with the occurrence of lesions.

Growth Patterns and Nutritional Needs

The most rapid growth periods for bone development are:

• From 3 months before until one month after birth

• Between 6-12 months of age

• Just after puberty

To support rapid bone growth, mares need to be correctly fed during the last 3 months of pregnancy. Quality hays are good feeds for broodmares, but the amino acid composition must be considered. The effects of amino acid imbalance are most pronounced during the last 90 days of pregnancy and the first 90 days of birth - when bone growth spurts occur.

Essential amino acids like lysine, found in lucerne hay, are crucial for building bone and muscle. Hay can be supplemented with linseed, soybean, or other oil meals to provide adequate protein.

Monitoring Growth and Development

Correctly fed yearlings achieve greater gains in wither height, reach mature height earlier, and deposit less fat because their essential amino acid requirements are met. Although body weight is an indication of growth, it is not sensitive enough to reveal the effects of lowered amino acid absorption on skeletal or muscle development. Height is more indicative.

OCD and Insulin Levels

A connection has been found between OCD and insulin levels after feeding. Diets high in energy and containing sweet feed or other high sugar concentrates increase the incidence of DOD. These diets can cause high blood glucose, insulin, and acid levels for up to 4 hours after feeding. Foals of mares fed high starch/sugar feeds have been found to be relatively insulin resistant/glucose intolerant between 3 and 12 months of age – the most common age for OCD lesions to develop.

Biomechanical Forces and Joint Vulnerability

Many cases of OCD are likely the result of excessive biomechanical force on developing cartilage, disrupting blood supply and preventing its conversion to bone. Factors include:

• Specific joints have 'windows of vulnerability'; for example, hocks are most vulnerable from birth to 3 months of age, stifles from 5 to 8 months.

• Genetic predisposition for rapid skeletal and muscle development. Foals may develop greater muscle mass than the rapidly growing bone can support – genetics is thought to contribute 25% of the risk for DOD, management 75%.

• Overfeeding leading to joint overload.

• Conformation defects causing uneven and abnormal loading of the joint surface.

• Reduced bone density due to restricted exercise after an illness. When returned to pasture, the bone is not strong enough to support a normal amount of exercise, and the joint cartilage collapses.

• Sudden changes in exercise patterns can lead to fatigue and joint overload.

Lesions of osteochondrosis develop during a specific and limited window of time, namely when the epiphyseal cartilage is nourished by vulnerable blood vessels. Therefore, preventive measures should be concentrated on this critical period, which can vary depending on the species and the specific joint involved.

The Importance of a Balanced Diet

A balanced diet providing the correct amount and ratio of minerals, essential amino acids, and energy is crucial in preventing lesions resulting from defective cartilage formation. The incidence of DOD is much higher in horses fed unbalanced diets. Proper dietary management helps regulate growth and prevent excess condition, especially for biomechanical-induced forms of DOD and in foals with a genetic potential for rapid growth.

Feeding Guidelines for Foals

The primary guideline (unless advised otherwise by a veterinarian) is to avoid letting the foal eat the mare's feed or providing the foal with feed until it is 2-3 months old. Foals that are thriving on their mother's milk do not require additional feed until they reach 2-3 months of age. To ensure this, place the mare's feed bins out of the foal's reach or use a nosebag for the mare’s feed.

Recent Findings on Magnesium and OCD

Recent international research has found a link between OCD and magnesium deficiency in growing horses. Supplementing foals with magnesium from birth to 12 months of age resulted in a 50% reduction in OCD of the hock and fetlock at 5 months of age, and a 14% reduction in stifle OCD at 12 months of age.

From the very first steps taken by each new foal, skeletal injuries and issues can affect their future soundness. Careful management of nutrition and growth rate can pay big dividends in future soundness and longevity.

Dr Jennifer Stewart
BVSc BSc PhD Equine Veterinarian and Consultant Nutritionist