8 minute read
Windows of Opportunity
Dr Jennifer Stewart BVSc BSc PhD Equine Veterinarian, CEO Jenquine and Consultant Nutritionist Equine Clinical Nutrition
Although affecting 20-25% of all 3-12 month old foals, the term developmental orthopedic disease (DOD), was coined in 1986 by the American Quarter Horse Association to describe skeletal problems in growing horses - limb deformities, bone cysts, contracted tendons, club feet, joint enlargements, wobblers, osteochondritis dessicans (OCD) and physitis (Figures 1 and 2).
Osteochondritis dissecans (OCD) is weak, malformed cartilage that results in flaps of cartilage and bone that either remain attached or break off and float around the joint. It is linked to rapid growth, large body size, high energy diets, mineral imbalances, insulin and thyroid fluxes.
Figure 1. OCD lesion
Most lesions are not detectable (even with Xrays) until the damage causes lameness or joint swelling. Signs of stifle DOD are commonly found in horses from 6m-2yo; hocks 6m-3yo and shoulder OCD or bone cysts 1218mo. However, the disease is present long before signs appear and begins during periods of vulnerabilty.
Physitis is swelling around the growth plates due to overload on the bones, weakened cartilage. It is associated with excess energy, high body weight, conformational defects, calcium deficiency and excess exercise. It often occurs in well-grown, fast-growing, heavy-topped foals during the summer when the ground is dry and hard. It is characterized by flaring of the growth plate, giving a typical “boxy” appearance to the affected joints.
Figure 2. Physitis
The period from 3 months before birth to 5 months of age is one of turbulent change for the musculo-skeletal system. Many dynamic remodelling processes take place, which profoundly influence the strength and integrity of bones and joints. From 5 to 11 months, growth and development continue at a slower rate and the final constitution of joint cartilage has been formed by 5 months of age. Because these processes are only active during the first year of life, DOD lesions only develop during this period – even though they may not show up for months or years later.
Windows of vulnerability: Specific joints have ‘windows of vulnerability’. The hock joint is vulnerable from before birth. Hock DOD can develop before birth and is usually present at 1 month of age, stifle lesions develop from 3-8 months of age. DOD lesions are formed in a very limited window of time and the pathologic insult to a susceptible joint is short (weeks), occurring randomly during the period of joint vulnerability. The age at which periods of irregularity occur varies for each joint. During periods of irregularity (our eyes are a blunt instrument and even Xrays can’t identify these periods) the joint is susceptible to the combined effects of nutritional imbalances, body weight and growth rate. Managment will determine whether the normal variations in cartilage thickness resolve spontaneously – or progress to DOD. And, this provides us with an opportunity to guide and regulate growth and to protect soundness.
Windows of opportunity: because the window of opportunity for sound bone and joint development is open for a short time, correct nutrition of the pregnant mare and young horse is essential for soundness. Prevention should be sought before and after birth and must be the priority for the foal, weanling and yearling.
Risk factors for DOD: With the exception of early fetlock lesions, OCD is generally associated with increased bodyweight and a faster rate of growth. Rapid growth, diet excesses and imbalances, high starch/sugar feeds in mares and foals and higher body weights. High weight gain in foals increases the compressive load on the immature, growing skeleton, which disrupts the blood supply to the cartilage and prevent its conversion to bone.
Hock DOD occurs more frequently in taller and heavier-born foals; those that grow faster in height/ weight and are >5kg above average at 4 weeks of age and >14kg at 8 months. Foals that are intermittently upright in the pasterns also have more frequent and severe lesions. Taller foals; being 5.5kg heavier at 25 days and 17kg at 120 days, and faster weight gains from 3-5 months increase the chance of stifle and/or shoulder lesions. Wobblers are often taller and heavier with higher body weight, wither and hip height from birth to 12 months old, and faster weight gain at 1 - 2 months, 4 - 5 months and 7 – 8 months of age.
Feeding the pregnant mare: There are two basic rules when feeding mares from 8 to 11 months gestation:
Feeding the pregnant mare: There are two basic rules when feeding mares from 8 to 11 months gestation:
Avoid feeds with over 12% starch + sugar (also called NSC). The reasons for this are that pregnant mares become insulin-resistant which predisposes them to laminitis. Although this is an evolutionary adaptation to divert nutrients away from the mares body and send them to the growing foal, it increases the risk of laminitis. Feeds with >12% NSC produce additional swings in blood glucose and insulin that escalate the threat. Added to this is increased body weight of the pregnant mare which places additional stress on the hooves.
A second reason is that increased NSC intake in mares and foals has been shown to cause DOD and a connection has been found between OCD and insulin levels after feeding. Sweet feeds and high sugar/ starch feeds can cause high blood glucose and insulin and low blood pH for up to 4 hours after feeding in foals between 3 and 12 months of age - the most common age for OCD lesions to develop. High insulin predisposes the foal to abnormalities in cartilage growth and the risk of DOD.
Ensure trace minerals (including iodine, zinc, copper, manganese) meet recommended levels. During the last trimester of pregnancy, the foal builds stores of these minerals in its liver. Foals with lower liver copper concentrations have worsening OCD scores between 5 and 11 months, while those with 50% higher liver copper concentrations had significant improvement in
OCD lesions in the stifle by 11 months. Copper is important for early repair and spontaneous resolution of early OCD lesions. Excess zinc can induce a copper deficiency - so although supplementation of the average diet is required, the inclusion of multiple supplements +/- a prepared feed can lead to excesses and imbalances.
Feeding the foal
Foals don’t need feeding! Unless the mare has poor milk production, foals don’t need hard feed and should not have access to the mares feed. Creep feed can be introduced around 2 – 3 months but before this mares milk meets all their needs.
Major Periods of Bone, Muscle and Fat Growth
BONE: 3 months before until 9 months after birth
MUSCLE: 2 to 22 months of age
FAT: 6 months onwards
Feeding the weanling
Foals do not grow only in weight and height – specific tissues have well-defined periods of maximum development. Maximum bone growth occurs from 3 months before birth to around 9 months of age. Maximum muscle development is from 2 until 22 months of age.
Because of the link between above average weight gains and the onset of bone diseases, muscle growth should not be pushed forward while the bones and joints are vulnerable. A lighter, leaner weanling with appropriate height (remember height is an indication of bone growth, not muscle) is the ideal. These growth periods provide the opportunity for determining body composition and for achieving genetic potential in terms of bone and muscle. Foals with a genetic potential for rapid growth, correct dietary management will assist in regulating growth and preventing excess condition. Avoiding feeds with >12% starch and sugar and providing a supplement with correct protein and minerals are the guiding principles.
DOD is a complex, multifactorial disease. Biomechanical influences (excess weight), exercise, nutritional imbalances (75% of the risk) and genetic influences (25% of the risk) have been investigated and all are interrelated. Diets based on fibre and oil, balanced with a vitamin/mineral supplement rather than any feedstuff with >12% starch and sugar will protect the developing youngsters soundness. Extruded and micronized feeds must be used with caution as their increased digestibility can produce more rapid and profound effects on blood glucose and insulin.
ABOUT THE AUTHOR –
Dr Jennifer Stewart
CEO BVSc BSc PhD Dip BEP
Equine Veterinarian and Consultant Nutritionist
Dr Jen Stewart has been an equine veterinarian for more than 40 years and an equine nutritionist for more than 10 years. Jen has been developing premium formulas for studs, trainers and feed companies in Australia and around the world and regularly consults to leading international studs and trainers in various countries.
Jen has spent a fair bit of time researching and being involved in nutritional management of developmental orthopaedic diseases, colic, tying-up, laminitis, performance problems, post-surgery and other conditions. And is currently the only practicing equine veterinarian and clinical nutritionist in Australia. Jen’s promise is to continue to BRING SCIENCE TO YOUR FEED BIN
