Equine Developmental Orthopaedic Disease or DOD is a collective term used to describe certain growth anomalies associated with bone and cartilage formation in young horses. The term is often referred to by its initials or simply as DOD’S. In the strictest sense it is not really a disease but an “umbrella” term for a number of disorders in the growing horse that involve a fault in the development of the bone and cartilage for various different reasons. DOD’S include such things as angular limb deformities, flexural limb deformities, physitis, osteochondrosis and subchondral bone cysts.
Normal Development Of Bone in the Young Horse
The development of bone begins in the mare’s uterus, the skeleton is initially laid down as cartilage and calcium begins to be deposited and ossify the structure around 75 days. Growth continues slowly until around 240 days when the foal begins a rapid growth prior to birth.
In the case of angular limb deformities and flexural limb deformities a foal could be born with the condition. Generally development of these disorders tends to occur in horses between 3 and 9 months of age, however the signs of DOD may not appear until the animal is older and starting work but the damage has usually occurred during this rapid development stage.
Nutrition of the Foal Can Play a Role.
Up until 3 months the foal is generally heavily reliant on the mare’s milk for nutrition, at this stage it is very rich but at about 3 months its nutritional value begins to decline and the foal becomes more independent often grazing or may be receiving supplementary feeding in preparation for weaning.
The nutrition of the foal can have a large impact on the occurrence of DOD particularly in the areas of carbohydrate intake and vitamin and mineral balances. So this is a crucial stage in the development of a sound skeletal structure that will stand the training the animal will have in the future.
Although of a primary concern nutrition is not the only factor that can influence the development of DOD’s. Some breeds are more susceptible to bone growth disorders than others, so genetics can play a role. As too can trauma or injury, excessive exercise and hormonal factors such as high levels of insulin that can be linked to feeding excessive amounts of carbohydrates; all have been shown to contribute to the development of DOD’s.
Rapid Growth Can Play a Role in DOD’s.
It is widely agreed that rapid growth is a major contributing factor to DOD in young horses. Excessive growth and growth spurts should be avoided, It is important that optimal growth rates are obtained, not maximum, when producing young horses.
Over feeding of carbohydrates to produce a big weanling or yearling for sale has been shown to cause skeletal abnormalities and upset the growth plate of the bone and cartilage. As, can suddenly feeding a smaller less grown horse in an attempt to produce it faster.
Growth should be steady and monitored. Although energy requirements should not be ignored it is protein that is needed for growth. Energy can be supplied in the form of fats and oils rather than sugars and starches to avoid disrupting insulin levels.
Feeding the correct amount of minerals and vitamins to help build good strong bone is also vital.
Calcium, phosphorous, magnesium, copper and zinc are all needed but in the correct balance. Over feeding of a mineral is just as bad as under feeding. Feeding too high a level of Phosphorous can disrupt the calcium/phosphorous ratio that is vital for the strength of the bone.
Over feeding Zinc can affect the uptake of Copper as both these minerals use the same transport mechanism in the gut.
Vitamins are important too, Calcium needs vitamin D to enable the body to utilize it; fortunately it is synthesized by the skin in the presence of sun light and can be stored in the body.
Research on the function of Vitamin K1 has found it has an integral role in the formation of strong, dense bone. A protein in the bone called oestocalcin needs Vitamin K1 to stick the major components of bone together, Collagen and Hydroxyapatite.
Supplementation of young growing horses with K1 has shown a reduction in the incidence of bone diseases. A balanced diet and even growth curve is vital in the young growing horse to avoid problems, unfortunately in some cases these conditions are present at birth.
Angular Limb Deformity.
In the case of Angular Limb Deformities (ALD) or crooked legs; Foals born with this disease have legs that deviate either out or in from normal line of the limb. The deviation is most common at the knee joint, but not limited to here and in some it can be in the fetlock or hock joint as well.
Cause and Treatment of ALD.
ALD is associated with unequal growth of long bones where one side of the bone grows faster than the other, this results in the angulations of the limb away from the normal straight position. This abnormal growth occurs in the growth plat near the joint.
Treatment depends on how bad the condition is and how quickly it is identified.
Restricted exercise and diet adjustment combined with corrective hoof trimming are recommended. Serious cases would lead to possible splints, casts or surgery.
Other causes of ALD that develop later include lameness (which causes abnormal weight-bearing), physitis (inflamed growth plates) excessive body growth in relation to bone/joint growth, and growth plate injury.
Flexural Limb Deformities (FLD) are more often known as “contracted tendons.
These can occur inter uterine or be acquired during growth. In contracted tendons as the name suggests the flexor tendons appear to contract, this causes the heel to be pulled up off the ground and the foal to stand more on the toe of the foot instead of a flat bearing surface of the hoof.
Cause of FLD.
FLD can occur because of bad positioning inside the uterus, not enough exercise, overfeeding, and dietary imbalances.
Physitis
Physitis is characterized by inflammation of the growth plate of a bone. It usually occurs in the long bones of the legs, so very common, but not limited to, the fetlock area.
Causes of Physitis
There are a few theories about why physitis occurs, poor nutrition, uneven hoof growth, and pressure on the growth plate can all be contributory. Compression of the growth plate seems to be the most likely cause, physitis is most commonly seen in foals that are big for their age, have grown quickly, and ended up top-heavy.
It is most often seen when the ground is hard during a summer dry spell. It is also linked to diets where the calcium/phosphorus ratio is unbalanced.
Treatment of Physitis
- Treatment includes reducing feed to reduce weight and/or slow growth rate.
- The diet also needs to be assessed to ensure that the calcium/phosphorus ratio is correct (for treatment purposes it should be around 1.6:1) and the amount of protein and carbohydrate is not excessive.
- Restricting exercise to a small paddock with a cushioned footing is also recommended.
- Regular attention to hoof balance will help also.
Osteochondrosis
Osteochondrosis (OC) occurs when cartilage fails to harden into bone. Osteochondrosis dissecans, (OCD) is similar but more serious.
In the case of OCD the cartilage fails to harden into bone, and a piece of cartilage remains on the articular surface of the cartilage in the joint or at the growth plate.
As different joints close at different ages each joint has a vulnerable growth stage, during which time the cartilage is forming into bone and can be affected.
If the condition is not addressed during this period then surgery at a later date is generally the only option. It is possible for many mild cases of OC to resolve themselves over time. However diet is of primary importance for this recovery to take place.
Subchondral bone cysts are small holes in the bone.
They are common in the stifle and are often not detected until the horse starts to work and is asked to use the stifle in a more athletic way.
They tend to form when the cartilage has excessive pressure or stress placed on it (such as happens during quick growth). Some cases are successfully treated with extended rest of 4-6 months and treatment with non steroidal anti inflammatory drugs.
Other options are surgery, since surgery has very good results, sometimes it is preferred over rest and allows the animal to return to normal routine much quicker.
**Subchondral bone cysts are small holes in the bone. They are common in the stifle and are often not detected until the horse starts to work and is asked to use the stifle in a more athletic way.
In Conclusion
The cause of a DOD’s is by no means straight forward; even an undetected stone bruise can cause a foal to change how it bears weight on a limb and put pressure on the growth plate causing an uneven growth.
Growth rates and diet are a huge factor. Except for those caused by trauma or injury, they all appear to be linked to (and possibly caused by) excess energy intake in growing horses.
It does not appear to make a large difference what the diet is composed of, if they are fed in excess of energy requirements. Unbalanced vitamins and minerals, even if the animals have normal weight gain can be contributory. Calcium, phosphorous and Copper are important elements as is vitamin K1.
Foals and weanlings appear to be at a larger risk for developmental orthopaedic disease followed by young horses that are fed to achieve quick and excessive growth.
Modern feeds are formulated to address the needs of the growing horse so chose an appropriate formulation from a reputable company and feed according to the individual requirements of your animal.
Pay attention to your grazing and address deficiencies in the soil as these will be mirrored in the grass. Monitor the growth of young stock against growth curve charts available from feed companies and adjust your feeding accordingly. Be observant of foals joints and movement. The quicker an issue is addressed the better so work with your vet, it can be too late to correct these things when the animal is older.
This Article was originally published in the November 2019 Issue of Irish Sport Horse Magazine.