Dr. Judith L. Mulholland

In my experience, certain breeds, families and body types (phenotype) are much more predisposed than others to developing laminitis on certain farms at certain times. I can almost predict the ponies or horses that will have a problem.

A couple of little studies of my own revealed a high risk to females in comparison to males, and that some ponies are innately resistant to the disease. Then I read an article in Equus magazine by Dr. David Kronfeld of the Middleburg Agricultural Research and Extension Center in the USA. A study of 160 related ponies into risk factors for laminitis indicates that metabolic changes in the body associated with obesity and pregnancy, combined with genetics and pasture changes resulting in insulin resistance, contribute to laminitis. Blood tests taken when all the ponies were healthy revealed that those who had previously developed laminitis showed elevated levels of  triglycerides (fats) and increased insulin and insulin compensation. Of this group 11/13 later developed laminitis. Genetically a link was also present with a dominant mode of inheritance that was partially suppressed in males. This genetic predisposition may have given a survival advantage in very harsh conditions in which ponies evolved, but creates disaster when overloaded with rich feed. During the time of the study (spring), pasture testing revealed a two fold increase in starches and sugars known to increase the risk of laminitis. Plasma cortisol was also studied, and this stress hormone has been previously linked to hyperlipemic laminitic syndrome.

Pre Laminitic Metabolic Syndrome (PLMS) is a set of risk factors that can be identified in healthy ponies. Fat tissue plays a role by producing hormones that hamper the action of insulin which is trying to get glucose from the blood into cells for energy production. In other cases, insulin resistance causes the animal to get fat. This process is yet to be fully understood. The body fat of insulin resistant horses forms in patterns e.g., hard, thick, cresty neck, spongy fat on the shoulders, tail base, over the eyes, and in the sheath of males. In my experience, these animals are at risk of developing laminitis with any slight increase in energy in the ration. I can now see the pattern of elevated insulin, cortisol and triglycerides in hypercortisolaemia (Cushing's like syndrome), hyperlipemia and laminitis. These three diseases are all fatal in severe form.

Relevant conclusions from Dr. Kronfeld's study were:

• A genetic connection and dominant mode of inheritance for pasture laminitis with partial suppression in males.
• A phenotype e.g., refined head, cresty neck, rounded body (the show pony type) with a distinctive metabolic profile.
• Characterisation of a pre-laminitic metabolic syndrome in apparently healthy ponies, and exaggerated compensated then decompensated insulin resistance during laminitis.
• Evidence of association between the onset of laminitis and increased pasture starch content due to rapid growth of clover (the trigger factor for at risk ponies or horses).

The researchers offered the following advice:

• Blood test ponies and horses at risk of  Pre-Laminitic Metabolic Syndrome.
• Test for elevations of triglycerides and insulin. This is most relevant to overweight animals related to those with laminitis.
• I suggest that cortisol or ACTH should also be included if you are blood testing.
• If your animal is healthy but shows signs of PLMS, it is time to act to change the metabolic profile and improve insulin sensitivity.
• Begin daily exercise, a controlled weight reduction program, eliminate large grain or molasses based feeds, and begin administering antioxidants and omega 3 and 6 fatty acids.
• Sample your pasture grasses and test your ponies when nutrient levels are high. Feed seedless grass hay and consult your veterinarian for further advice.

Suggested reading:
Kronfeld. D., PASTURE LAMINITIS BREAKTHROUGH, EQUUS 342, April 2006, pp. 47 - 63.
THE ROLE OF GLUCOSE IN LAMINITIS, Medical Front, EQUUS 341. Equus Magazine

Normal reference ranges for insulin and glucose:
(variation between laboratories):
normal reference range for equine insulin 4.0 - 7.1 mu/L  or
normal reference range for equine insulin 10 - 30 uU/ml
normal reference range for equine glucose 3.4 - 7.7 mmol/L
normal reference range for equine cortisol 83 - 360 nmol/L
normal reference range for equine triglycerides 0.1 - 0.9 mmol/L