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A unification theory
Researchers speculate horses may be more inclined to develop resistance to insulin that can lead to adult-onset diabetes


Increased cortisol levels can also be found in severely stressed or systemically ill horses. Many veterinarians have dealt with "stress founders" and with founder following uterine or other infections. Pollitt suggests that all these cases of laminitis share a cause relating to insulin resistance rather than to strict endotoxemia. Other researchers are also postulating that in cases of grain overload founder, elevated glucose levels cause a "toxicity" that causes temporary insulin resistance (so much glucose floods the system in such a short time that the body cannot produce enough insulin to compensate). This insulin resistance is intensified by elevated cortisol levels induced by endotoxemia seen in these cases. If this part of the theory is proven correct then the best method of treating or preventing laminitis is by treating or preventing conditions that lead to insulin resistance. There are many current projects looking at methods of doing that.

Insulin inhibits the breakdown of triglycerides (fat) into free fatty acids (FFA) and glycerol. In the absence of insulin this process of lipolysis can occur and FFA are released.

FFA are transported to the liver, packaged as very low density lipoproteins (VLDL) and stored in adipose tissue for potential later use. The cresty necked horses or overly fat ponies seen in veterinary practices may well be insulin-resistant animals and their body condition may be the first clue to their underlying condition. The unification theory would suggest that it is not difficult to now explain why these animals are at risk for laminitis in conditions that other horses seem to tolerate well.

Just as in humans, there appears to be a genetic component to type 2 diabetes and ponies, Morgans, other gaited breeds and some Arabian bloodlines may be affected; though this research is just beginning.

Promising The practical advice available to veterinarians and their clients from these new theories is the most promising aspect of this research.

First an accurate diagnosis must be made. Appropriate testing for Cushing's disease and hypothyroidism should be done as these conditions may give a similar clinical appearance. Excellent treatment is available in the form of Pergolide or thyroid replacement hormone for these horses.

Blood glucose, insulin levels and a glucose tolerance test may be needed to accurately diagnose a horse as being insulin intolerant or diabetic. As in humans with type 2 diabetes, the first step in treatment is management of the diet and an increase in exercise. Many diabetics can be controlled for long periods of time with these simple steps (see box). Additional drugs to control glucose levels are being investigated and new treatment protocols will soon be suggested.

The most important point is to realize that these horses are different and that their failure to respond to normal treatments indicates the presence of a more important underlying condition-insulin resistance. Success in treating any of the clinical signs-laminitis, obesity, lethargy-will only come from addressing this problem as well.

Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.


Source: DVM360 MAGAZINE,
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