The case of the foundered pony

by George Looby, DVM  
Horse owners are bedeviled by many ills, such as founder or laminitis. This condition has been recognized by owners and veterinarians for many years and attempts to successfully treat it have too often been met with disappointing results. The lack of successful treatment may be attributed in large measure to a lack of understanding of what was really going on in the affected animals’ body.
Some of the possible causes cited in some of the older textbooks include concussion, exhaustion, overexertion and certain types of feed. It was well recognized that chunky ponies and spring pasture oftentimes made for a poor combination but the underlying cause(s) remained something of a mystery. Treatment was largely symptomatic, standing them in a brook seemed to provide some relief as did a variety of anti-inflammatories.
The laminae are delicate structures within the hoof wall that provides stability and nutrition to the inner structures of the hoof itself. One of the functions is to provide stability to the coffin bone, the last bone in the leg. When the laminae are disturbed in any way their ability to maintain the coffin bone in its proper position is compromised, allowing the end of the bone to tip forward — or downward — putting pressure on the inner surface of the sole of the hoof. Once it happens it is essentially irreversible.
It seems that there are certain members of the horse family that are predisposed to the development of this condition, the obese pony being the classic one. Some breeds of horses are more likely to be affected than others. These include the Morgan, Arabians, Peruvian Pasos and the Paso Finos, among others. This group may all be predisposed to a rather recently discovered cause of laminitis which has been given the name insulin resistance. Horses who are affected often have a body score that might be considered to be at the high end of the scale with fatty deposits along the crest, the rump and above the eyes. These animals show a tendency to drink and urinate excessively and have voracious appetites.
Almost any alteration in the normal gait or signs of discomfort should be a cause for close follow-up to ensure that it does not persist. Signs of soreness over the back and shoulders without an apparent cause should be watched carefully. The white line of the hoof should be watched for signs of widening or bruising. Know what is normal in order to recognize the abnormal. Once more obvious signs of laminitis develop, reversing the damage already done may prove to be very difficult. These signs may include, but not be limited to, divergent hoof wall growth rings, founder rings as well as the digital artery having a characteristic pounding sensation which can be felt above the lateral cartilage. Affected horses tend to rock back on their rear hooves to take the pressure from the more commonly affected front hooves and shift weight from one front hoof to the other.
Insulin resistance probably has been around for a long time but it took a long time for researchers to identify it. In all species insulin is produced in the pancreas by structures within that gland called the Islets of Langerhans. Insulin is required for the transport of glucose across cell membranes and is responsible for the formation of either glycogen or fat. What happens in the case of insulin resistance is that the cells of the body become resistant to the glucose uptake action of insulin. When this happens, more insulin is needed to keep the blood glucose levels within normal limits after a starchy or high sugar meal. If this condition becomes severe enough blood glucose levels may become abnormally high. This means that body cells are not getting enough energy and the high levels of insulin, if present for too long a time, can have harmful effects on various body systems. In the case of the horse this includes the hooves.
At this point in time the exact cause is simply not known. Even though the exact cause is still unknown there are some factors that probably do contribute to its development. Diet, obesity, breed and history of laminitis have already been mentioned. In addition it has been observed that horses over 20 years of age seem to be more prone to developing insulin resistance. The onset seems to be secondary to Cushings Disease which is quite common in older horses, especially mares.
As with so many conditions, prevention is preferred to treatment and many of them are common sense measures that can be instituted once the underlying contributing causes are understood. If a horse is chunky, restrict time on lush spring pasture which tends to be high in sugar. Feed mostly grass hay or legume-grass mix hay with little or no grain, especially if the horse tends to be somewhat on the chunky side. Test forages, both hay and pasture, for the amount of sugar contained. If high in sugar, soak the hay in hot water for 30 minutes or cold water for an hour before feeding. Maintain the animal in good physical condition through a good exercise program.
If an owner is unfortunate enough to have a horse founder there are some measures that can be taken to help in correcting the condition with the understanding that some damage has already been done. They are in fact very similar of those used in a preventative program, just done after the fact. Get the animals weight down to an acceptable level through a program of correct diet and exercise. Limit carbohydrate intake by eliminating grain and high sugar feed. Restrict pasture if the growth is lush.
Good basic dietary management can do much to help control the onset of this most insidious condition. Know your horse and understand if it is one of those predisposed to laminitis. If it is, discuss it with your veterinarian. They are there to help and provide assistance in avoiding a problem that at best is never easy to manage.

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