Page 254 - The Veterinary Care of the Horse

digested in the small intestine spills over into the large intestine. Here it is rapidly

fermented by Lactobacilli spp and Streptococcus bovis producing lactic acid. The
VetBooks.ir increased acidity upsets the normal balance of microorganisms in the hind gut and some
of these die and release toxins. It also increases the permeability (leakiness) of the gut

wall so toxins enter the circulation and reach the laminae, causing inflammation.
• Access to lush grass. In the process of photosynthesis, plants use energy from the

sunlight to manufacture sugars including glucose, sucrose and fructose from carbon
dioxide and water. Any excess sugar is converted to a carbohydrate called fructan and

stored in the plant stem. The plants use fructans as an energy source for growing and
metabolism. In the spring and autumn, when conditions are sunny but the temperature is

cool, growth and metabolism slow down and fructan levels can build up. Until recently it
was thought that high levels of fructan pass undigested through the small intestine and

into the hind gut of the horse where they are fermented by bacteria causing increased
acidity, death of normal gut bacteria, release of toxins and laminitis. However, this is now
disputed. The current thinking is that grass-induced laminitis in over 90% of animals is

linked to underlying hormone disease such as EMS or PPID. When these horses and
ponies eat grass containing sugars, starch and fructans, they produce abnormally high

insulin levels, which trigger the pasture-associated laminitis.
• Stress, e.g. frequent travelling of overweight show animals, being sold or losing a

companion, can precipitate laminitis in vulnerable animals. Chronic stress increases
levels of circulating cortisol.

Horses and ponies that have suffered previous bouts of laminitis are at increased risk.

Clinical signs

Any number from one to all four feet may be involved. However, the two front feet are most

commonly affected.
• Stance Many laminitic animals develop a characteristic stance. The forelimbs are

extended forwards and most of the weight is taken on the heels to relieve the pressure at
the toe. The hind limbs may be positioned forwards under the body so they can take more

of the horse’s weight (Figures 6.41a and b). When all four feet are involved, the horse
may spend long periods of time lying down.