Laminitis Fact Sheet
To understand laminitis it is necessary to understand how the inside of the foot is constructed and how horses bear weight. The force created by the horses weight is transmitted down through the bones of the leg to the coffin bone (third phalanx or P3), which sits within the hoof wall. The force is transferred from the bone to the wall, and then from the wall to the ground. The laminae (or laminar junction) connect the outside of the coffin bone to the inside of the hoof wall: it is their presence that prevents the bone from descending through the sole. In effect, the coffin bone (and therefore the horses weight) is suspended from the inside of the wall by these delicate structures. Each lamina runs from the top of the foot to the bottom, one set of laminae projecting out as thin sheets of tissue from the coffin bone (Fig. 1a), a second set projecting inwards from the hoof wall (Fig. 1b).
The main laminae are called primary laminae. Each primary lamina has lots of mini-laminae projecting from its surface, known as secondary laminae. The end result of all these primary and secondary laminae is a great increase in the surface area bonding the outside of the bone and the inside of the wall. This increase in surface area reduces the stress across the junction. The laminar junction contains specialised blood vessels known as arterio-venous anastomoses (AVAs) which allow blood to pass rapidly from arteries to veins without passing through the small blood vessels (capillaries) of the foot (Fig. 2). It is only when blood is in the capillaries that it can nourish the tissues. AVAs are probably present in the foot to act as pressure-relief valves so that the capillaries do not burst when the foot hits the ground.
What is laminitis?
Laminitis is a weakening or destruction of the laminae, thought to result from lack of blood supply to the affected region. The development of the disease is not completely understood, however, and other mechanisms may prove to be involved. The lack of blood supply may result from inappropriate prolonged opening of AVAs which shunt blood away from the capillaries. Mild cases of laminitis may go unnoticed, with no permanent damage to the foot. In more severe cases, the lack of blood supply causes the laminae on the outside of the bone to unzip from the laminae on the inside of the hoof wall. In the worst cases, the weakened laminar junction can no longer support the weight of the horse, and the coffin bone moves relative to the wall. Movement of the bone can further reduce blood supply to the laminae by squashing blood vessels, compounding the damage. The coffin bone can move in two main ways: if the laminae at the front of the foot are affected more severely than those at the heels (a common finding) the bone will rotate (Fig. 3a). If the entire laminar junction is affected the bone may move directly downwards (Fig. 3b). This is a sinker. The position of the bone can (and should) be determined by X-ray. Both rotation and sinking may cause the bone to penetrate the sole of the foot.
Causes of Laminitis
Occasionally a horse or pony will get laminitis for no obvious reason. These are the exceptions, however. The factors listed below all significantly increase the chances of an animal becoming affected.
Excessive feed intake: Overfeeding increases the risk of laminitis. Ponies allowed unrestricted access to pasture, particularly lush spring grass, are at extremely high risk. Grass intake should be strictly limited in these circumstances, the aim being to maintain ideal body weight. Such restrictions may seem cruel, but are infinitely preferable to letting the pony get laminitis. Extreme care should be taken if concentrates are fed to ponies, and thought should be given to eliminating them from the diet completely in high-risk animals. Seek veterinary advice. “Overfeeding” laminitis can also occur in horses, where it is most commonly due to excessive concentrate intake. The classic case is the horse which escapes and gains access to the feed bin. If this happens, call a veterinarian immediately: the horse is at very high risk.
Toxemia: Horses which have toxins in their bloodstream are at high risk of laminitis. Common causes of toxemia are severe diarrhea, pleuritis, some types of severe colic and retention of the placenta (afterbirth) after foaling. The placenta should always be inspected to ensure it is complete. If any pieces are missing, however small, contact your veterinarian.
Trauma: Laminitis occasionally occurs in horses or ponies which exercise excessively on very hard ground.
Excessive weight-bearing: Horses which are very lame in one leg (e.g. due to a fracture) may develop laminitis in the opposite leg due to excessive weight-bearing. If a horse with severe lameness suddenly appears to be more comfortable, this could be because the opposite leg is now painful. If this happens, call your veterinarian.
Steroids: Steroids (more precisely, corticosteroids) can cause laminitis. Although steroids are extremely useful and effective drugs, their use should be kept to a minimum in horses. Short-acting rather than long-acting preparations should be used whenever possible, and particular care should be taken in animals which are already at high risk of laminitis (e.g. fat ponies). Steroids should never be used to treat laminitis.
Cancer of the pituitary gland: One of the more common types of cancer in older horses is benign cancer of the pituitary gland. The most obvious signs are excessive drinking and development of an excessively long coat. These animals are also prone to laminitis. Hairy, thirsty horses should be seen by a veterinarian, who will be able to confirm the diagnosis and advise on management.
Signs of Laminitis
The signs of laminitis vary from mild lameness visible only at a trot, to an animal that stands rooted to the spot, refusing to move, to one that is down and blowing in pain. The main signs are listed below:
What to Do
If you think that your horse or pony has laminitis it is important to call a veterinarian immediately. Laminitis is not a “wait and see” condition.
Prevention of laminitis resolves around careful, sensible horse management. Some cases of laminitis are not preventable, but the majority of cases would never occur if the following guidelines were followed.
Equine Research Centre
Guelph, Ontario, Canada