The term laminitis refers to the swelling of the ‘laminae’ within the hoof capsule. The laminae bond the hoof wall to the pedal bone in the foot thus transmitting the entire weight of the horse from the hoof to the skeleton.

The weight of the horse tries to shear the hoof wall from the pedal bone which, in the normal horse, is resisted by the laminar attachments in between the two. In a healthy foot, the laminae are more than strong enough to cope with this.

When laminitis occurs the intertwining laminar attachments, become weakened, resulting in a partial or complete failure (depending on severity) to oppose this shearing force, damage is caused and this results in pain for the horse. In severe long standing (chronic cases) the laminae may not be able to support the pedal bone sufficiently and it can rotate away from the hoof wall and break through the sole of the foot.

Unfortunately, there are many factors which can cause laminitis. Much research into the exact mechanisms is ongoing but the most common identified causes of laminar swelling are stated below:

• Obesity – Increased weight is thought to cause mechanical damage to the laminae by an increased downward force on the pedal bone putting more strain on the laminae. Simply put, the horse is too heavy for its own laminae.
• Grass associated – Usually associated with turnout on to a fresh lush pasture or when a horse eats lots of first flush spring grass, the exact reason for this is yet to be determined but it has been linked to the high level of a specific sugar being ingested. One theory suggests that bacteria within the gut multiply rapidly in the abundance of sugar. These bacteria release waste products which are toxic to the laminae. Normally, low levels of toxin are kept within the gut and excreted in the faeces but when the numbers of bacteria increase rapidly these waste products leak from the gut into the circulation.
• Endotoxaemia (internal toxins) – Circulating endotoxins (internal blood poisons) result in changes in blood flow to the laminae causing swelling and damage to the laminar attachments. Endotoxaemia can result from grain overload, uterine infections and severe colic episodes.
• Steroids – High levels of substances in the horse called steroids are known to cause laminitis.

Types of Laminitis

• Acute onset – the horse with acute or rapid onset laminitis will be depressed and often will not eat. They are reluctant to move and stand awkwardly with their weight shifted to the hind legs. There can be an increase in heart and respiratory rates, with bounding pulses at the back of the pasterns (digital pulses). Usually both forelimbs are involved, though all limbs can be affected in cases of endotoxaemia. One limb may be affected in cases of mechanical laminitis where the opposite limb is injured or non-weight bearing.
• Sub acute – This tends to be a less dramatic onset with less severe signs; usually the first sign is shifting weight lameness or stiffness in gait.
• Chronic (long standing) -This usually occurs after an episode of one of the above, there is usually visible changes noted in the hooves, which can be mis-shapen, with a seedy toe (wide and weakened white line is visible) and a history of chronic recurring foot abscesses. The horse/pony often has a longstanding history of lameness.

How can I prevent laminitis?

Prevention of laminitis is complicated due to the varied inciting causes. However, by preventing horses susceptible to developing laminitis from over eating lush pasture and not overfeeding with concentrates, it can be controlled. Also, in such individuals, extra vigilance should be employed when feeding and with hoof care so any episode can be detected early and thus treated more effectively. Your veterinary surgeon will be able to offer further advice on all aspects of laminitis.

In some chronic cases of laminitis the laminae will have degenerated to the extent that the pedal bone has rotated away from the hoof wall. Taking radiographs is not a treatment for laminitis but it enables the veterinary surgeon to visualise any long-term changes that may affect his/her treatment decisions in the future. Unfortunately where significant rotation has occurred it may be unrealistic to expect the pony to fully recover its previous quality of life and in such circumstances euthanasia may be advised.