Laminitis Explained

By Richard Vialls DEP, MEPA(UK)

May 2007

Laminitis is a very well known condition that has been documented for thousands of years. However, despite this, we still know very little about what causes it and how the disease progresses. The strict definition of laminitis is inflammation of the laminar corium, but we tend to use the term to refer to the whole chain of events that follows on from that inflammation.

There are known to be many possible causes/triggers of laminitis but for the purposes of this article I'll stick to the horse that gets laminitis while eating what we would normally expect to be a suitable diet (e.g. grass plus a small hard feed per day). This kind of laminitis is typically known as pasture laminitis.

The current mainstream view of laminitis describes various stages in the progression of a typical pasture laminitis case:

Pre-Laminitic Metabolic Syndrome (PLMS)

Recent research has indentified a condition that is being called Pre-Laminitic Metabolic Syndrome (PLMS). PLMS seems to pre-dispose a horse to getting laminitis. In this condition the horse's metabolism seems to be significantly different from healthy horses. A key part of PLMS is that the ability of the horse to deal with a rise in blood glucose level (e.g. from eating a feed) seems to be impaired. Normally the horse would produce insulin which in turn controls the mechanisms that store away excess glucose bringing the blood glucose levels down to normal again. The problem in a PLMS case is that the cells that are supposed to respond to the insulin become less sensitive and so the body needs to produce more insulin to get the same result. This problem is called Insulin Resistance (IR) and has been associated with pasture laminitis for a few years now.

There is no guarantee that a horse with PLMS will go on to develop laminitis, but the risks are significantly higher. It is also possible for a horse without PLMS to develop laminitis, but it looks likely that this only happens when something catastrophic happens to the horse (e.g. breaking into the feed store and eating a whole bag of oats, being given an incorrect drug dose by a vet, etc.).

While there are tests that can be done in research labs to identify horses with PLMS, there is still work to be done to produce a simple blood test that can be done in the field to indentify it. In the meantime, many cases show obvious signs of Insulin Resistance which owners can use as a warning that laminitis is a high risk.

The Developmental Phase

When the worst happens and a horse gets pasture laminitis, there is an initial developmental phase. The mainstream view is that this phase typically lasts 30-40 hours. During this period, some process that we have yet to fully understand (there are a number of theories but I won't go into them here) causes the laminar corium to become seriously inflammed. The laminar corium is similar to the nail bed of a human finger nail and is responsible for attaching the hoof wall to the pedal bone. It has to be able to carry a significant amount of weight - especially in shod horses where the shoe transfers all the horse's weight onto the bottom of the hoof wall. Once the inflammation in the laminar corium reaches a certain level, the laminar attachment between the hoof wall and the pedal bone starts to fail.

The Acute Phase

During the acute phase, the hoof capsule and the pedal bone start to come apart causing massive amounts of damage both to the hoof capsule itself and to the laminar corium that attaches it to the pedal bone. By this stage, the horse will be in significant pain. The acute phase typically lasts a few days.

The Chronic Phase

The chronic phase starts when the damage inside the foot starts to be visible from outside the foot in terms of distortions in the hoof capsule. With the laminar attachment of hoof wall to pedal bone failing, the two get ripped apart. The laminar corium will desperately try to form a reattachment, but in vain - resulting in a wedge of damaged horn (often with voids and some degree of blood incorporated into it) between the pedal bone and the hoof wall. Most commonly, the pedal bone rotates away from the hoof wall (although it is perhaps more helpful to view this as the hoof wall rotating away from the pedal bone). The result is that the front wall of the hoof is forward of where it should be. Commonly the sole then starts to become damaged as well due to the excess weight being place on it and this further adds to the pain for the horse.

The standard terminology doesn't distinguish well between a case where ongoing damage is happening because the underlying inflammation is still present and one where the inflammation has gone but the hoof capsule damage has yet to grow out - both are typically described as having chronic laminitis. However, when treating a case, this is a very important distinction.

This foot is typical of a laminitis case in the chronic phase. The major damage to the hoof capsule is clearly visible. In this case the damage was healing out well (the horse hadn't had any active laminitis for a few weeks) so the very top part of the hoof looks more normal.

To be continued...