What causes Laminitis?

The laminae are tiny finger-like projections on the inside of the hoof and the outside of the pedal bone. The laminae on each side interlock with each other, suspending the pedal bone in the hoof. In this way, these tiny strands of tissue carry the weight of the entire horse, spread across its four feet. In order to maintain a tight bond, the laminae require a good blood supply, lots of energy, and to be free of inflammation and toxins.

Laminitis occurs when the laminae are compromised, become inflamed and ultimately fail – allowing the pedal bone to break away from the hoof wall, rotate and sink. This causes acute and often extreme pain, with potentially long term consequences. The laminae become damaged for a variety of reasons including:

  • Lameness in one limb putting extra stress on the laminae in the other limbs.
  • Repeated trauma on hard ground.
  • Certain medications.
  • Incorrect farriery.
  • Severe illness.

For this article we will concentrate on the nutritional and endocrine causes:

  • Excess intake of non-structural carbohydrates (sugar and starch).
  • Insulin resistance.
  • Obesity.
  • Metabolic syndrome.
  • Cushing’s disease.

The role of Insulin

Insulin, one of the body’s most important hormones, is released in response to sugar ingested in or released from the diet.

Insulin instructs cells in the body to take up the sugar from the blood to use as energy, or store in muscle and fat. Under certain conditions the insulin level in the blood can be abnormally high, which in horses can cause laminitis.

The exact mechanism for this is still being studied but it is thought that high levels of insulin affect the blood supply to the laminae cutting them off from vital nutrients and causing oxidative damage and inflammation.

High levels of insulin in the blood are seen broadly in two circumstances: in response to a high sugar meal; and due to insulin resistance.

Diet

It’s widely known that laminitis often occurs during periods of fertile grass growth, when the grass contains a lot of sugars and starch.

It can also be triggered by certain feeds: which are also naturally high in sugars and starch or are highly molassed. When large quantities of sugar are absorbed into the blood stream there is a spike of insulin release – intended to drive the sugar into cells for energy production or storage.

However, in extreme circumstances, especially when there is underlying insulin resistance, this can cause laminitis.

Insulin resistance

Prolonged elevation of the blood insulin level is seen when the body’s cells fail to respond to it. If the cells don’t respond to insulin, blood sugar levels stay high, and more insulin is produced to bring them down. This is called insulin resistance (IR) and is increasingly common in our horses, with some breeds being more prone than others. There are a number of conditions which increase the risk of IR:

  • Obesity and Equine Metabolic Syndrome (EMS)

Various hormones stop insulin from working and contribute to insulin resistance. Such hormones are released constantly by fat tissue, which is what links obesity and insulin resistance. A condition which is termed Equine Metabolic Syndrome. Affected horses are not always generally obese: fatty deposits in certain areas (such as the neck and rump) may be enough to cause EMS.

  • Cushing’s Disease

Cushing’s disease – or Pituitary Pars Intermedia Dysfunction (PPID) – is caused by an age related change in the brain, leading to the release of cortisol. Cortisol, which is also released during periods of stress, stops insulin from working, causing insulin resistance.

  • Inflammation

There is one final important mechanism triggering insulin resistance which again links diet with laminitis: the anti-insulin effect of inflammation. Inflammation and the release of inflammatory markers and hormones into the blood contribute to IR. A key example of this is seen during a hind gut dysbiosis.

  • Hind Gut Dysbiosis 

Horses have a limited capacity to digest starch and dietary levels can overload the stomach and small intestine (where they should be digested), causing “overspill” into the hind gut. Here they are broken down quickly by health-negative micro-organisms which thrive on this readily fermentable food. The bad-bacteria multiply, causing an imbalance in the hind gut (known as a dysbiosis). It is believed that these changes in the gut cause inflammation and the release of toxins in the bloodstream producing IR.

During periods of fertile growth grass contains high levels of fructans, a starch which cannot be broken down in the foregut and spills into the hindgut causing dysbiosis and disease. Stressed grass (caused by overgrazing or frost) also contains lots of fructans, so beware the laminitis paddock especially on a frosty morning.

We can see a picture developing with a number of risk factors for laminitis all feeding into a state of insulin resistance. In this state one inappropriate meal can trigger an episode.

The key to laminitis prevention is to reduce insulin resistance, and feed appropriately.  This can and should be achieved through manipulation of the diet and exercise regime.  

You can read about the best management practices to prevent laminitis here.

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