Laminitis: In Depth

Kirsten Voigt, DVM

Why does my horse have laminitis?

Laminitis simply means inflammation of the lamina, which is the tissue that attaches the hoof to the underlying bone.  There are several causes of laminitis and on-going research reveals more of the risk factors all the time.  Think of a horses' susceptibility to laminitis as a boat being filled with boulders.  The ability of the boat to stay afloat depends on the number and size of the boulders. Some horses start with an empty boat in which case it would take one very big boulder to sink the boat.  Others carry boulders at all times which means it could take surprisingly little to put them over the edge.  These "boulders" will be discussed but first we need to learn a little about the laminar tissue and how it works. 

The laminae are a little bit like Velcro with one side being the outside of the coffin bone (lowest bone in the foot) and the other side being the inside of the hoof capsule.  They are made of thin, interlocking tissues leaflets with a good blood supply.  Their job is to suspend the bone inside the hoof and provide shock absorption.  Laminar tissue is highly dependant on blood and nutrient supply because it is constantly growing and repairing.  Alterations in this supply can quickly lead to an inflammatory episode.  The laminae are also unusually sensitive to substances carried in the blood because of the network of very small blood vessels that supply the tissue.  Once the laminae are inflamed it is hard to get them back to normal because even the weight of the horse standing can be enough to continue the damage and inflammation cycle.  Now let's take a more detailed look at the causes of laminitis. 

Trauma

Trauma refers to the amount of stress put on the lamina from bearing weight, shock absorption (repeated concussion), and prying forces placed on the tissue as the hoof wall pushes off the ground.  When the laminar tissues are taxed there is some level of damage done.  If there is enough damage the result is inflammation.  Obesity, over grown hooves and hard or rocky ground all increase the amount of trauma on the feet. 

Toxins

Horses can have toxins in the blood during a time of illness.  This happens often with serious colics involving injury to the gut, retained placentas, severe respiratory infections or grain overloads.  Inflammation is the way the body responds to toxins.  The multiple small vessels in the lamina act as a strainer causing a lot of the inflammatory products get lodged there and start acting on the laminar tissue.  The key is to treat for the initial cause to get rid of the toxins, prevent absorption and to prevent the body from developing the inflammatory response.  The earlier the treatment, the more effective. 

Vascular Disturbances

Immune mediated diseases, vaccination reactions and being anesthetized for surgery can cause vascular disturbances in the lamina.  These disturbances create an imbalance in the supply and demand of the lamina which is a cause for inflammation.   

Insulin Resistance

Insulin resistance (a.k.a. metabolic syndrome) is a newer concept in relation to laminitis.  This is a condition where tissues no longer respond well to insulin.  Insulin is a hormone that is required to allow blood sugars to enter the tissues so that it can be used for energy.  The laminae are very dependent on blood sugar for energy production.  Insulin resistance causes the tissue to be deprived of the energy it needs which leads to inflammation.  Persistently high blood sugar stimulating increased insulin production is what causes insulin resistance.  When a high level of insulin is in the blood too often, the tissues eventually tune it out, similar to the mother who no longer hears her kids calling her name after the hundredth time.  Obesity and Cushing's disease are both causes of persistently high blood sugar and the overproduction of insulin.

Horses with insulin resistance have excessive fat deposited the crest of the neck and over the head of the tail.  They are typically the ones who develop laminitis readily from sugars and starches in the diet such as sweet feed type grains or grasses with a high sugar level. It can be tested for by blood tests that look at insulin and/or glucose levels.  Insulin resistance can be reversed with weight loss and regular exercise. 

Equine Cushing's Disease

Cushing's is a disease of certain neurons in the brain that results in the over production of several hormones one of which causes high blood sugar and one that causes increased insulin production.  This leads to susceptibility to laminitis as discussed. 

The easiest way to tell if a horse has Cushing's is to look at its hair coat.  Eighty percent of horses with the disease have a long shaggy hair coat that fails to shed all year round.  For horses without the long hair coat but for whom Cushing's is still suspected, a blood test called a dexamethasone supression test can be performed to confirm the presence or absence of the disease.  A medication called pergolide is available which greatly helps alleviate the symptoms including the laminitis susceptibility.  

Why is it so hard to get an obese horse to lose weight?

Fat tissue itself is not just a storage tank.  It is responsible for the production of many chemical signals in the body relating to appetite and the use of fat for energy.  If an excessive amount of fat is stored in the tissue, it interferes with the chemical signaling.  The fatty tissue actually tells the brain to be hungry all the time, to store more fat and to slow down the body's metabolism.  This is a vicious cycle to break since the obesity causes obesity.

Exercise and thyroid supplements help speed up the metabolism and burn fat and a reduced feed intake will encourage the body to use its fat stores. 

Why do we test for thyroid function?

T4 is a product of the thyroid gland which is found in the blood.  It is a good indicator of how fast a horses' metabolism running.  A slow metabolism means the horse is an easy keeper who is prone to obesity.  As mentioned earlier the chemical signaling for fat use or storage is greatly altered in a state of obesity.  The thyroid at times is caught in the crossfire and produces less T4 causing the metabolism to slow down.  If the T4 is low we can give a thyroid supplement to help speed up the metabolism in order to achieve a desired weight.  Once an ideal weight is reached the horse can be weaned off of the medication.  

The more you know about laminitis the easier it is to prevent and treat.  Recovery times are extremely variable and depend on the initial cause.  The earlier and more aggressive the treatment is, the better the prognosis will be.