Causes and Triggers for Laminitis
Acute laminitis must be treated as a veterinary emergency - your prompt actions in the first few hours can make a significant difference to the prognosis and future life of your pony. Always call your vet and hoofcare professional as soon as you suspect laminitis.
Laminitis is caused by reduced bloodflow to the foot. Some of the factors that can trigger acute laminitis are outlined below.
For a horse that is chronic laminitic, there are a number of other factors that can trigger an episode and I'll touch on them at the end of this section.
Endotoxemia
Certain bacteria can cause more harm when they die than when they are alive. When these bacteria die, their cell walls break down, releasing molecules called endotoxins. If large amounts of endotoxin are absorbed into the bloodstream, it causes a dramatic drop in blood pressure and blood flow to the tissues.
The feet are particularly sensitive to the effects of endotoxin, so laminitis is a common consequence when endotoxin enters the bloodstream.
The conditions that can result in endotoxemia—excess of endotoxin in the bloodstream—and hence laminitis can include:
Colic that involves interruption of the blood supply to any part of the bowel, e.g. twisted bowel
Colitis, which is inflammation of the large intestine, resulting in profuse, watery diarrhea
Pleuropneumonia, which is bacterial infection of the chest cavity and lungs
Endometritis, which is infection of the uterus—mares that do not expel the placenta within a couple of hours of foaling are especially vulnerable
The endotoxin- causing bacteria are normally found in large numbers in the bowel. Usually, the bowel can limit endotoxin absorption into the bloodstream. However, when the bowel wall is damaged by severe inflamation or disruption in its blood supply, endotoxins can flood into the bloodstream.
Carbohydrate Overload - Raids on the feed store! Carbohydrate overload and excess oligofructose (fructans) , such as:
Al these can result in endotoxemia and laminitis. This is because Carbohydrate overload creates an acidic environment in the bowel. The trigger may not be a single loading of these carbohydrates or fructans - it can simply be sufficient to overwhelm the horse's capability to process them. If the horse has been operating near to its tollerance limits then a small amount of addditional carbohydrate or fructans may be all that is needed to push it over the edge.
The bowel bacteria are very sensitive to acidity, so are killed in large numbers. The normally adequate protective mechanisms in the bowel are overwhelmed. So even though the bowel wall is not damaged, endotoxins enter the bloodstream.
Severe Dehydration or Shock
Severe dehydration results in a reduction in blood volume, blood pressure, and hence blood flow to the tissues, particularly the extremities (including the feet).
Shock, which is circulatory collapse, also results in a dramatic drop in blood pressure and blood flow. As well as directly affecting blood flow in the feet, a severe reduction in blood pressure can also reduce blood flow to the bowel wall. This allows endotoxin to enter the bloodstream. Thus, any condition that causes the blood pressure to drop dramatically could cause laminitis by reducing blood flow both to the feet and the bowel.
Corticosteroids
Corticosteroids, or "cortisone," effectively block the production of anti inflamatory substances released by damaged cells - by blocking these substances corticosteroids can promote healing.
Corticosteriods can make the horse more prone to laminitis because these drugs make the blood vessels in the feet more sensitive to the effects of adrenaline.
Adrenaline is a natural substance that is produced by the adrenal glands, near the kidneys. It causes blood vessel constriction, which is essential for maintaining normal blood pressure and blood flow in the body. In high doses, corticosteroids can make adrenaline-induced blood vessel constriction either so profound or prolonged that laminitis develops.
Pituitary Gland Dysfunction
Old horses often develop a benign tumor (adenoma) of the pituitary gland, which is located at the base of the brain.
The pituitary gland normally produces hormones that direct the function of several other glands. Among other things, pituitary gland dysfunction causes the adrenal glands to produce too much cortisol, the body's natural "cortisone." Laminitis occurs in these horses for the same reason as it does with corticosteroid use.
Horses with pituitary adenoma typically have very long, shaggy or wavy haircoats that are not shed in the summer. These horses tend to drink and pee more than normal. Despite good apetite and the best of diets, they still look in poor body condition. They are also more prone to infections such as colds, skin conditions, foot abscesses, eye infections, tooth and gum infections etc.
Supporting Limb Lameness and Concussion
Horses are designed to distribute their body weight over each of the four feet. When one limb cannot bear weight due to severe injury, the opposite limb must take more weight. For example, if the horse is unable to bear weight on its left forelimb due to trauma or soft tissue damage, the right forelimb must take more of the load. If the horse will not bear weight on one limb, the load on the opposite foot cannot be relieved unless the horse lies down—which many are reluctant to do.
The blood supply to the front of the pedal bone and hoof wall comes from blood vessels that run underneath the pedal bone, through the tip, and up the front of the bone. Constant overloading of the foot can compress these vessels, restricting blood flow to the sensitive laminae of the hoof wall. Therefore, laminitis in the weight-bearing foot is a very common consequence of a severe, nonweightbearing lameness.
Chris Pollitt BVSc, PhD states that the case for ischaemia as the cause for supporting limb lameness is clear cut. When a limb is normally loaded, blood perfusion through the major blood vessels is normal, but when it is in the loaded position on a hard surface for too long then the blood perfusion in the foot below the level of the coronary band falls to zero. We'd call that a "foot gone dead"! Chris Pollitt states that a chronic lack of perfusion over several days can eventually trigger lamellar pathology that is indistinguishable from that initiated from other causes.
Pollitt's recommendations are that the horse should be provided with a deep bed of wood shavings or sand so that it can lie down comfortably and allow blood to circulate through its feet. Deep, compliant bedding also allows the horse to find a foot position that promotes the circulation.
"Road founder" or "concussion laminitis" is cause by repeated riding of a vulnerable horse with thin walls or soles on hard surfaces whcih can cause sufficient damage to the lamellar connections to induce laminitis. Concussion laminitis may also be due to compression of the blood vessels at the bottom of the pedal bone.
We have a laminitic pony that we use for driving and he trots without any problems for many miles on tarmac - but he is barefoot. I wonder whether concussion originated laminitis might be more common in shod horses asked to trot or canter for extended periods on hard tarmac?
Insulin Resistance and Obesity
Laminitis as a metabolic disease has many similarities to human Type II diabetes mellitus. This is the type of diabetes that does not necessarily require insulin injections, but rather comes later in life and is usually controlled by diet and glucose lowering medications.
Diabetes mellitus (DM) is a group of diseases in which blood glucose levels are elevated because of deficient insulin and/or abnormal insulin action. Laminitis, according to the insulin resistant (IR) viewpoint, also has elevated glucose in the bloodstream yet is considered to have elevated insulin as well.
Insulin resistance is defined as the failure of tissues to respond appropriately to insulin. There are numerous ways in which tissues become insulin resistant, including a reduction in the density of insulin receptors on the cell surface, malfunction of insulin receptors, defective internal signaling pathways, and interference with the translocation or function of glucose transporter 4 (GLUT4) proteins.
Endocrinopathic Laminitis, Obesity-Associated Laminitis, and Pasture-Associated Laminitis, Nicholas Frank, DVM, PhD, Diplomate ACVIM
Obese horses are more likely to suffer from insulin resistance, and insulin sensitivity decreases when obesity increases. Insulin resistance can be diagnosed by measuring blood insulin concentrations, which are usually elevated in affected horses. Insulin resistance may be the key factor in laminitis susceptability in horses, and may explain why only specific horses in the herd develop laminitis and other animals who may also be overweight do not.
Obesity-associated laminitis is a useful term because obesity is easily recognized, and owners can address this issue to reduce the risk of laminitis. However it is not clear whether obesity in itself raises the individual's risk of laminitis, or if the disease is caused by insulin resistance which is more common in obese horses.
Certainly evidence is there to suggest that obesity, inflamation and insulin resistance are associate in humans becuase a chemical tumor necrosis factor (TNF) is secreted from adipose (fat) tissue as body mass increases. This inflamation producing chemical inhibits insulin receptor signalling which in turn lowers insulin sensitivity (or increases insulin resistance). Researchers found blood TNF was higher in obese horses, and detected elevated TNF within adipose tissues collected from obese insulin-resistant horses. {Vick MM, Adams AA, Murphy BA, et al. Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse.}
Insulin resistance may be the link between obesity and laminitis in horses
Equine Metabolic Syndrome (EMS) is a term that has been adopted to describe a clinical syndrome of obesity and/or regional adiposity (cresty neck, fat pads on shoulders and rump, fatty deposits above eyes) ,IR, and laminitis. This term is useful because it ties laminitis to IR.
Horses of certain breeds and bloodlines are genetically predisposed to EMS e.g. Morgans, Arabians, Warmbloods, pony breeds. A more efficient energy metabolism seems to predispose animals to obesity and consequently IR. Young to middle-aged horses are affected, and there is an important correlation with management practices. Horses become obese and develop IR when they are fed too many calories in the form of grains or rich grass.
It is as yet unclear and more research is needed to determine whether horses with PPID (pituitary gland dysfunction) are predisposed to laminitis because corticosteroid excess weakens hoof tissues over time, or whether insulin resistance is also key risk factor in developing laminitis in these patients
Lush Pasture
Grass-associated laminitis typically develops after the pasture grass grows rapidly in the spring or after a heavy rain. Pasture grazing contributes to obesity in metabolically efficient horses because large amounts of energy are consumed when grass is abundant.
Sugars (fructans) from pasture grass also exacerbate insulin resistance. It should also be recognized that lush pasture challenges the gastrointestinal tract because grass is consumed in large quantities over a relatively short period of time. This increases the mass of feed passing through the intestinal tract and therefore the total amount of carbohydrate entering the large intestine.
The carbohydrate content and composition of the grass also varies markedly over time. Grasses store carbohydrates when nutrients and sunlight are plentiful or when they are preparing for drought or winter conditions. Pasture grazing contributes to the development of obesity and IR, and represents a dynamic factor that may trigger laminitis.
Laminitis has been experimentally induced by creating a situation of carbohydrate overload within the large intestine. This has the same effect as the lush grass and raid on the feed bins. It alters the microbial flora, enhancing lactic acid production, lowering the gut pH, and increasing intestinal permeability. Exotoxins, endotoxins, and bacterial byproducts subsequently move into the blood and initiate a systemic inflammatory response that triggers laminitis.
Chronic Laminitis
Chronic laminitis is a complex of foot problems that are the direct or indirect result of an episode of acute laminitis. Once a horse or pony has had an acute laminitis event and has recovered, the chronic stage begins.
The consistent or recurrent problems can occur through:
pressure on the sole from a rotated or sunken pedal bone
breakdown of the hoof wall to pedal bone bonds
permanent changes to the blood supply to the hoof
All of these changes are down to pedal bone rotation - it tears the blood vessels that supply the hoof wall to pedal bone bond and when the top of the pedal bone tips forward it compresses the coronary corium between the inside of the hoof wall and the top edge of pedal bone. As a result of these anomalies the horse is more prone to:
Solar bruising
Subsolar abscesses
Flaring and separation of the wall at the toe
Infection beneath the separated walls
Hoof wall cracks
Lameness due to overloading of the heels in order to protect the toe
Slower hoof wall growth at the front of the foot - the hoodf wall grows more rapidly at the heels - hence "slipper foot"
A chronic laminitic can experience laminitic events caused by metabolic stress - for example:
Dehydration
Getting too hot
Being too cold - I've seen an episode of footiness after one of my ponies got cold and wet and was shivering very badly.
Having access to too much grass
Having access to ANY grass in the case of a very sensitive horse
Having access to too many sweet succulents or treats
A good stampede around the padock can trigger footiness
Lack of exercise allowing the horse to become overweight or obese
A real risk that I have to be aware of is that since my ponies are doing pretty well is that I slip into complacency around exercise or relax the regime of dietary control and they react to that change in regime!
Both of my chronic laminitics are leading full, active lives and taking part in strenuous exercise and apart from the odd footy day with one particularly sensitive pony, are having a great life! There are many more success stories in the UKNHCP member's sites of ponies and horses who have sucessfully recovered from laminitis and gone on to a very useful athletic life.