Endocrinological Disorders

Endocrinological disorders in the horse
The endocrine system is a complex system of glands and hormones that maintain homeostasis within an animal. An imbalance or change in the levels of these hormones can cause disruption to metabolic processes that occur in the body, and can also lead to diseases that can be classed as endocrine disorders. Examples that are prevalent in the horse are Diabetes, Hyperinsulinaemia (too much insulin in the blood), and Equine Metabolic Syndrome (EMS). These can lead to further complications such as laminitis that need to be managed with appropriate diet, exercise and veterinary care. 
Diabetes Mellitus
Diabetes Mellitus is a relatively rare disorder in the horse, and originally was associated with cases of Hyperinsulinaemia and Hyperglycaemia (too much glucose in the blood); however it has now been shown that it is a very complex disorder, with various strains of the disease. The two main strains of diabetes are insulin dependent and non-insulin dependent diabetes.
Insulin dependent diabetes is thought to be caused by a failure to produce or to release insulin. This form of the disease is very rare in horses, and is often thought to be secondary to destruction of the pancreas, often due to a parasitic infection. This leads to the loss of function of the Islets of Langerhans (which produce insulin) in the pancreas, and therefore the disruption in production of insulin. Clinical signs of the disease are weight loss of the horse, depression, polyuria (producing a lot of urine) and polydipsia (excessive drinking). It is often diagnosed using an Intravenous (IV) Glucose Tolerance Test.
Non-Insulin dependent diabetes is usually seen in obese ponies, these ponies tend to have large fatty deposits on their crest. Often they show no other abnormal physiological or clinical signs, other than their increased weight. There is a progressive decline in their insulin sensitivity. 
Cases of diabetes are diagnosed using glucose tolerance tests, either orally or via IV. Bloods are taken at intervals during the test and glucose and insulin concentrations are tested. If a diagnosis is confirmed often the only treatment is administration of parenteral insulin, where the horse or ponies own insulin levels are supplemented. 
Hyperinsulinaemia occurs in both horses and ponies, and has been linked in recent scientific research to Laminitis. 
Hyperinsulinaemia occurs when there are abnormally high levels of insulin in the circulating blood, it is also known as Insulin Resistance, where there is a reduction in sensitivity of tissues to the metabolic effects of insulin. The disease is often secondary to hyperadrenocorticalism (excess adrenal steroids), when an increased production of adrenocorticotrophic hormone inhibits the function of insulin, leading to it being in excess in the blood. The mechanism for Hyperinsulinaemia has not yet been identified however it may be due to stimulation of pancreatic β cells, or problems involving insulin target receptors.
Horse owners and vets should attempt to increase insulin sensitivity in at-risk ponies. In horses and ponies that are obese weight loss and increasing exercise may promote insulin sensitivity. Also increasingly more common is the use of drugs manufactured for humans; however there is very little research into the effectiveness and use of these on horses. 
Link to the Development of Laminitis
Laminitis can be loosely defined as acute degeneration of the laminae that connect the distal phalanx and the hoof wall. The condition endocrinopathic laminitis has been linked to diseases such as Diabetes and Hyperinsulinaemia, as it is caused by hormonal dysfunction as opposed to inflammatory and intestinal conditions.
Picture comparing a healthy hoof to a laminitic hoof https://www.equi-ads.com/learning-about-laminitis/
In a diabetic horse there is an increased level of glucose in the blood stream, as insulin is not able to monitor and control the levels in the blood. This is similar to pasture induced laminitis which is caused by obesity from eating rich grasses and feeds. There is an increased ingestion of grass that is rich in starch, fructan and sugar as this often leads to obesity in horses and ponies.
A link between insulin resistance, or Hyperinsulinaemia, and laminitis was first documented in the 1980s, and this link has been further established since. Hyperinsulinaemia has been shown to increase laminae dysfunction. The increased insulin in the system of these horses is thought to be the cause of the laminitis, and the destruction of the laminae in the foot.
It has been shown in a study by Asplin et al (2007) that giving insulin to healthy ponies with no prior history of laminitis has led to them contracting laminitis. 5 ponies given insulin all contracted the disease within 72 hours, whereas the 4 ponies given the control solution (no insulin) did not. This could potentially show that it is too much insulin, as opposed to insulin resistance, that is a cause of laminitis. 
Pain and Inflammatory Pathways Associated with Laminitis
It is evident that ponies and horses suffering from laminitis are in a great deal of pain, we can see this as horses will show persistent nociception, evident by them limiting weight  bearing on a sore foot, and increased stress, shown by clinical signs such as heart rate variability and hypertension.
The hoof has a sensory innervation system consisting of myelinated A-nerve fibres, which carries the sensory information from the area of the solar dermis of the heel. This area and the dermis of the dorsal hoof wall receive information from unmyelinated afferent neurons. These are normally high threshold fibres, and transmit information regarding pressure on the foot and potential injury to the foot. 
It is thought that there are two processes that cause the pain that arises in laminitis cases, events arising from inflammation and nerve injury. The Laminitic hoof contains components of the blood that suggest that there is a level of inflammation in the hoof, such as platelets, lymphocytes, neutrophils and macrophages. Pain from the inflammation in the foot arises as C fibres that are present are responsive to all of these blood components, and it sends a message to the brain that pressure should be relieved from the foot, so as to minimise the pain from the inflammation.
It has been shown in recent research that lateral digital nerves associated with laminitis decrease in number of both myelinated and unmyelinated axons, and the nerves that are present have increasing sensitivity to excitatory products that would activate the nerve, such as those released by local inflammatory cells. 
Treatments for Laminitis Pain
The combination of inflammatory and nervous pain pathways suggests that Laminitic pain needs a combination of pain killers for inflammatory conditions and neuropathic to manage the pain.
Managing the pain in Laminitic horses is an important ethical issue. If any horse or pony is thought to be in pain, it is essential that a vet is called, and if the pain is unmanageable and great discomfort is being caused then the possibility of euthanasia needs to be discussed to ensure minimal suffering for the animal.
Suggested reading:
Asplin K.E. Sillence, M.N., Pollitt, C.C., and McGowan, C.M. (2007) Induction of laminitis by prolonged Hyperinsulinaemia in clinically normal ponies The Veterinary Journal 174 (3) pp. 530 – 535
Higgins and Snyder (2006) The Equine Manual 2nd ed. London: Elsevier 
Current therapy in equine medicine Robinson and Sprayberry (2009)