Disorders of the endocrine system *H&H VIP*

In the past few years, there has been a huge growth in research into equine hormonal disease, something we now commonly recognise in both horses and ponies in the UK.

Researchers from around the world gathered earlier this year at the Havemeyer Foundation International Summit in Florida, USA, to discuss their progress, shedding valuable light on the cause and treatment of these complex conditions.

There are two common hormonal (or endocrine) conditions that affect horses: PPID (pituitary pars intermedia dysfunction, also known as Cushing’s disease) and EMS (equine metabolic syndrome). The conditions share some similarities and both have links with laminitis, but there are also important differences between them.

PPID usually occurs in ageing horses, especially from the mid-teens onwards, although younger cases are recognised. The pituitary gland, found dangling just below the brain, produces a range of different hormones that work as messengers in the body, controlling different processes such as growth, breeding cycles and response to stress.

When PPID develops, part of the pituitary gland becomes overactive and produces too many hormones.

Signs of PPID can vary, ranging from abnormal hair growth or delayed coat shedding to muscle loss, excessive drinking and urinating, laminitis, lack of energy and recurring infections. We can all spot the telltale signs of severe, advanced PPID — the long curly coat and the pot belly, but vets often use blood tests to identify PPID in horses with more subtle signs. Accurate diagnosis is important, because some of these issues could also be caused by other conditions.

Adrenocorticotropic hormone (ACTH), produced by the pituitary gland, can be measured in blood samples to help diagnosis. Cases of PPID usually have high ACTH levels, but sometimes a further test is needed in which the horse receives an injection of another hormone (thyrotropin-releasing hormone or TRH) to see how the pituitary gland responds.

Horses with PPID are often treated with a drug called pergolide. Many also benefit from dietary changes, vaccination and deworming protocols, plus clipping of their long coats.

Insulin control

Key to EMS is that the normal fine balance between blood sugar levels and the hormone insulin becomes disturbed. An important consequence is that EMS cases are prone to developing high insulin levels in the blood, a situation which can trigger laminitis.

EMS cases are usually overweight or have unusual deposits of fat, such as a cresty neck. These fat deposits can produce hormones that affect the way the horse produces and responds to insulin.

Vets can investigate insulin control in several different ways, the simplest being to measure the levels in a blood sample. More involved testing is sometimes needed, however, such as measuring how insulin levels respond when the horse eats sugary food, drinks a sugary syrup or is given a small infusion of sugar directly into the bloodstream.

EMS affects adult horses and ponies and is much more common in certain breeds. They usually put on weight easily and are often described as “good doers”.

A horse’s diet, exercise and turnout all contribute to the likelihood of him developing EMS, as well as the genes he inherited from his parents. Because these genes will never change, horses with the condition require long-term dietary care and management.

What have we learned?

Groundbreaking news at the conference included the results of research into the hormone ACTH and its use in the diagnosis of PPID.

We have always considered this a simple test, yet we are learning more about how ACTH levels can vary in different situations. Recent work has shown that some breeds, such as the Shetland, have naturally higher levels of ACTH. Healthy older horses (over 20) can also have higher levels of ACTH that would be abnormal in a younger horse.

We are also discovering how the pituitary gland changes through the seasons. Levels of ACTH are naturally lowest in the spring but increase in response to longer summer days, peaking in the autumn and then falling. Researchers discussed whether some types of lighting could be used to change the activity of the pituitary gland and whether pergolide dosing could be refined.

Another interesting study investigated PPID in older sports horses, suggesting a link between the disease and suspensory ligament degeneration.

Similar advances have been made in our understanding of how the various insulin control tests can be used and improved. Some of those developed in horses need to be adjusted for ponies, while certain tests are less reliable than was previously thought.

Several pieces of research focused on how different horses and ponies react to exactly the same diet. Just like people, some ponies find it harder to lose weight than others — even when they eat the same things. We’ve recently learned that differences in insulin control at the start of a diet indicate how easy it might be for an animal to lose weight.

There are important differences in insulin control between breeds. Ponies often have very different responses to a variety of feeds, compared to horses, and researchers are trying to understand how we can alter these responses to help our ponies lose weight and how to prevent laminitis.

A drug called metformin has sometimes been used to treat horses with EMS in an attempt to control insulin levels. A recent trial indicated that horses that received the drug lost more weight than those who didn’t. Further work is needed to clarify the usefulness of metformin; in this recent trial insulin levels didn’t change, although this may have been due to the timing of drug administration.

Other research from the USA suggested that contamination of the environment by some types of pollution could also affect insulin levels in horses and make them prone to EMS.

Over the next few years, researchers will continue to investigate how PPID and EMS may be related and will try to learn more about the changes that can occur in a wide range of other hormones as part of these conditions.

Laminitis will always be a main focus, as we strive to understand more about exactly how the condition develops and how it can be prevented and treated more effectively.

Ref Horse & Hound; 7 April 2017