The changing way we view laminitis and its cause

Paradigm shifts have occurred in our understanding of equine laminitis, according to researchers, with tell-tale hoof-rings a potential sign of trouble ahead.

It is now widely accepted that most cases of laminitis are a clinical sign of systemic disease in the horse.

Laminitis associated with endocrine disease is the most common form, presenting primarily as lameness. In this form, there is a variable subclinical phase associated with gross changes in the hoof capsule. Many horses also present with gross hoof wall alterations that include divergent rings, increased cap horn or a wider/separated white line, and flat or convex soles.

Crucially, under the microscope, endocrine laminitis can be seen to cause an elongation of lamellar cells in the hoof as opposed to severe basement membrane failure, which had been proposed by some researchers to explain severe cases.

University of Liverpool researcher Cathy McGowan and her colleagues, in a review published recently in The Veterinary Journal, discussed the changes in the way we now think about this crippling condition.

Laminitis is one of the most serious diseases of horses, ponies and donkeys. It is a painful condition of the tissues (lamellae) that bond the hoof wall to the pedal (coffin) bone in the horse’s hoof. Severe and recurring cases can cause chronic painful lameness or result in the horse being destroyed to prevent further suffering.

For the past decade, researchers led by McGowan, a professor in the university’s Department of Equine Clinical Science and Institute of Aging and Chronic Disease, have systematically investigated laminitis caused by hormonal dysregulation – the form known as endocrine laminitis.

She led ground-breaking research that showed laminitis was directly caused by insulin, an important hormone involved in dietary glucose control, which overturned previously held theories of laminitis.

This new knowledge has paved the way for improving veterinary science’s understanding of the disease and improving future research and treatment.

McGowan’s understanding of abnormal insulin regulation stemmed from her work as a veterinary specialist where she treated many horses and ponies with endocrine disease. She noticed that what they had in common was abnormal insulin regulation.

The researchers highlighted three major advances in the understanding of this disease.

First, laminitis is now considered to be a clinical syndrome associated with a disease that affects a number of organs and tissues, or affects the body as a whole (systemic) or altered weight bearing rather than being a discrete disease entity.

Next, laminitis associated with the endocrine system − endocrine laminitis − is now believed to be the predominant form in animals presenting primarily for lameness.

These simple but important paradigm shifts have several implications, the main one being that an accurate diagnosis of the associated systemic disease (most commonly endocrine disease) would be pivotal for laminitis management, prognosis and the prevention of recurrence.

Third, from a series of studies, McGowan, pathologist Janet Patterson-Kane and doctoral student Ninja Karikoski showed that, under the microscope, the changes in the hoof lamellae were subtle in comparison with previous descriptions and, most importantly, there was evidence of a prolonged subclinical phase in at least some horses, as evidenced by the development of divergent hoof rings visible on the hoof wall.

These hoof rings may signify a vital window of opportunity for horse owners and their veterinary surgeons to recognise and apply therapeutic intervention before painful laminitis occurs.

These rings would have taken longer to develop (about three months) than the known duration of laminitis (as a clinical sign). These macroscopic changes typically have been associated with previous clinical episodes of laminitis or prolonged chronic laminitis. Management of endocrine disease at this crucial stage might slow, prevent or even reverse lamellar pathology, with the prevention of lamellar stretching the goal.

Under the microscope it was clear that instead of severe basement membrane failure (as had been proposed based on experimental models in severely systemically ill horses), stretching and elongation of the lamellar cells is an early and key event in the disease and this knowledge will inform research directions in the future.

“These findings completely change the way we think about a very important disease in horses,” McGowan said. “This is very important to the equine industry and veterinary profession and will be the basis of future research directions.”

In their review, McGowan, Patterson-Kane and Karikoski, said future investigations should focus on the earliest changes in the disease.

“Basic research at the cellular/molecular level is likely to be necessary to determine precisely how hyperinsulinaemia drives the key and possibly preventable lesion of lamellar stretching,” they said.

Paradigm shifts in understanding equine laminitis
J.C. Patterson-Kanea, N.P. Karikoski, C.M. McGowan
https://doi.org/10.1016/j.tvjl.2017.11.011

The review, published under a Creative Commons License, can be read here. 

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