It's had many names--from azoturia to Monday morning sickness, but, despite the disease being well-recognized for hundreds of years, the causes of the equine muscle disorder most commonly known as tying up, have been elusive. Stephanie Valberg, DVM, PhD, Dipl. ACVIM, Dipl. ACVSMR, explained the history of this disease from ancient cart horses to modern genetic testing and took the audience at the Frank J. Milne lecture at the AAEP Convention into the current understanding of equine myopathies.

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This mare from the University of Minnesota Equine Center's PSSM herd is experiencing a tying-up episode with associated muscle stiffness, profuse sweating, and a reluctance to move.
Image courtesy of Dr. Stephanie Valberg

All horse people know the signs, even if we haven't seen a horse in the act of "tying up," we know to look for the muscle tremors, the shivering, the firm muscles. Nowadays, we even know to be on the lookout for more subtle signs such as reluctance to move forward, a stilted gait, or exercise intolerance. But there is still a lot of misunderstanding as to the causes and management of the conditions that result in "tying up."

Yep, conditions. As Dr. Valberg said in the conclusion of her presentation, "Tying up" is not one specific disease. It's a clinical term, like "colic" covering multiple myopathies. The understanding that multiple conditions can present in the same manner allows researchers to pursue the many factors that impact whether a horse will tie up.

Dr. Valberg began with the early history of research into equine muscle disease, noting that while the signs and even risk factors for the disease have been well-described for several centuries, names such as azoturia, black water, Monday morning disease, and others reflected a lack of ability to discover the causes of the condition.

Even when I was in college and veterinary school in the early-mid 1990s, it seemed as though we heard a different mechanism for "tying up" or "exertional rhabdomyolosis" every week. One of the more prevalent theories was that the condition was due to lactic acidosis, a build-up of lactate in the cells. It turns out that prevailing wisdom, taken for decades as gospel, was based on one 1932 study--showing once again, that we don't always know what we think we know.

Thanks to the efforts of a number of researchers including Dr. Valberg, we now know that multiple conditions can cause tying up-like signs in many breeds of horse. Some of these conditions such as polysaccharide storage myopathy (PSSM) in Quarter Horses and a number of draft breeds are genetic while others such as the highly fatal seasonal pasture myopathy affecting horses in the North American Midwest are caused by a plant toxin.

The good news is that knowing more about the causes and mechanisms of a disease helps to develop better treatment protocols. For years, rest was recommended for horses who were prone to tying up. It turns out, however, that many of these horses experience abnormalities in their muscle metabolism that are exacerbated by confinement and that improve with exercise and dietary management.

Advances in equine genomics, specifically the sequencing of the equine genome, has allowed the development of genetic testing for Type 1 PSSM and malignant hyperthermia (MH) in Quarter Horses. Development of a test for recurrent exertional rhabdomyolosis in Thoroughbreds has been slowed by the fact that the gene implicated in those horses is located on a poorly understood region of the chromosome.

However, genetic testing doesn't predict whether a horse will tie up, said Dr. Valberg, only whether it is at risk for tying up. So many factors, such as diet, exercise, fitness level, even nervousness, play into whether a horse will tie up, that a genetic test can only tell you if the likelihood of a given disorder exists.

But, even as more conditions and causes of equine muscle disease are discovered, one thing we do know, said Dr. Valberg is the answer to "Does lactic acidosis cause tying up?" The answer is a resounding no.