Managing muscle problems in horses *H&H VIP*

Taking himself for a gallop around his field was nothing out of character for Irish cob gelding Bobby. What concerned his owner, Sarah Healing, was what he did next.

“He suddenly stopped and immediately began digging with a front hoof,” explains Sarah, who was watching Bobby from her kitchen window. “He dug a hole and got down to roll. His lip was quivering and he was obviously uncomfortable so I called the vet, thinking he had colic.”

However, after further investigation, colic was ruled out and Bobby soon settled with an injection of muscle relaxant. It appeared that he had suffered an episode of muscle cramping called “tying-up”, which was confirmed when a blood test revealed high levels of muscle enzymes.

Stiffness and spasms

Tying-up, also known as azoturia or set-fast, is the name given to a syndrome of muscle discomfort that occurs during or soon after exercise.

“A horse that has tied-up may be reluctant to move and may want to lie down,” says Rosie Naylor MRCVS, who explains that the effects can range from slight stiffness to more debilitating muscle spasms. “Sweating and trembling are often observed, while the muscles may also feel hard and warm to the touch. The horse may become distressed and show signs of agitation such as scraping at the floor — signs that may easily be mistaken for colic.

“In some cases, depending on the horse’s management and training regime, the clinical signs may be mild and amount to little more than poor performance or a reluctance to collect when ridden.”

Rosie explains that there are many different conditions that can cause a horse to tie-up.

“A genetic condition can affect normal muscle function,” she says. “A typical example is type one polysaccharide storage myopathy (PSSM), which impairs the ability of muscle fibres to use energy stored as sugars. Another is recurrent exertional rhabdomyolysis (RER), which affects the release of calcium in thoroughbreds. This stimulates muscle contraction, after which the muscles cannot relax.

“Horses with these genetic conditions may be prone to tying-up in the future, or may have already experienced previous episodes.”

Not all cases of tying-up are caused by an underlying genetic condition. An excess of dietary energy relative to the horse’s requirements can contribute to the development of clinical signs, hence the old term “Monday morning disease”, which derived from when working horses tied-up after being given a rest day without a reduction in rations.

“Over-exertion beyond fitness levels may be another trigger factor,” adds Rosie. “Anti-oxidant deficiencies or electrolyte imbalances have also been suggested, but this is difficult to confirm, especially after the event.

“A recent viral infection may predispose a horse to tying-up, while eating certain toxins — such as those found in sycamore leaves, which are known to cause atypical myopathy — can, in rarer cases, elicit the condition.”

“Diagnosis of tying-up is usually reached by confirming marked increases in the levels of muscle enzymes in a blood sample.

These can be monitored during recovery, and the rehabilitation programme adapted according to how these change over time.

“Discolouration of the urine is also highly supportive of the diagnosis,” says Rosie. “When a horse has marked muscle damage, the muscle proteins released are excreted in the urine and can cause it to turn a reddish-brown colour. These breakdown products from the muscle are toxic to the kidneys and in rare cases can induce renal failure. If discolouration of the urine is present, intra-venous (IV) fluids are usually recommended to flush toxins out of the kidneys.

“A horse that has tied-up is usually treated with anti-inflammatory drugs such as phenylbutazone (bute), which also provide pain relief,” adds Rosie. “If he’s very distressed, he may also be given a mild sedative.”

Root cause

Further investigations may be recommended to identify genetic conditions, which can involve collecting a muscle biopsy or blood samples.

“Genetic testing can also be carried out from DNA extracted from hair roots,” Rosie explains. “It’s important that hairs are pulled, not cut, to ensure that the ‘bulb’ of the hair containing the DNA is left intact. A minimum of 30 hairs from the mane is usually recommended.

“Many horses prone to tying-up can be managed effectively with a modified diet and exercise regime,” she adds. “This involves feeding a diet low in sugars and providing any additional calories required in the form of fat, ensuring that energy requirements match workload. Good-quality forage should form the bulk of the diet. Dietary restrictions may be recommended if the horse is overweight.

“Regular daily exercise and paddock turnout are essential, while rest days, sudden changes in routine and other causes of stress should be avoided. Medical management may also be recommended for horses with RER, where the abnormal release of calcium in the muscle can be modified using medication.”

Rosie explains that prognosis will be determined by the severity and frequency of tying-up episodes, along with the response to treatment.

“With the appropriate management, many horses can fulfil an athletic career,” she says. “The presence of irreversible changes on a muscle biopsy, however, such as scarring, may warrant a poor prognosis.”

‘We were panicking at first’

Following his tying-up episode, Bobby’s enzyme levels remained elevated.

“He was rested, but each time he returned to work the levels would rise,” explains Georgie Critchley MRCVS, of Brown Moss Equine Clinic. “They never completely returned to normal. Blood testing then identified the genetic marker for type one PSSM. The condition is slightly over-represented in draught breeds and usually tends to manifest itself at around six to seven years old, which ties in with Bobby’s profile.”

“I had never heard of PSSM,” says Sarah. “We were panicking at first, wondering whether he might have to be put to sleep, but we were reassured that the condition is often controllable with management changes.

“We were advised to switch him to a low-starch, high-energy diet and started an exercise plan to build his work levels back up,” she adds. “We now have to ride him every day to keep his muscles moving, which can be quite difficult. He also has to live out 24/7; we would prefer to keep him in at night, but he doesn’t seem bothered.”

Bobby has had a busy summer, eventing at BE80(T) level, and has just started hunting.

“He needs a feed that gives him energy yet suits his requirements, as he can be lazy,” says Sarah. “As long as we keep on top of things, he’s fine — although we’d be back to square one if he tied-up again. I’m just glad we caught it early and we can manage it well.”

If you suspect that your horse is displaying signs of tying-up:

• Contact your vet immediately and keep the horse as still as possible; movement may worsen the condition. Leave him in a comfortable stable if you can, with plenty of bedding in case he chooses to lie down.
• Avoid transporting him long distances as this will increase the energy demands placed on the affected muscles.
• Keep him warm while waiting for the vet. A cooler rug will help dry him off if he is sweating heavily.
• Offer water to help rehydrate him. Providing hay might calm him if he becomes anxious, but remove all feed if there is any suggestion of colic.
• Have a suitable container ready in case he urinates — identifying a urine colour change can help confirm a diagnosis.

Ref Horse & Hound; 10 January 2019