By Hayley Kosolofski

There are two simple words that describe painful, exercise-associated muscle cramping in a horse: tying up. While the traditional tying up usually occurs after a long hard ride, some horses can tie up repeatedly for no immediately obvious reason. 

Regardless of the underlying cause, the clinical signs are similar. And in most cases, affected horses require immediate veterinary care, says Dr. Fabienne Uehlinger of the Western College of Veterinary Medicine (WCVM) in Saskatoon, Saskatchewan. 

“The horse becomes very stiff — it’s usually reluctant to move, it may show signs of colic and be sweating, and it may be breathing hard,” says Uehlinger, a board-certified specialist in large animal internal medicine.

Affected horses often look like they are experiencing colic or laminitis (founder). In severe cases, a horse may pass very dark urine as substances released from damaged muscles are flushed through its kidneys. Damaged muscles release myoglobin, a muscle protein which is toxic to the kidneys and can cause permanent damage.

More often than not, tying up occurs as a one-time event due to physical exertion beyond the horse’s fitness level or due to an underlying condition, explains Uehlinger. For example, a tying-up episode can occur if a horse is being exercised after it has been on stall rest, if it has suffered a recent bout of equine influenza, or if it has been exercised too strenuously outside its regular exercise routine. 

As long as the horse recovers well from the episode, Uehlinger says the animal should be fine without long-term damage.

“When we think back to those episodes, we can usually find a reason for why they have happened and we try not to do it again in the future.”

However, there are times when a horse ties up repeatedly for no apparent reason. For these chronic cases, veterinarians usually consider two conditions: polysaccharide storage myopathy (PSSM) and recurrent exertional rhabdomyolysis (RER). In some cases, these conditions can be inherited and passed down between generations.

Recurrent exertional rhabdomyolysis (RER) is typically diagnosed in racing breeds such as Standardbreds and Thoroughbreds, and tends to occur in young racehorses in training. Affected horses are often described as nervous or high strung.

“PSSM refers to abnormal storage of sugars in the muscle,” says Uehlinger. “That sugar is providing the fuel necessary for the muscle to work. When the horse exercises or moves around, it is not actually able to use that sugar appropriately. It doesn’t have the energy to do what it needs to do, and it ties up.”

PSSM is divided into Types 1 and 2. Type 1 most commonly occurs in Quarter Horses and related breeds such as Appaloosas and Paints. Type 1 is also common in draft horse breeds of European descent, such as Belgians and Percherons, as well as in some warmblood breeds. There is a known genetic mutation that is inherited by the offspring of affected horses. 

Type 1 PSSM occurs most commonly in Quarter Horses and related breeds, as well as draft breeds of European descent and some warmblood breeds. A genetic mutation is inherited by the offspring of affected horses. Type 2 PSSM tends to occur in light horse breeds such as Thoroughbreds and Arabians as well as Quarter Horses, and the genetic mutation that causes it has not been identified.

Type 2 PSSM tends to occur in light horse breeds such as Thoroughbreds and Arabians, but it may also occur in some Quarter Horses. So far, researchers have not identified the genetic mutation that causes this type of PSSM.

RER is typically diagnosed in racing breed horses such as Thoroughbreds and Standardbreds. It affects females more often than males, and the condition tends to occur in young racehorses in training. Trainers typically describe affected horses as nervous or high strung.

“Acute signs can be very similar to sporadic tying up,” says Uehlinger. “So again, your horse may look like it’s colicky — it’s sweaty, it’s reluctant to move.”

However, tying up episodes due to RER are not usually associated with overtly exerting events — such as racing — but may occur during light training.    

Horses with PSSM or RER can also experience milder episodes that may present as lameness, back pain, an inability to engage the hind end, or poor performance. 

Unlike sporadic tying up, episodes of muscle cramping associated with PSSM or RER can occur after short periods of light exercise, trailering, or pasture turnout. These episodes are recurrent. In all cases, an acute episode of tying up is considered a veterinary emergency.

If your horse ties up repeatedly, Uehlinger recommends talking to your veterinarian about investigating the condition’s underlying cause. As a starting point, your veterinarian may conduct an exercise test by collecting an initial blood sample before lightly exercising your horse for 15 minutes. A second blood sample will be collected about four to six hours after exercising.

“If a horse has a tendency to tie up, we can demonstrate substances that are being released from the damaged muscle in the blood [samples],” says Uehlinger. Specifically, veterinarians measure muscle enzymes that are being released into the horse’s blood. 

Once your veterinarian has established that your horse has a tendency to tie up, further diagnostics can help identify which condition is affecting your horse. Since a genetic abnormality has been identified for Type 1 PSSM, a genetic test can be run on a hair or blood sample. For Type 2 PSSM and RER, a muscle biopsy is necessary to make a definitive diagnosis. 

“Unfortunately, we cannot heal these horses of PSSM or RER,” says Uehlinger. “The predisposition to tie up will always be present and we have to implement lifelong management strategies to prevent these episodes.”

There are two key components to managing affected horses: diet and exercise. Horses diagnosed with PSSM or RER benefit from a diet that is low in sugars as they are not able to properly use sugar for energy in their muscles. The primary energy source in their diet should be in the form of fat and fibre.

Related: The "Gen-ethics" of Equine Breeding

Horses with PSSM and RER should have a diet that is low in sugars, and the foundation of their diet should be a good quality grass hay like timothy or brome. Photo: Robin Duncan Photography

The basis of the diet should be hay — ideally a good quality grass hay like timothy or brome. Horse owners can provide additional energy through vegetable oil or feeds that are specifically formulated with a high fat and fibre content.

Uehlinger also recommends that affected horses receive daily exercise. The exercise doesn’t need to be strenuous, but it does need to be consistent. This routine is important to maintain energy metabolism in the muscle. If you are unable to ride every day, daily turnout can help keep your horse moving. The most important thing is to try and avoid lay-ups and stall rest. 

Horses diagnosed with PSSM or RER should be exercised daily to maintain energy metabolism in the muscle. Although the exercise does not need to be strenuous, it is important to keep the horse moving and avoid stall rest.

“The good news is that there are specific management strategies that we can implement in order to prevent the majority of these tying-up episodes,” says Uehlinger. “With a regular exercise and feeding routines, many horses that tie up are able to continue to perform and enjoy their work.”

Hayley Kosolofski of Sherwood Park, Alberta, is a fourth-year veterinary student at the Western College of Veterinary Medicine (WCVM). 

Related: Understanding PSSM in Horses

Main Photo: Tying up typically occurs after a long hard ride, often resulting from physical exertion beyond the horse’s fitness level. The horse becomes very stiff and reluctant to move, it may be breathing hard and appear to be experiencing colic or laminitis. Photo: ©Dreamstime/Chris Van Lennep