Since the age of horse drawn plows, tying up or Monday Morning Sickness in horses has been described.
Recent research and genetic studies from Dr. Stephanie Valberg, professor and director of the
University of Minnesota Equine Center, and others, have defined specific muscle disorders
in horses including exertional rhabdomyolysis, the technical name for tying up. Horses with
this complex of problems acutely develop a short, stiff, stride, anxiety, pain as demonstrated
by sweating and trembling, and eventually inability to move. Dark urine, azoturia,
may develop as muscle pigment is broken down and excreted in urine. Chronic signs
include reluctance to move, bad attitude to work, lack of energy, and stretching as though
to urinate—mild signs of colic.
Classic tying up occurs in horses fed grains, worked consistently, but then rested—commonly on Sundays, historically, then worked on Mondays. Type 1 PSSM
or polysaccharide storage myopathy,
is this classic muscle disease occurring in Belgians, Percherons, Irish Drafts, Cob, Connemara, 6-10% of Quarter Horses, Paints, and Appaloosas, and 18% of warmbloods, but not Hannovarians.
Interestingly, it occurs in 28% of halter Quarter horses, and rarely in racing stock, and is almost nonexistent in Arabians, Standardbreds and Thoroughbreds. Diagnosis is by clinical signs,
high elevations of muscle CK enzymes in the serum after an episode, and now genetic testing performed on whole blood or hair root samples at UMN.
Type 2 PSSM occurs more commonly in high performance Quarter Horses—cutters, reiners, barrel horses, and less commonly in pleasure and halter horses. Other warmbloods,
Standardbreds,and Thoroughbreds may be affected, and Arabians have a slightly different presentation. Muscles may atrophy after attacks and even foals may show a stiff gait or
difficulty standing. Horses may not perform well, be reluctant to perform after 5 or 10 minutes of exercise, and may be reluctant to collect, canter, or jump. Warmbloods seem
to be older at the onset of signs—8 years or more. Diagnosis is by muscle biopsy. A heritable predisposition is suspected but not proven.
Recurrent Exertional Rhabdomyolysis (RER) occurs in about 5% of Thoroughbreds and Standardbreds with abnormal muscle contractions and is associated with excitement most
commonly in fillies with a nervous temperament, usually when these race horses are held back from maximal exercise.
Managing horses with tying up from PSSM is twofold—diet and exercise.
Horses should not be stalled
, but should have free access to moving in fairly large areas. Exercise daily.
All feed needs to be low carbohydrate: hay should have a very low NSC (nonstructural carbohydrate) ideally below 12%. (Test annual hay supply at Equi-analytical Lab.) Concentrates should be low starch and at least 20% of energy calories should be provided by fat. This will reduce excitability, insulin, glucose, and cortisol concentrations.