Monday Morning Disease in Horses

Equine exertional rhabdomyolysis, historically called Monday morning disease, causes painful muscle breakdown in horses and requires careful management to prevent and treat.

Key Facts

Exertional rhabdomyolysis results from muscle fibre breakdown during or after exercise
The condition was historically common in draught horses rested over weekends with unchanged high grain rations
Affected horses show severe muscle stiffness, pain, reluctance to move, and myoglobinuria
Sporadic and chronic recurrent forms exist; the latter often has a genetic basis
Polysaccharide storage myopathy and recurrent exertional rhabdomyolysis are common causes of chronic disease
Diagnosis involves measurement of muscle enzymes CK and AST in blood

Welfare Considerations

Exertional rhabdomyolysis causes acute and intense muscle pain that constitutes a significant welfare emergency. Affected horses experience severe distress and should not be exercised further once signs appear. Myoglobin released from damaged muscle threatens kidney function, adding systemic welfare concerns. Chronic recurrent cases live with repeated painful episodes that disrupt training, impair quality of life, and create ongoing stress. Management through diet modification, exercise consistency, and in some cases genetic testing and targeted dietary supplementation can dramatically reduce episode frequency and severity.

What You Can Do

Never continue exercising a horse showing signs of tying-up; stop and call your vet immediately
Work with your vet to identify the underlying cause and implement a management plan
Ensure exercise levels and grain intake are balanced consistently, avoiding feast-and-fast cycles
Consider genetic testing for PSSM or RER if your horse has recurrent episodes
Support equine welfare research into the genetics and management of rhabdomyolysis