Seasonal pasture myopathy in horses in the midwestern United States: 14 cases (1998-2005).

The study aimed to examine and understand the symptoms, diagnostic results, tissue tremetone concentrations, and outcomes in horses affected by an intense and nonexertional muscle breakdown disease, initially thought to be due to white snakeroot poisoning. The disease was observed during unfavorable weather conditions and displayed symptoms like generalized weakness, muscle tremors, lethargy, and an extended period of lying down. However, the study found no tremetone in the horses and observed similarities with a particular type of muscle disease in Europe.

Methods & Sample

• Fourteen horses exhibiting the muscle disease over seven years (1998 to 2005) were included as the subjects of the study.
• The primary criteria for inclusion were serum creatine kinase (CK) activity greater than 45,000 U/L, severe muscle necrosis in the proximal postural muscles or signs of weakness without any noticeable firmness in the muscles during the physical examination.
• Vitamin E and selenium concentrations were measured in six of the horses, while tremetone concentration was measured in seven of the horses.

Findings

• The disease symptoms surfaced during unfavorable weather conditions.
• The horses showed signs of generalized weakness (11 horses), muscle spasms (10 horses), lethargy (6 horses), and prolonged lying down (4 horses).
• The serum CK activity ranged from 46,487 to 959,499 U/L, while the standard range is 82 to 449 U/L. Aspartate transaminase activity was found to be greater than 1,500 U/L where the standard range is 162 to 316 U/L.
• Two horses survived the disease with a rigorous antioxidant and fluid treatment.
• Past death examination of the horses revealed severe muscle necrosis with lipid accumulation, mainly in the neck, proximal fore and hind limbs, intercostal (between the ribs) muscles, and diaphragm muscles. Damage to the heart muscle was observed in seven horses.
• There were no abnormal readings in vitamin E and selenium concentrations, and no detection of tremetone in liver or urine samples.

Conclusion

• The study could not confirm that white snakeroot toxicosis, associated with rhabdomyolysis, was the cause of the disease, since tremetone was not detected in the horses.
• Interestingly, the study found similarities to cases of a specific muscle disease, known as “atypical myopathy“, found in Europe.