6 Toxic myopathies

Skeletal muscle is a highly vascular and metabolically active tissue and is therefore somewhat susceptible to a variety of toxins. The most commonly encountered toxins in veterinary species are ionophore antibiotics and plant associated toxins. Livestock and horses are most frequently affected.

6.1 Ionophore toxicity

Ionophore antibiotics are commonly found in the feed of livestock and are used as growth promoters. Horses are particularly susceptible to ionophore toxicity, requiring only low concentrations of the drug to cause severe damage, and may be exposed either through errors at feed mills or by consuming feed formulated for other species. Monensin is the most frequently implicated ionophore. Ionophores affect membrane permeability to electrolytes, and results in calcium overload and subsequent death of skeletal and cardiac muscle. Affected horses may show sings of colic, lethargy, stiffness, and/or weakness. Lesions are often seen within 48 hours. The heart is often most severely affected, showing pale, white to tan, often streaked areas of necrosis (Figure 6.1); other muscles may show similar changes, though to a lesser degree. Horses usually only require one dose to become severely affected, and therefore show acute, multifocal, monophasic muscle injury.

Figure 6.1: Equine heart demonstrating marked myocardial necrosis, manifesting as pale white streaks across the ventricle. Photo: C. Martin

6.2 Plant toxicities

Gossypol from cottonseeds (Gossypium spp.), senna or coffee senna plant (Senna occidentalis, formerly Cassia occidentalis or C. obtusifolia), or coyotillo (Karawinskia humboldtiana) are the most common sources of plant toxin-induced myopathy (Figure 6.2). The latter two produce a rapidly progressing disease characterized by a swaying, stumbling gait followed by recumbency, myoglobinuria, and elevated CK. Mortality is high. Gross lesions include ill-defined pallor of many muscles, that under light microscopy demonstrate multifocal, monophasic myonecrosis. Cattle, horses and pigs are particularly affected.

Gossypol toxicity occurs mostly in pigs. Cottonseed, the source of gossypol, is added to swine feed as a protein supplement, and toxicity occurs when excess cottonseed (> 10 % of feed) is consumed. Lesions take weeks to 1 month to appear, and include skeletal and cardiac myonecrosis.

6.2.1 Seasonal pasture myopathy of horses

Ingestion of box elder and/or sycamore maple tree seeds containing hypoglycin A causes seasonal pasture myopathy of horses. This disease is characterized by rhabdomyolysis and myoglobinuria, typically occurs in the fall, and can be fatal. Gross findings are non-specific, typically consisting of pale, necrotic muscles affecting multiple different muscles.

Figure 6.2: Left: A coffee senna plant. Photo by Jee and Rani Nature Photography (License: CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=9452256). Right: Box elder (Manitoba) maple seeds.