5 Nutritional myopathies

5.1 Selenium and vitamin E deficiency

Deficiency in selenium, and to a lesser extend, vitanim E, is a very important disease of young cattle, sheep, swine, and horses. Neonatal animals are particularly susceptible, as they rely on stores accumulated in utero, which may be sub-optimal. The disease is rarely seen in carnivores. This disease is also colloquially known as white muscle disease. Selenium and vitamin E levels in animals are directly related to dietary intake, it is thus important that animals are fed well balanced rations. Complicating matters, vitamin E degrades over time, and feed that was originally properly formulated may wind up being low in vitamin E if stored for prolonged periods.

Selenium and vitamin E are components of antioxidants that play important roles in the control of free radicals and oxidative damage. Recall that a free radical is a molecule with an unpaired electron, and as such are highly reactive and have enormous potential to damage cell and mitochondrial membranes (lipid peroxidation). Antioxidants are protective against free radicals: vitamin E directly scavenges free radicals, while selenium is a component of an enzyme, glutathione peroxidase, that reduces free radicals.

When levels of selenium and/or vitamin E are low, free radical damage accumulates, particularly in skeletal and cardiac muscle. The high oxygen requirement and contractile activity of these two tissues renders them particularly susceptible to oxidative damage. Free radical damage to cell and organelle membranes contributes to a loss of ionic gradient and increased sarcoplasmic Ca²⁺ concentrations, leading to hypercontraction, myofiber necrosis, and potentially mineralization.

Grossly, the necrotic muscle appears white, though, if you recall, this is a non-specific finding, and could be the result of a number of etiologies. When performing a necropsy, pay close attention to muscles that tend to be the most active: the heart (particularly the left ventricle of calves, right ventricle of sheep, the tongue, diaphragm, and intercostal muscles). Histologically, there is multifocal, polyphasic necrosis with regeneration. The damage tends not to affect the basal lamina or satellite cells, so myofibers are able to regenerate and recover, depending on the severity of the damage. Although regeneration is possible, it is not infrequent for animals to die or be euthanized due to this condition.