Camelidae and Cervidae Selenium-Responsive Myopathy Study Guide

Overview and Clinical Importance

Selenium-responsive myopathy, also known as white muscle disease (WMD) or nutritional myodegeneration (NMD), is a degenerative muscle disease caused by deficiency of selenium and/or vitamin E. This condition affects both skeletal and cardiac muscle and is clinically significant in camelids (llamas, alpacas, dromedary and Bactrian camels) and cervids (deer, elk, moose). Understanding this disease is critical for the NAVLE as it tests knowledge of pathophysiology, clinical recognition, species-specific considerations, and management in exotic and production animal species.

Although overt myopathy is rare in South American camelids compared to other livestock, selenium deficiency remains a disease of concern in many regions of North America, Europe, Australia, and New Zealand. Cervids, particularly farmed deer and elk, are susceptible to WMD, especially in selenium-deficient geographic regions.

Etiology and Pathophysiology

Role of Selenium and Vitamin E

Selenium is an essential trace element that functions as an integral component of the enzyme glutathione peroxidase (GPx). This selenoenzyme catalyzes the reduction of hydrogen peroxide and lipid hydroperoxides to water and alcohols, respectively, using glutathione (GSH) as a cofactor. The main reaction is: 2GSH + H?O? → GSSG + 2H?O.

Vitamin E (alpha-tocopherol) is a lipid-soluble antioxidant that functions as a free radical scavenger within cell membranes. It protects polyunsaturated fatty acids (PUFAs) in membrane phospholipids from peroxidation by reactive oxygen species (ROS).

Mechanism of Muscle Damage

When selenium or vitamin E is deficient, reactive oxygen species accumulate and cause oxidative damage to cell membranes through lipid peroxidation. Muscle cells are particularly vulnerable because they have high metabolic activity, generate significant ROS during contraction, and contain high concentrations of calcium stored in the sarcoplasmic reticulum. The sequence of events includes increased sarcolemmal permeability, calcium influx into myocytes, activation of calcium-dependent proteases, mitochondrial dysfunction, and ultimately myocyte necrosis and calcification.

Geographic and Dietary Risk Factors

Species-Specific Considerations

Camelidae (Llamas and Alpacas)

South American camelids have unique selenium metabolism compared to ruminants. Llamas and alpacas have greater selenium-dependent glutathione peroxidase activity in serum, resulting in less difference between serum and whole blood selenium concentrations compared to cattle, sheep, and goats. This is clinically important when interpreting diagnostic values.

Camelid Selenium Status Parameters

Cervidae (Deer and Elk)

White muscle disease in cervids, particularly red deer (Cervus elaphus) and elk (Cervus canadensis), is well-documented and can be fatal, especially in neonates. Congenital WMD occurs when pregnant dams are selenium-deficient, resulting in fetal/neonatal myodegeneration. White-tailed deer, mule deer, and moose are also susceptible. Selenium deficiency can lead to reduced fecundity, juvenile survival rates, and population growth in wild cervid populations.

Clinical Presentation

Forms of White Muscle Disease

WMD presents in two main clinical forms: cardiac (peracute) and skeletal (subacute). These may occur independently or concurrently.

Species-Specific Clinical Presentations

Camelids: Typically, both hind legs are symmetrically affected. Tongue and heart muscles are commonly involved in neonates. Newborns with tongue lesions have difficulty nursing and may present as "dummy" crias. Ill-thrift, infertility, neuropathy, and immunodeficiency syndromes may be associated with subclinical deficiency. Skeletal muscle swelling and pain upon walking, inability to stand, and trembling when held upright are characteristic.

Cervids: Congenital WMD in deer calves often presents at birth with weakness and inability to nurse. Sudden death during exercise is common if cardiac muscle is involved. Capture myopathy in deer shares pathophysiologic similarities with WMD and may be exacerbated by selenium deficiency. Clinical signs include depression, muscle stiffness, tremors, tachypnea, tachycardia, dark-red urine (myoglobinuria), and metabolic acidosis.

Diagnosis

Clinical Pathology

Exam Focus: In polyphasic (ongoing) muscle injury like WMD, both CK and AST are persistently elevated. In monophasic (single insult) injury, CK may normalize while AST remains elevated if samples are taken more than 24 hours post-injury. This pattern helps differentiate nutritional myopathy (polyphasic) from single-event trauma or ionophore toxicity (monophasic).

Gross and Histopathologic Findings

Gross Pathology

At necropsy, affected muscles appear pale pink to white with bilateral symmetry. Characteristic findings include white, chalky streaks or patches (dystrophic calcification) most prominent in cardiac muscle (especially left ventricle and interventricular septum) and postural skeletal muscles (tongue, neck, shoulder, gluteal, semimembranosus, semitendinosus). Intercostal and diaphragm muscles may also be affected. The tissue may feel dry and gritty due to calcium deposition.

Histopathology

Microscopic examination reveals polyphasic, polyfocal degeneration and necrosis of myofibers. Key histologic features include: hyaline degeneration (Zenker's necrosis) with swollen, hypereosinophilic, homogeneous sarcoplasm lacking cross-striations; floccular necrosis and myofiber fragmentation; loss of nuclei and sarcolemmal collapse; dystrophic mineralization/calcification of necrotic fibers; influx of macrophages and satellite cell proliferation (regeneration); and in chronic cases, fibrosis and evidence of regenerating myotubes with central nuclei and basophilic cytoplasm.

Differential Diagnosis

Treatment

Acute Management

Treatment success depends on the form of disease and severity of muscle damage. The skeletal form responds favorably to treatment if initiated early, while the cardiac form is often refractory to therapy.

Prevention

Supplementation Strategies