Musculoskeletal System

The musculoskeletal system consists of bones, cartilage, muscles, ligaments, and tendons. Primary functions of the musculoskeletal system include support of the body, provision of motion, and protection of vital organs. The skeletal system serves as the main storage system for calcium and phosphorus and contains critical components of the hematopoietic system. Many other body systems, including the nervous, vascular, and integumentary systems, are interrelated, and disorders of any one of these systems may also affect the musculoskeletal system and complicate diagnosis.

Congenital and inherited anomalies can result in the birth of diseased or deformed neonates. Congenital disorders can be due to viral infections of the fetus or to ingestion of toxic plants by the dam at certain stages of gestation. The musculoskeletal system can also be affected by certain congenital neurologic disorders.

The principal causes of osteodystrophies are deficiencies or imbalances of dietary calcium, phosphorus, and vitamin D, as well as dysregulation of parathyroid hormone (PTH) activity. Their interrelationships are complex and not easily defined.

In sarcocystosis, the endothelium and muscles and other soft tissues are invaded by Apicomplexan protozoans of the genus Sarcocystis. As the name implies, Sarcocystis spp (from Greek sarkos: muscle and kystis: cysts) form cysts in muscles of various intermediate hosts—people, horses, cattle, sheep, goats, pigs, birds, rodents, camelids, wildlife, and reptiles. The cysts vary in size from a few micrometers to centimeters, depending on the host and species. Most Sarcocystis spp infections are distributed worldwide.

Also see Lameness in Cattle and Lameness in Horses and Lameness in Sheep and Lameness in Goats and Lameness in Pigs.

The lesions that cause lameness in dairy cows result in intense pain and are a major animal welfare issue. Lameness also causes stress, which debilitates and reduces productivity. The financial impact of lameness includes losses from decreased production, cost of treatment, prolonged calving interval, and possibly nursing labor. Loss of milk of 1.7–3 L/day for up to 1 mo before and 1 mo after treatment (because of pain) plus milk discarded because of antibiotic therapy must also be considered. Lame cows are more reluctant to use automatic milking systems and show visible signs of stress when forced to do so.

Abnormality of gait is a sign common to many diseases and conditions. A complete history is important for diagnosis and should include incidence and duration in the herd, nutrition, feed changes, method of rearing, and recent introductions to the herd. (Also see Health-Management Interaction: Goats.)

Lameness is defined as an abnormal stance or gait caused by either a structural or a functional disorder of the locomotor system. The horse is either unwilling or unable to stand or move normally. Lameness is the most common cause of loss of use in horses. It can be caused by trauma, congenital or acquired disorders, infection, metabolic disorders, or nervous and circulatory system disease.

Lameness has been an issue in swine production for many years and continues to be a problem worldwide. Although lameness can be caused by congenital or developmental abnormalities, most lameness in production animals is caused by pain associated with infections, trauma-related injuries, or underlying metabolic diseases. As such, it has become an area of focus for swine farm audits of animal well-being. It is also an economic issue, because an increasing prevalence or incidence of lameness in a herd is likely to affect viability, growth, or reproduction of pigs. Pig flow may be affected if farrowing targets are not met because of high rates of breeding stock removal or if growth of grower/finisher pigs is slowed by high lameness incidence. As with diseases of other body systems, lameness problems in a swine herd require a comprehensive approach if a diagnosis (or diagnoses) is to be reached so that preventive or curative measures can be instituted.

Lameness in sheep may be caused by a number of systemic diseases, some of which include navel/joint ill (Escherichia coli and Erysipelothrix), tetanus, white muscle disease, frostbite, chlamydial polyarthritis, rickets, enzootic ataxia (copper deficiency), mastitis, orchitis, nutritional osteodystrophies, selenium toxicosis, laminitis, dermatophilosis, bluetongue, ulcerative dermatosis, and in some countries, foot-and-mouth disease. Weakness, ataxia, and neurologic problems may be misinterpreted as lameness in diseases such as scrapie, listeriosis, and visna. Additional information on differential diagnosis, treatment, and prevention can be found under the specific topics (see Musculoskeletal System Introduction and {blank} Nervous System Introduction).

Muscle disorders in horses present with a variety of clinical signs ranging from muscle stiffness and pain to muscle atrophy, weakness, exercise intolerance, and muscle fasciculations. The most common clinical presentation is muscle pain, stiffness, and reluctance to move due to rhabdomyolysis. Rhabdomyolysis, defined as disruption of striated skeletal muscle, can broadly be grouped into causes associated with exercise (exertional rhabdomyolysis) and causes unrelated to exercise.

Recumbency in cattle is caused by numerous metabolic, traumatic, infectious, degenerative, and toxic disorders. If treatment of the underlying cause of recumbency is not successful and cattle are unable to rise for >24 hr after initial recumbency, they may develop a secondary recumbency from pressure damage to muscles and nerves, often termed “downer cow syndrome.” An alert downer cow does not show signs of systemic illness or depression, is able to eat and drink, and remains in sternal recumbency for no apparent reason. A nonalert downer cow appears systemically sick and depressed. Downer cow syndrome also describes the pathology of pressure-induced muscle and nerve injuries after prolonged recumbency. The most important pathophysiologic event that develops during prolonged recumbency is a pressure-induced ischemic necrosis of the thigh muscles that frequently affects both hindlegs.

Signs of musculoskeletal disorders include weakness, lameness, limb swelling, and joint dysfunction. Motor or sensory neurologic impairment may develop secondary to neuromuscular lesions. Abnormalities of the musculoskeletal system may also affect other organs of the endocrine, urinary, digestive, hemolymphatic, and cardiopulmonary systems. Evaluation of musculoskeletal disease is aimed at localizing and defining the lesion(s). Diagnosis requires accurate review of the signalment, history, and physical status of the animal. A lameness examination is critical to determine a diagnosis. Useful ancillary tests include radiography, ultrasonography, arthrocentesis, arthroscopy, arthrography, electromyography, and tissue biopsy and histopathology. For subtle lesions, advanced imaging techniques, including bone scans, CT, and MRI, are being used with greater frequency in specialty clinics and university hospitals.

Many arthropathies are developmental, including aseptic necrosis of the femoral head, patellar luxation, osteochondrosis, elbow dysplasia, and hip dysplasia. Other arthropathies are degenerative, infectious or septic, immune-mediated, neoplastic, or traumatic.

Myopathies can be congenital, hereditary, idiopathic, inflammatory, metabolic, neoplastic, traumatic, or due to nutritional imbalances.

Osteopathies can be developmental, infectious (osteomyelitis), idiopathic (eg, hypertrophic osteopathy), nutritional, neoplastic, or traumatic.