Virus-Associated Myopathy
Steven M. Parish
Necrosis of skeletal and cardiac muscle occurs frequently in association with some viral diseases. In most situations, viral- induced muscle damage represents a component of systemic multiple organ system involvement.
For example, myocarditis occurs in association with foot-and-mouth disease, equine influenza, and equine infectious anemia. Other diseases causing myocarditis or skeletal muscle manifestations include bovine ephemeral fever, malignant catarrhal fever, bovine viral diarrhea, and bluetongue. Equine influenza A2 and equine herpesvirus 1 have been reported to induce primary muscle stiffness and clinical signs resembling those seen in horses with rhabdomyolysis.108,109 Details concerning specific clinical manifestations can be found in other sections of this text.Sarcocystosis
Steven M. Parish • Stephanie J. Valberg
Cysts of the sporozoan parasite Sarcocystis are commonly seen in routine histologic sections of the heart, esophageal, and skeletal muscle of cattle, sheep, goats, and horses. More than 90% of horses older than 8 years of age have sarcocysts in their esophageal muscles. Cysts may pose no problem, but with heavy infestations multisystemic dysfunction occurs. Experimentally induced acute disease is characterized by fever, mild anemia, chronic myositis, and muscle wasting.110,111
■ Epidemiology The life cycle of the parasite involves two hosts: carnivores as the definitive host and cattle or horses as the intermediate host. Three species of sarcocysts, Sarcocystis cruzi, Sarcocystis hirsuta, and Sarcocystis hominis, are known to infect cattle; Canidae, Felidae, and primates are the definitive hosts for these species. Three sarcocyst species have been described in horse muscle: Sarcocystis bertrami, Sarcocystis equicanis, and Sarcocystis fayeri.112,113 Dogs have been identified as the definitive host of these equine sarcocyst species.
In sheep and goats, Sarcocystis ovicanis and Sarcocystis capracanis have been described, with Canidae as the definitive host. The most common mechanism for natural infection in cattle is by ingestion of feeds contaminated with infected carnivore feces. Feedlot workers using feed bunks as toilets may be a source of exposure for feedlot cattle.■ Clinical Signs Although low-level natural infection is common in cattle, when administered experimentally, a dose of 200,000 sporocysts of S. cruzi is necessary to cause severe clinical disease.110 Within 4 weeks the animal develops fever (39.4° C/103° F), anorexia, salivation, weight loss, weakness, muscle fasciculations, severe depression, and sometimes death. Fever is the earliest sign and is biphasic relative to two periods of parasitemia, one occurring at 15 to 19 days and another at 25 to 42 days after inoculation. During the second febrile episode, affected calves frequently develop other clinical signs, particularly anemia. Extravascular hemolysis occurs, and hemorrhage into many tissues is common. The mechanisms involved in the hemolytic and hemorrhagic phases are likely to involve immune mechanisms. Mortality is greatest during this phase of the disease. Laboratory analysis may reveal elevations in serum urea nitrogen and bilirubin concentrations, sorbitol dehydrogenase, LDH, CK, and AST activities. If animals survive, these laboratory values usually return to normal in about 2 weeks. Animals surviving this phase commonly continue to be inappetent and have decreased weight gains, muscle atrophy, and hair loss on the neck, rump, and tail. These changes are mediated by alterations in a variety of pathways, the net result being a partitioning away of nutrients that are used for growth.111 The anemia is ameliorated by a regenerative process, with normal hematologic values obtained in 1 to 2 months after clinical recovery. Similar clinical findings are seen in sheep and goats. Abortion is common.
A syndrome similar to that described in cattle has been reported in two horses with malaise, fever, and muscle atrophy.112,113■ Diagnosis Diagnosis of sarcocystosis requires history, clinical signs, laboratory and serologic evaluation, and the demonstration of immature cysts in muscle biopsies. It is important to differentiate between the muscle cysts caused by Sarcocystis and those produced by toxoplasmosis because toxoplasmosis does not cause clinical disease in cattle.
■ Treatment and Control Specific treatment is effective only in the early stages of sarcocystosis in food animals. Experimental therapy with amprolium or the ionophore antibiotics before the second stage of parasitemia frequently prevents development of clinical sarcocystosis in cattle.111 Successful treatment of one horse with sarcocystosis using phenylbutazone, trimethoprim-sulfamethoxazole, and pyre- methamine has been reported.113 Control involves preventing gross contamination of cattle and equine feeds with carnivore feces. The common use of ionophore antibiotics (e.g., growth promotants and coccidiostats) in cattle is also likely to help reduce the incidence of sarcocystosis.114