Infectious myositis is an acute, subacute, or chronic infection of skeletal muscle. Once considered a tropical disease, it is now seen in temperate climates as well, particularly with the emergence of HIV infection. [1, 2] In addition to HIV, other viruses, bacteria (including mycobacteria), fungi, and parasites can cause myositis. For a detailed discussion of HIV-associated myopathies, refer to HIV-1 Associated Myopathies.
Single or multiple muscle groups in the limbs can be involved, a notable exception being trichinosis, which commonly involves orbital muscles. In most instances, involvement of proximal muscles is predominant. Characteristic myopathic features and findings of polymyositis, including inflammatory infiltrates, may be seen.
Viruses: Viruses implicated in the pathogenesis of myositis include HIV-1, human T lymphotrophic virus 1 (HTLV-1), influenza, coxsackieviruses, and echoviruses. As in the non–HIV-infected population, HIV-associated polymyositis is most likely autoimmune in origin. Influenza myositis could be due to direct viral invasion or autoimmune response.
Pyomyositis: The pathogenesis is unclear, but trauma, viral infection, and malnutrition have been implicated. Although most cases of pyomyositis occur in healthy individuals, other pathogenetic factors include nutritional deficiency and associated parasitic infection in tropical climates. In the temperate climates, pyomyositis is seen most commonly in patients with diabetes, HIV infection, and malignancy.
Lyme borreliosis: Musculoskeletal manifestations are noted frequently in Lyme borreliosis. The disease is transmitted by the bites of ticks of the Ixodes genus that carry the spirochete (see image below). The animal reservoirs are the white-footed mouse in the Eastern United States and the wood rat in California. Human infection results from the bite of infected ticks in the late spring and early summer. Lyme myositis may result from direct invasion of muscle by the spirochete Borrelia burgdorferi or by autoimmune mechanisms. 
American trypanosomiasis: The causative organism is a protozoan, Trypanosoma cruzi. The insect vectors are reduviid bugs such as Rhodnius prolixus ("vinchuca"), Triatoma infestans, and Panstrongylus megistus. The insect defecates on the host's skin as it feeds, contaminating the bite wound with feces containing the parasites. T cruzi occurs in 2 forms in humans, the intracellular amastigote and the trypomastigote form in blood, which is ingested by the insects (see image below). The parasite reproduces asexually and migrates to the hindgut. In humans, the parasite loses its flagellum and transforms into the amastigote form, which may enter muscle and multiply, resulting in myositis.
Cysticercosis: Myositis also can occur in cysticercosis, which represents an infection by the larval stage of the intestinal tapeworm Taenia solium. Human infection results from ingestion of raw or incompletely cooked pork. Another mode of infection is by contamination of food and water by feces containing the eggs of the tapeworm. The larvae migrate throughout the body and may form fluid-filled cysts in a variety of tissues, including muscle.
Pyomyositis: Approximately 676 cases have been reported in the US literature since 1971.
Lyme disease: Endemic areas include the Northeast, mainly Connecticut, Massachusetts, Maryland, and New York; the North-Central region, mainly Wisconsin and Minnesota; and the West Coast, especially Northern California.
In eastern Uganda, 400-900 cases of tropical myositis occur per year; it is rare in western Kenya.
Cysticercosis is most prevalent in India, Eastern Europe, Central America, and Mexico.
In endemic areas of Latin America, 8% of the population is seropositive for American trypanosomiasis.
A potentially life-threatening complication of pyomyositis is toxic shock syndrome.
Rhabdomyolysis can complicate influenza and, rarely, coxsackievirus myositis.
Race-, sex-, and age-related demographics
In Hawaii, muscle abscesses were noted to be confined to the Polynesians. In the French Pacific islands, the disease is not seen in the French settlers.
Infectious myositis has a male predominance and is typically seen in young adults.
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