Infectious Myositis



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.[3]

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Ixodes scapularis (dammini), tick vector for Lyme disease. Courtesy of Centers for Disease Control and Prevention.

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.

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Trypanosoma cruzi in blood smear. Courtesy of Centers for Disease Control and Prevention.

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.



United States



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.



See the list below:


See the list below:


Known pathogens include the following:

Laboratory Studies

See the list below:

Imaging Studies

See the list below:

Other Tests

Trichinosis: Skin test with trichinellar antigen is unreliable.


See the list below:

Histologic Findings


Muscle biopsy is required to confirm diagnosis. Findings in the acute stage of larval invasion of the muscles include segmental necrosis and interstitial infiltrates composed mainly of eosinophils. The Trichinella species larvae sometimes can be seen in the muscle biopsy. However, encapsulated cysts (without larvae), granulomas, and focal calcification are more likely to be encountered.


Widespread necrosis of muscle fibers, perimysium, and blood vessels is noted. Pleomorphic inflammatory response consisting of both neutrophils and lymphocytes is noted.

Streptococcal myositis

Muscle necrosis and gram-positive bacteria in chains is noted.


The viable larvae produce little or no tissue reaction. However, rupture or death of a cysticercus (mature larva) evokes an acute inflammatory response with a pleomorphic exudate composed of neutrophils and eosinophils. Over time, fibrous tissue encapsulates the cysts. A chronic granulomatous response may surround the cysts.

Lyme myositis

Muscle biopsy shows atrophic fibers and an infiltrate consisting of lymphocytes, plasma cells, and macrophages. Borrelia burgdorferi can be detected in muscle fibers by the modified Dieterle silver stain method.

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Histopathology: Borrelia burgdorferi spirochetes in Lyme disease, with Dieterle silver stain. Courtesy of Centers for Disease Control and Prevention a....


Sections of infected tissues may reveal clusters of amastigotes in muscle cells surrounded by acute or chronic inflammation.

Influenza myositis

Muscle fiber necrosis without inflammatory change is observed. Influenza viral particles have been identified in muscle fibers under electron microscopy. Muscle fiber regeneration is seen in some, with an inflammatory response consisting of mononuclear and polymorphonuclear leukocytes.

Fungal myositis

Muscle biopsy is needed to confirm diagnosis. Budding yeast and pseudohyphae may be seen.

Medical Care

All medical care should be provided in conjunction with an infectious disease specialist and the primary care physician.

Surgical Care

Pyomyositis: During the suppurative phase, abscess aspiration under ultrasonic or CT guidance may be required. Surgical drainage is especially necessary for large abscesses.

Fungal myositis: Focal fungal abscesses may require surgical drainage/debridement.


Consultations with a neurologist and/or infectious disease specialist may prove useful.

Medication Summary

Treat the underlying cause of infectious myositis. Use appropriate antibiotics for pyomyositis. Prednisone may be effective to treat HIV-1–associated polymyositis.[10]

Prednisone (Sterapred)

Clinical Context:  Can be used for HIV-1–associated polymyositis. Use in combination with thiabendazole for trichinosis.

Class Summary

These agents decrease inflammatory reactions by reversing increased capillary permeability and suppressing PMN activity.

Thiabendazole (Mintezol)

Clinical Context:  Treats trichinosis infections; inhibits helminth-specific mitochondrial fumarate reductase; alleviates symptoms of trichinosis during invasive phase. Little value in disease that spreads beyond lumen of intestines; absorption from GI tract is poor.

Mebendazole (Vermox)

Clinical Context:  May be useful in early stages of trichinosis. Causes worm death by selectively and irreversibly blocking uptake of glucose and other nutrients in susceptible adult intestine where helminths dwell.

Class Summary

Parasite biochemical pathways are sufficiently different from those of the human host to allow selective interference by chemotherapeutic agents in relatively small doses.

Tetracycline (Sumycin)

Clinical Context:  For treatment of Lyme myositis. Treats gram-positive and gram-negative organisms as well as mycoplasmal, chlamydial, and rickettsial infections. Inhibits bacterial protein synthesis by binding with 30S and possibly 50S ribosomal subunit(s).

Ceftriaxone (Rocephin)

Clinical Context:  Drug of choice for most neurologic manifestations of Lyme disease; third-generation cephalosporin with broad-spectrum, gram-negative activity; lower efficacy against gram-positive organisms; higher efficacy against resistant organisms. Arrests bacterial growth by binding to penicillin-binding proteins.

Cefazolin (Ancef)

Clinical Context:  Can be used for treatment of pyomyositis. Semisynthetic cephalosporin effective against: S aureus (including penicillinase-producing strains), Staphylococcus epidermidis, group A beta-hemolytic streptococci, and other strains of streptococci.

Cephalexin (Keflex, Biocef)

Clinical Context:  Indicated for treatment of infections by S aureus (including penicillinase-producing strains) and streptococci

Vancomycin (Vancocin)

Clinical Context:  For treatment of severe infections caused by methicillin-resistant (beta-lactam-resistant) staphylococci; and for treatment of staphylococcal infection in individuals allergic to penicillin or cephalosporins.

Class Summary

Empiric antimicrobial therapy must be comprehensive and should cover all likely pathogens in the context of the clinical setting.

Further Inpatient Care

Pyomyositis: Hospitalize for systemic antibiotics.


Prevent trichinosis and cysticercosis by adequately processing pork.

When traveling in endemic areas of Latin America, apply insect repellents such as N -diethyl-meta-toluamide (DEET) to avoid American trypanosomiasis. Pyrethrin insecticides also may be used to kill insect vectors. Using bed nets to keep away insects is advisable.


Complications can include the following:

In the case of trichinosis, heavy infestation may be fatal because of myocarditis and/or diaphragmatic involvement. Myocarditis can lead to cerebral embolism.


Prompt administration of antibiotics can result in complete resolution of pyomyositis.

Most patients with trichinosis and myopathic involvement recover after several weeks. Infection may be fatal if severe and involving other organs systems (eg, cardiac, pulmonary, CNS).

Patient Education

Travelers visiting area of endemic trichinosis should be educated on the hazards of eating raw or undercooked pork.

Educate traveling diabetic patients concerning the need for prompt treatment of cutaneous infections and infected insect bites and to avoid strenuous activity.

The Myositis Association of America serves as a resource for patients and the medical community.

For patient education resources, see the Bites and Stings Center.


Mohammed J Zafar, MD, FAAN, FACP, FASN, Associate Clinical Professor of Medicine, West Michigan University School of Medicine; Neurologist, Clinical Neurophysiologist and Neuroimager, Kalamazoo Nerve Center, PLLC

Disclosure: Nothing to disclose.

Specialty Editors

Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Florian P Thomas, MD, PhD, MA, MS, Chair, Neuroscience Institute and Department of Neurology, Director, Multiple Sclerosis Center and Hereditary Neuropathy Centers, Hackensack University Medical Center; Founding Chair and Professor, Department of Neurology, Seton Hall-Hackensack Meridian School of Medicine; Professor Emeritus, Department of Neurology, St Louis University School of Medicine; Editor-in-Chief, Journal of Spinal Cord Medicine

Disclosure: Nothing to disclose.

Chief Editor

Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM, Adjunct Associate Professor of Neurology, University of Missouri-Columbia School of Medicine; Medical Director of St Mary's Stroke Program, SSM Neurosciences Institute, SSM Health

Disclosure: Nothing to disclose.

Additional Contributors

Roberta J Seidman, MD, Associate Professor of Clinical Pathology, Stony Brook University; Director of Neuropathology, Department of Pathology, Stony Brook University Medical Center

Disclosure: Nothing to disclose.


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Ixodes scapularis (dammini), tick vector for Lyme disease. Courtesy of Centers for Disease Control and Prevention.

Trypanosoma cruzi in blood smear. Courtesy of Centers for Disease Control and Prevention.

A patient with trichinosis and ocular involvement. Courtesy of Centers for Disease Control and Prevention and Dr. Thomas F. Sellers, Jr.

Histopathology: Borrelia burgdorferi spirochetes in Lyme disease, with Dieterle silver stain. Courtesy of Centers for Disease Control and Prevention and Dr Edwin P. Ewing, Jr.

Ixodes scapularis (dammini), tick vector for Lyme disease. Courtesy of Centers for Disease Control and Prevention.

Trypanosoma cruzi in blood smear. Courtesy of Centers for Disease Control and Prevention.

A patient with trichinosis and ocular involvement. Courtesy of Centers for Disease Control and Prevention and Dr. Thomas F. Sellers, Jr.

Histopathology: Borrelia burgdorferi spirochetes in Lyme disease, with Dieterle silver stain. Courtesy of Centers for Disease Control and Prevention and Dr Edwin P. Ewing, Jr.