Non-hereditary Myopathies

Infection-induced inflammatory myopathies

Rapid globalization has led to a dramatic increase in movement of people across countries and even cross-continental mobility. Owing to the availability of an ever-widening array of foods and flavors, people are increasingly being exposed to inadequately cooked meat, fish, or game. These trends have resulted in an increasing incidence of uncommon infections including infectious myositis.

1. Parasitic infection

Taenia solium causing cyticercosis
T. saginata
Diphyllobothrium causing sparganosis
Hookworm infection causing ancylostomiasis

(1) Toxoplasmosis

This infection occurs following the consumption of under-cooked meat or contaminated food or water containing feces of domestic fowl. Vertical transmission from mother to fetus is known. Infection of the skeletal muscles typically manifests as an acute or subacute myositis with fever and lymphadenopathy. Other organs may also be infected leading to cardiomyopathy and skin rash simulating dermatomyositis. Immunocompromised patients, including those diagnosed with AIDS are more susceptible than the normal population. Serological tests and imaging of skeletal muscle samples for detection of Toxoplasma cysts can be helpful. We reported infiltration of CD4+ T cells and macrophages in a biopsied muscle sample (Matsubara, 1990).

(2) Malaria

Malaria remains an endemic disease; 220 million people were infected and 430,000 malaria-related deaths were reported in 2017, and most deaths were reported from Africa. Domestic infection of Plasmodium vivax was previously reported in Japan; however, all cases in recent years are in patients infected abroad. Symptom severity depends on the species of Plasmodium causing malaria. Malarial infection causes weakness and myalgia, and rhabdomyolysis has been reported in patients infected by P. falciparum (Knochel, 1993). Although the exact mechanism causing muscle injury remains unclear, it has been hypothesized that abnormal aggregation of red blood cells induces ischemic necrosis of muscle cells (Taylor, 1990).

(3) Taenia solium and cysticercosis

T. solium migrates to various organs and grows to form an encysted larval stage (Cysticercus cellulosae) within 2 months and causes cysticercosis. Human infection typically occurs following the ingestion of undercooked cysticerci-infected pork. After ingestion, embryonated eggs hatch in the intestine, penetrate the intestinal wall and spread to various organs. Skeletal and cardiac muscles, the brain, and eyes are predisposed; however, any organ can be infected. Patients may develop various gastrointestinal symptoms, anemia, epileptic seizures, and meningoencephalitis. Notably, chronic infection may be observed in asymptomatic individuals. Stool examination, blood tests to detect eosinophilia, serological tests, and CT and MRI are useful for diagnosis. Histopathological findings of muscle specimens may show cysts and pericystic reactive inflammation.

(4) Sparganosis

The life cycle of Diphyllobothrium needs two intermediate hosts. The first host is cyclops (water flea) and the second is fish, reptiles, and amphibians. Humans can be infected by ingestion of these intermediate hosts, typically after the consumption of water containing cyclops infected by the plerocerocoid larva or after the consumption of infected frogs or snakes. After ingestion, the eggs penetrate the intestinal wall and undergo systemic dissemination. Patients usually manifest with subcutaneous swellings, and ocular sparganosis can lead to loss of vision.

(5) Trichinellosis

Trichinellosis is caused by ingestion of meat containing the cysts of Trichinella, which typically occur in undercooked pork or game. Meat of infected bears is known to cause this condition in Japan. Myalgia and fever with eosinophilia are observed approximately two weeks after ingestion at the time of larval migration to the muscle. Myocarditis, pneumonitis, and encephalitis may occur in severe cases. Larval encystment in the muscle causes myositis and inflammatory reactions, and the disease manifests with myalgia and weakness. Extraocular, jaw, and neck muscles are frequently affected.

(6) Hookworm

Necator americanus and Ancylostoma duodenale are similar in shape except the former is smaller in size. Infection occurs through oral transmission or through the skin. These parasites migrate to the lungs and later reach the small intestine where they remain in the mucosa causing tissue injury. In most cases hosts remain asymptomatic. Occasionally, they may cause anemia. Histopathological findings in skeletal muscle tissue may show larvae with perilarval inflammation.

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