We investigated 17 adult cases (14 males and 3 females) of myalgia induced by human parechovirus type 3 (HPeV3) infection, treated during the summers of 2008, 2011, 2014, and 2016. The patients were aged between 21 and 50 years. The limbs and trunk of all patients were affected, and severe myalgia, muscle weakness, and decreased grip strength were observed. In addition to myalgia and muscle weakness, symptoms included fever in 14 (82%), upper respiratory inflammation in 8 (47%), gastroenteritis in 4 (24%), and scrotal pain in 4 (29% of males) patients. Tendon reflexes were preserved, and serum creatine kinase level increased in all but 1 patient. Spinal MRI was performed for 3 patients, with normal results. Musculoskeletal MRI scans showed abnormal signals in the femoral muscles in 2 of 5 patients. In a nerve conduction test, the frequency of F wave appearance in the median nerve was 40% or less in 5 of 9 patients, and repeater F waves were seen in 2 patients. Of these, 7 patients had infants in their families, and developed fever around the same time; they may have been infected by these infants. All patients recovered within 1-2 weeks. HPeV3 infection is characterized by severe myalgia, and is frequently observed in summer every 2-3 years.
Keywords: HPeV3-M; HPeV3-associated myalgia/myositis; epidemic myalgia; human parechovirus type 3; myositis.