G.P.78: Acute Myositis due to Sarcocystis nesbitii infection

Abstract

Human muscle sarcocystosis is usually asymptomatic but can present monosymptomatically as focal muscle pain and/or swelling or rarely, with acute systemic symptoms in an outbreak. We describe features of acute myositis seen in an outbreak of Sarcocystis nesbitii infection in Malaysia in 2012. Infected patients presented acutely with fever and generalised myalgia, which in some included the face and jaw muscles. In about half the patients, myalgia was moderate to severe. There were 17 patients in whom myositis could be confirmed either by the presence of painful muscle swelling or hyperintensity on MRI STIR sequences suggesting muscle inflammation. Muscle swelling and inflammation on MRI were multifocal rather than diffuse and there was predominant involvement of the facial and masticatory muscles suggesting a possible predilection of the sarcocysts for these muscles. Muscle biopsy confirmed the diagnosis by demonstrating the presence of intracellular sarcocysts which was confirmed to be S. nesbitii by molecular methods. Inflammatory changes did not surround the infected muscle fibres but were seen focally with necrosis elsewhere within the muscle. Tissue eosinophilia was not prominent. Laboratory investigations revealed elevated serum creatine kinase and eosinophil count only in a third and half of the patients respectively. Even without specific treatment, muscle pain and swelling gradually subsided over a period of weeks to months. In summary, acute S. nesbitii myositis presented with fever and multifocal muscle pain and swelling. The multifocal nature of muscle inflammation was confirmed on MRI. The intracellular location of the sarcocysts does not elicit a direct immune response but sarcocysts may secrete chemical antigens that result in inflammation elsewhere.