Most infections are asymptomatic. In symptomatic cases, gastrointestinal symptoms, including diarrhea, vomiting, and abdominal pain, develop within the week after ingestion of contaminated meat. These symptoms usually last for less than a week but can occasionally persist for much longer. During the following week, symptoms and signs related to migrating larvae are seen. These findings include, most notably, fever, myalgias, periorbital edema, and eosinophilia. Additional findings may include headache, cough, dyspnea, hoarseness, dysphagia, macular or petechial rash, and subconjunctival and retinal hemorrhages. Systemic symptoms usually peak within 2–3 weeks, and commonly persist for about 2 months. In severe cases, generally with large parasite burdens, muscle involvement can be pronounced, with severe muscle pain, edema, and weakness, especially in the head and neck. Muscle pain may persist for months. Uncommon severe findings include myocarditis, pneumonitis, and meningoencephalitis, sometimes leading to death.
The clinical diagnosis is supported by findings of elevated serum muscle enzymes (creatine kinase, lactate dehydrogenase, aspartate aminotransferase). The erythrocyte sedimentation rate is usually normal, which may help distinguish trichinosis from autoimmune myopathies. A commercial ELISA assay is available in the United States. Serologic tests become positive 2 or more weeks after infection, but cross-reactivity can be seen with other parasites. Rising antibody titers are highly suggestive of the diagnosis. Muscle biopsy can usually be avoided, but if the diagnosis is uncertain, biopsy of a tender, swollen muscle may identify Trichinella larvae. For maximal yield, biopsy material should be examined histologically, and a portion enzymatically digested to release larvae, but evaluation before 3 weeks after infection may not show muscle larvae (eFigure 35–42). Serum and muscle biopsy analysis are available from the CDC.