The larvae of this small roundworm are usually ingested with raw or insufficiently cooked pork or insufficiently cooked meat products contaminated by infected pork. During the first week after ingestion, symptoms consist of nausea and diarrhea; these may be minimal or absent. About 7-8 days after ingestion there is onset of severe muscle pain, which sometimes begins in the face. Bilateral periorbital edema often develops. Eosinophilia may begin as early as 10 days after ingestion and, with muscle pain and periorbital edema, forms a very suggestive triad. Eosinophilia is present in about 90% of patients and reaches its peak during the third week. Most patients have a slight or mild leukocytosis. Most patients display elevation of creatine phosphokinase (CPK) enzyme of varying degree due to involvement of skeletal muscle. Hypoalbuminemia is also frequent. The most helpful laboratory procedures are bentonite flocculation (BF) or LA slide tests. Bentonite flocculation takes about 3 weeks after infection for results to become positive; peak titers develop in about 3 months, and titers remain elevated for several years. Reports indicate 80%-95% sensitivity by the time of maximal titer. The LA slide test detects antibody earlier than bentonite, becoming detectable in about 20%-30% of cases by day 7 after onset of symptoms and 80%-92% in 4-5 weeks. False positive results have been recorded in polyarteritis nodosa (also in tuberculosis, typhoid, and infectious mononucleosis, but these are not ordinarily considered in the differential diagnosis). ELISA is also available; peak sensitivity is about 88%. In one study, all three methods detected significantly more patients (BF 52%, LA 36%, and ELISA 16%) when repeated 3 weeks after the initial testing.

Muscle biopsy is occasionally useful. It is considered best to wait until at least 3 weeks after infection to do this procedure in order to allow the larvae time to encyst. A painful area of a skeletal muscle has been recommended as the preferred site for biopsy.