Which parasite’s larval migration is classically associated with Loeffler syndrome (eosinophilia and transient pulmonary infiltrates)?

• A. Ascaris lumbricoides
• B. Strongyloides stercoralis
• C. Trichuris trichiura
• D. Enterobius vermicularis

Answer: (A)

Loeffler syndrome happens when migrating larval nematodes trigger eosinophilic inflammation in the lungs, producing transient infiltrates and coughing. The classic example is Ascaris lumbricoides because its larvae travel from the intestine to the lungs during their life cycle. In the lungs, they provoke an eosinophilic response, then move up the airways, are swallowed, and mature in the gut. This pulmonary migratory phase is what leads to the eosinophilia and temporary lung infiltrates seen in Loeffler syndrome. Other parasites mentioned typically do not have a lung migration phase in their usual life cycles (whipworm and pinworm stay in the gut), and while Strongyloides can occasionally cause similar lung findings, the textbook association for this pattern is Ascaris lumbricoides.