Paragonimus

Paragonimus is a genus of digenetic trematodes in the family Paragonimidae. The species most relevant to humans cause paragonimiasis, a chronic pleuropulmonary infection. The best known human pathogens are Paragonimus westermani and related species; in North America, Paragonimus kellicotti is a principal agent.

The life cycle involves two intermediate hosts. Adults inhabit paired lungs of mammals, including humans, and release operculated eggs into the bronchi, which are swallowed and excreted. In freshwater, eggs hatch into miracidia that infect a freshwater snail (the first intermediate host). After development, cercariae leave the snail and migrate into freshwater crustaceans (crabs or crayfish), where they encyst as metacercariae. Humans acquire infection by eating undercooked or raw crustaceans containing metacercariae. Metacercariae excyst, migrate to the lungs, and mature into adults.

Clinical features arise from pulmonary inflammation and tissue damage. Patients typically present with a chronic productive cough, chest pain, hemoptysis, and dyspnea. Eosinophilia is common. Radiographs and CT may show nodules, infiltrates, pleural involvement, or cavities. Eggs may be detected in sputum or stool, though sensitivity varies.

Diagnosis and treatment: Diagnosis relies on detection of eggs in sputum or stool, supplemented by serology and imaging. Praziquantel is the first-line therapy; alternative regimens include triclabendazole or, in some settings, bithionol. Prevention emphasizes avoiding raw or undercooked freshwater crustaceans and controlling intermediate hosts in endemic areas. Paragonimiasis occurs in parts of Asia, Africa, the Americas, and the Pacific, with regional species varied.