Pulmonary paragonimiasis is one of the most important endemic parasitic disease in Korea. Although it is well known that Paragonimus westermani invades into the lung tissue eventually resulting respiratory failure, the pathogenesis of inducing lung lesions is not fully elucidated. It is still debatable on the nature of morphological component of 'cyst' which is known to be the most consistent change in lung paragonimiasis. For the purpose of clarification of morphological changes associated with pulmonary paragonimiasis, an experimental study was carried out by giving metacercariae of P. westermani to 10 dogs. These dogs were sacrificed 2 months and 6 months after infection respectively, and light microscopic and electron microscopic observations were made. Following conclusions were obtained. Paragonimus lung lesions could basically be classified into two categories, i.e., direct mechanical effects by the worms and eggs, and the changes secondary to the worm infection. The relative importance of these two appeared almost same. Adult worms of P. westermani reside inside the lumina of the dilated bronchi in great majority of the cases. Only exceptions were secondary abscess formation and subsequent scar where no epithelial lining was detected around the worms. In 6 months group almost all P. westermani worms were found inside the bronchial lumina. Obliterative endobronchiolitis was another prominent feature among changes that were not directly associated with worms or eggs. This finding was more prominent in the distal segments of the bronchial trees that harbor the Paragonimus worms. P. westermani eggs were found as isolated and aggregated fashions, provoking characteristic granulomatous inflammation. Some of the egg granulomas were seen in the areas apart from the locations where the worms were physically located. Ultrastructural observations of the cyst wall showed fair numbers of plasma cells and mast cells. Some of the cystlining epithelial cells showed increased deposit of glycogen granules, suggestive of early metabolic alteration of the respiratory epithelium.
