A pathological study was done to elucidate sequential changes of the lungs in various time intervals following experimental paragonimiasis in 15 dogs and 15 cats. The dogs and cats were fed with 30-50 metacercariae of Paragonimus westermani, and were sacrificed at 15, 20, 30, 45, 60, 90 and 120 days after infection respecively. Autopsies were performed immediately after death. Gross and microscopic examination of the lungs showed following findings: There were no qualitative difference in pathological findings between dogs and cats. Pathological findings were first noticed at 20 days of infection in thoracic cavity, which consisted of fibrinous plueritis along with superficial hemorrhage. Although no worm was found in the lung parenchyma at this period, juveniles were seen in pleural cavity together with turbid effusion. Paragonimus juveniles were first recognized inside the lung parenchyma by 30 days of infection. This was the period when the lungs showed multiple areas of hemorrhage and probably active penetration by smaller worms. Hemorrhagic bronchopneumonia was quite pronounced from this stage through 45 days of infection. Paragonimus worm cyst was essentially composed of fibrous scar and heavy inflammatory cellular infiltrate. The lining epithelial cells were first became noticed by 2 months of infection. And these epithelial cells were thought to be probably transformed alveolar lining cells rather than bronchiolar epithelial cells. As the infection progress, the cyst wall became more stabilized and often showed squamous metaplasia. Fibrinous pleuritis with pleural effusion was very prominent finding in early periods of infection. Bronchiolitis and focal vascular sclerosis were often seen in experimental paragonimiasis.
