Human Pulmonary Distomiasis Caused by Paragonimus Westermanni

1. The morbidity of pulmonary distomiasis among the school children in the plains of the Prefecture of Shinchiku is 4.3 per cent, while in the mountainous regions among the savages it reaches in some districts 50 per cent. 2. Seventeen species of cercariae were discovered in fresh water mollusks in the Prefecture of Shinchiku, Formosa. But it was impossible to ascertain from morphological characteristics alone which of them developed into the pulmonary fluke. Consequently, the eggs of the pulmonary fluke after hatching into miracidia were allowed to come into contact with several species of fresh water mollusks, of which they infected two. But as it was difficult to keep the two spedes alive in the aquarium long enough to get cercariae, the second intermediate hosts of the pulmonary distomas were looked for in the severely infected villages of the savage tribes. 3. The miracidia of the pulmonary distomas leave the egg about 4 weeks after they are first set free in the water, and if they do not reach mollusks they soon die. 4. Three species of fresh water mollusks were found to act as the first intermediate host of the pulmonary distomas; viz., Melania libertina Gould, Melania tuberculata Mueller, and Melania obliquegranosa Smith. 5. The cercariae of the pulmonary distoma may be identified by their small size and a spine in the oral sucker. They develop in the liver of the three spedes of Melania mentioned above. 6. The second intermediate hosts of the pulmonary distoma,detected in the Prefecture of Shinchiku, are the following three species of fresh water crabs: Potamon (Geothelphusa) obtusipes Stimpson (native name, red crab), Potamon (Geothelphusa) dehaanii White (native name, dung crab), and Eriocheir japonicus De Haan (native name, hairy crab). In addition it was discovered that the following two species might act as intermediate hosts: Sesarma dehaanii Milne-Edwards and Potamon (Parathelphusa) sinensis Milne-Edwards. In Formosa four of the five species are the carriers of the cercariae. 7. The encysted cercariae are found in the gills, liver, and muscle, and have an elongated dark excretory vesicle in the middle of their bodies. They resemble the adult flukes. 8. Full grown encysted cercariae fed to dogs develop into mature pulmonary distomas and begin to lay eggs in about 90 days. 9. In the final host the parasites are taken into the alimentary canal as encysted cercariae. They liberate themselves from the cysts in the intestine and bore through the jejunum into the abdominal cavity. They then pierce the diaphragm, enter the thoracic cavity, and piercing the pleura reach the lungs. In the parenchyma of the lungs they form cysts and develop into adult forms. 10. The chief causes of pulmonary distomiasis are the eating of raw or insufficiently cooked crabs infected with the cercariae of Paragonimus westermanni, and the drinking of river water containing them.