Preliminary Notes on the Crustacean Vector of the Mammalian Lung Fluke (Paragonimus) in the Philippines

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The occurrence of human paragonimiasis in the Philippines was first reported by Musgrave (1907). Since that time additional cases have been encountered, but not all of them have been recorded in the literature, thus creating the impression that the infection is very rare in the Islands. As first suspected by Garrison (1908), the parasitism is not widely distributed, but appears to be confined to certain localities where the appropriate intermediate hosts are undoubtedly present and where the food habits of the people enable the infective stage of the causative agent to be introduced into the human body. In May and July, 1941, survey trips were made to Naga, Camarines Sur, Luzon, a suspected endemic place, with the object of gathering material for the study of the life history of the fluke. Numerous samples of snails (mostly Melania of various species), crabs (Potamon (Potamon) sp.), and prawns (3 species of Palaemon) were collected from small creeks near the foot of Mount Isarog. The snails were crushed and examined in the usual way for the presence of Paragonimus cercariae and the crustaceans were dissected and the soft parts examined separately between glass slides for the presence of encysted metacercariae. The snails and prawns were negative, but the crabs were found harboring encysted metacercariae which are whitish, globular and double-walled, the inner wall measuring 300 to 320 microns in diameter and the outer wall 340 to 400 microns. Of 216 crabs dissected, 60 per cent were positive, each crab harboring from 3 to 53 cysts. The cysts were most frequently encountered in the muscles of the legs, then in the gills, liver, and thoracic muscles. When fed to experimental rats and cats, the metacercariae developed into adult lung flukes which are morphologically indistinguishable from adult Paragonimus of local human origin. It may be presumed that human paragonimiasis is the result of the ingestion of the same kind of larvae, for it has been learned that river crabs are sometimes eaten uncooked by some of the people. The procedure consists in simply removing the legs, carapace and gills of the live crustaceans and eating the remaining soft parts with vinegar and salt. At other times the crabs are first killed by shaking them with ordinary salt and after 24 hours, during which time partial decomposition takes place, they are ready to be eaten. In order to determine the relation of these methods of eating the crabs to the epidemiology of paragonimiasis, observations were made on the effect of the condiments used on the viability of the cysts. In one experiment, the crabs were shaken with salt and it was noted that the crustaceans were either dead or moribund 7 hours later. When examined at this