Page:Tropical Diseases.djvu/854

798 that the miracidium, on being hatched out, entered some undetected mollusc; but further than this conjecture could not venture until Nakagawa, Kakami, Myairi, Yoshida, and other Japanese investigators showed that on escape from the egg the miracidium enters a fresh- water snail probably Melania libertina or M. obliquegranulosa—wherein it undergoes the usual developmental changes of sporocyst and redia, eventuating in the formation of cercariae. The cercarite in due time escape into the water, and, finding their way into certain species of fresh-water crabs—Potamon obtusipes, Eriocheir japonica, Potamon dehaanii—encyst themselves in the liver, muscles, etc., and especially in the gills of the crustaceans, and undergo developmental changes. Their further progress is not exactly known, but in dogs or cats fed on infected crabs the cercariae, on reaching the ileum of these vertebrates, escape from their cysts in from twenty-four to forty-two hours, penetrate the gut wall and pass into the peritoneal cavity, and a few of them, traversing the diaphragm, enter the lungs; a few pass into the liver, muscles, and other organs and tissues, and, after about ninety days, having attained sexual maturity, produce eggs. It is still doubtful as to how the encysted cercaria escapes from the crab, and as to how it obtains, in nature, access to man. However it may be in Japan, where raw fish is often eaten, neither the Koreans nor the Formosans eat uncooked crabs, and yet P. westermani is very common in certain districts of Korea and Formosa. Possibly encystment in the crab is not a biological necessity for the parasite; it may be that the cercarias, which can penetrate the cuticle or organs of the crab, can also penetrate those of man, and that man is infected with P. westermani in the same way as he is infected with the three species of schistosomes—S. haematobium, S. mansoni, and S. japonicum.

Diagnosis.—Diagnosis of endemic haemoptysis is at once established by the discovery of the characteristic ova in the almost equally characteristic sputum. Rales and other physical signs of lung consolidation are not usually discoverable. If the intestine or liver is implicated, ova may appear in the stools.

In the case of one-sided convulsions, or in hemiplegic affections occurring in a native of or in a visitor from the countries in which this trematode is endemic, the sputum should be examined on the chance of discovering evidence of the parasite. Should ova be found, there is a strong presumption that the cerebral trouble arises from trematode tumour in the brain.