Adaptability of Schistosome Larvae to New Hosts

Abstract

species which can become adapted for development to several different species of intermediate hosts have a much better chance of entering new localities than those which are absolutely specific in one mollusc. The literature on the cercariae shows a number of species which can develop equally well in different species and even different genera of molluscs. The best known example of the relation of adaptability in intermediate hosts to the spreading of a digenetic trematode to new localities is the sheep liver fluke, Fasciola hepatica, which has become widely distributed to various parts of the world, adapting itself to some species of the snail genus Lymnaea found in each new region. Although the larval stages of the human blood flukes have been known less than five years, the records already show a surprising lack of specificity in intermediate hosts. Leiper (1916:411) records the cercaria of Schistosoma haematobium from Egypt in Bullinus contortus and Bullinus dybowski, and the cercaria of Schistosomta mansoni from Planorbis boissyi. Cawston (1917:133) records the cercaria of Schistosoma haematobium in South Africa from Physopsis africana, and in Venezuela Iturbe and Gonzalez (1917) found the cercaria of Schistosoma mansoni in Planorbis guadelupensis Sowerby. The cercaria of Schistosoma japonicum has so far been described from only one host, the katayama snail, Blanfordia nosophora. It is significant that this species is an operculate snail belonging to a different order of the Gastropoda from the intermediate hosts of S. haematobium and S. mansoni. In addition to these at least two of the species of forked-tailed cercaria develop as described by Faust (1915:122 and 1918: 105) in more than one species of intermediate host. Cercaria gracillima is described from Lymnaea proxima Lea and Physa gyrina Say from the Bitter Root Valley, Montana, and Cercaria gigas from Planorbis trivolvis Say and Physa gyrina Say from Illinois. My own studies on the forked-tailed cercariae from the United States have shown several striking examples of lack of specificity in the choice of intermediate host. Cercaria douthitti Cort which was described (1915:49) from Lymnaea reflexa Say taken near Chicago,