HELMINTH LIFE CYCLES

Abstract

Although classical work continues, particularly on the trematodes, for the most part the basic life cycle patterns must be regarded as worked out. Helminthologists are increasingly turning attention to the mechanics of the life cycle with the result that parasitology is becoming an experimental science. Worms are now under study from the biochemical, physiological, and behavioral points of view. As a result, ideas of specificity have been altered, new physiological races of parasites discovered, and a number of new human helminthiases recognized, chiefly zoonotic and involving larval forms. The number of parasitic worms successfully adapted to laboratory propagation in animal hosts steadily increases, providing new and fruitful models for investigation. Even more striking is the great breakthrough in the last decade involving in vitro culture. Several cestodes and a variety of nematodes can now be reared through various stages, if not their entire life cycle, in partially defined media, and progress is being made with trematodes and acanthocephalans. Behavioral studies are beginning to shed light on how parasites find their hosts and how they locate each other inside the host. And in spite of long-held views to the contrary, evidence increases that certain activities of helminths may be under hormonal control.