A hundred years of controversy about the taxonomic status of Echinococcus species.

The parasitic diseases which we know today as cystic and alveolar echinococcosis are zoonoses known since antique times, and 1855, respectively. Whether the two clinically and morphologically distinct diseases were caused, according to a "unicistic" and a "dualistic" theory, by only one or two different cestode species was the subject of a fierce, 100 years long debate involving scientists from many countries. The natural life cycle of Echinococcus granulosus was fully clarified in 1855 after successful animal experiments. In contrast, the natural final and intermediate hosts of Echinococcus multilocularis remained unknown, and the advocates of either theory had to draw on a number of surrogate arguments to defend their positions. The seesaw of reasoning and mutual defeats of the two theories, and the final recognition of E. multilocularis as an independent species in the 1950s are described in this article.