The ‘bridge effect’ by intermediate hosts may explain differential distributions of Echinococcus species

Zoonotic cestodes of the genus Echinococcus show marked differences in their distribution patterns which have not been satisfactorily explained. Echinococcus multilocularis is limited to the Holarctic, whereas species in the Echinococcus granulosus sensu lato (s.l.) complex mostly have a worldwide distribution. We proposed and tested a `bridge effect' hypothesis stating that the low virulence of species in the E. granulosus s. l. complex for their intermediate hosts and a longer lifespan of infected hosts explain the differential distributions, particularly in hot and dry regions. This does not exclude the contribution of other factors such as human-mediated dispersal and intermediate host distribution. In the light of globalization and climate change, understanding what drives the distribution of zoonotic parasites is critical for preventing outbreaks of disease caused by these pathogens.