Host specificity in trematode parasites ranges from strict specificity, in which a species of trematode infects only one species of host, to moderate specificity, in which a species may infect a range of closely related host species, all the way to low specificity, in which a trematode may infect different orders of hosts (e.g. herons and pelicans) or even different classes of hosts (e.g. birds and mammals). Proof of specificity in the past relied on experimental infections of hosts. But recently, DNA technology has allowed us to prove that trematodes from different hosts are genetically identical. Brachylecithum mosquensis, a common liver fluke of American robins, was recently matched to liver flukes from a vagrant shrew, Sorex vagrans, from Montana. The common link between these dissimilar hosts is a fondness for snacking on carpenter ants.
See more here: Kinsella, J. M., and V. V. Tkach. 2009. Molecular identification of an avian dicrocoeliid, Brachylecithum mosquensis, in a vagrant shrew, Sorex vagrans, from Montana. Comparative Parasitology 76:287-289.
Image and post by Mike Kinsella.