Beavers (Castor canadensis) have been implicated in waterborne
transmission of Giardia (see Wilson et al., (1986) Waterborne
giardiasis outbreak - Alberta. Canada Diseases Weekly Report 8-20,
97-98) but Erlandsen et al. ((1988) Cross-species transmission of
Giardia spp.: Inoculation of beavers and muskrats with cysts of
human, beaver, mouse and muskrat origin. App. Envr. Microbiol. 54:
2777-2785) demonstrated that there is evidence for cross-species
transmission. It is likely that Giardia can exist in a wide variety
of mammalian hosts. As to species differences, there is Giardia
lamblia, Giardia muris, Giardia agilis, Giardia psittaci, Giardia
microtus, G. ondatrae, and G. ardae.... I don't know if the other
species are known to infect human hosts.
p.s. Beware of those 'gut' feelings when you are working with
intestinal protozoa - ha-ha. I know, I know - bad joke.