Background: Giardia spp. are flagellated protozoan parasites that infect humans and many other vertebratesworldwide. Currently seven species of Giardia are considered valid.

Results: Here, we report a new species, Giardia cricetidarum n. sp. in hamsters. Trophozoites of G. cricetidarum n. sp.are pear-shaped with four pairs of flagella and measure on average 14 μm (range 12–18 μm) in length and 10 μm(range 8–12 μm) in width. The trophozoites of the new species are generally larger and stouter than those of mostof the other Giardia spp. and exhibit the lowest length/width ratio (c.1.40) of all recognized Giardia species. Cysts ofG. cricetidarum n. sp. are ovoid and measure on average 11 μm (range 9–12 μm) in length and 10 μm (range 8–10μm) in width and are indistinguishable from the cysts of other Giardia species. Molecular phylogenetic analysesbased on beta-giardin, small subunit rRNA, and elongation factor-1 alpha loci all demonstrated that G. cricetidarum n.sp. is genetically distinct from all currently accepted Giardia spp. Investigation of the host range indicated that thenew species was only found in hamsters (including Phodopus sungorus, P. campbelli and Mesocricetus auratus), whileall the other described mammal-parasitizing species (G. muris, G. microti and G. intestinalis) each infect multiplehosts. Cross-transmission studies further demonstrated the apparent host specificity of G. cricetidarum n. sp. as itonly infected hamsters. Trophozoites were found in high numbers in hamster intestines (5 × 10 5 – 5 × 10 6 ) andwas rarely detected co-infecting with other Giardia spp. in the common hamster, suggesting it has someadvantages in parasitizing hamsters.

Conclusions: This study has identified a new species of Giardia, which appears to be specific to hamsters, andtogether with the three other mammal-parasitizing Giardia species with different host ranges, may be able to beused as a model system for the study of evolutionary divergence of host parasitism strategies in Giardia.