About 40 species have been described from different animals, but many of them are probably synonyms. Currently, five to six morphologically distinct species are recognised. Giardia lamblia (=G. intestinalis, =G. duodenalis) infect humans and other mammals, G. muris is found from other mammals, G. ardeae and G. psittaci from birds, G. agilis from amphibians and G. microti from voles. Other described, (but not certainly valid) species include:
Genetic and biochemical studies have revealed the heterogeneity of Giardia lamblia, which contains probably at least eight lineages or cryptic species.
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