The body is piriform to ellipsoidal, and bilaterally symmetrical. The anterior end is broadly rounded, and the posterior end is drawn out. There is a large sucking disc on the ventral side; the dorsal side is convex. There are 2 anterior nuclei, 2 slender axostyles, 8 flagella in 4 pairs, and a pair of darkly staining median bodies. The cysts have 2 or 4 nuclei and a number of fibrillar remnants of the trophozoite organelles. A synonym of this generic name is Lamblia Blanchard, 1888.
The names given the species of Giardia depend on the authorities concerned. Traditionally, it has been believed that Giardia is highly host-specific, and different names have been given to almost all the forms in different hosts. Thus, if we accept the names in Ansari's (1951, 1952) review, the species in cattle is G. bovis, that in goats and sheep G. caprae, that in the dog G. canis, that in the cat G. cati, that in the rabbit G. duodenalis, that in the guinea pig G. caviae, those in the Norway rat G. muris and G. simoni, and those in the house mouse G. muris and G. microti. However, Filice (1952) was unable to find any morphological difference between the giardias of the laboratory rat and a number of wild rodents, and on reviewing the literature discovered that almost no acceptable cross-transmission studies exist between some species. Altho he did not discuss them all, he appears to have concluded that there are only two species of Giardia in mammals, each with a number of races. G. muris occurs in the mouse, rat and hamster, and G. duodenalis in the rabbit, rat, chinchilla, ground squirrel, deermouse, pocket mouse, man and presumably ox, dog, cat and guinea pig, among others. The essential difference between these two forms is that the median bodies of G. muris are small and rounded while those of G. duodenalis are long, resemble somewhat the claw of a claw-hammer, and lie approximately transversely across the body.
In this connection, Hegner's (1930), Armaghanfs (1937) and Haiba's (1956) success in infecting laboratory rats with Giardia from man suggests that Filicefs view may eventually prevail. However, careful cross-transmission studies must be carried out before a decision can be made. In the meantime, it is more convenient to use different specific names for most of the forms from different hosts.
Associated with this nomenclatorial problem is an important epidemiological one. If it turns out that Giardia can be freely transmitted from one host to another, we shall have to revise our ideas about the danger to man of infections in laboratory and domestic animals, and of infections in one domestic animal to others. Here is an area of ignorance which deserves exploration.
Giardia has not yet been cultivated in artificial media, a fact which has hampered studies both of its epidemiology and pathogenicity. However, Karapetyan (1960) cultivated G. lamblia in chicken fibroblast tissue cultures.