Transfaunation Studies on the Host Specificity of the Enteric Protozoa of Rodents *

Breeding colonies of protozoa‐free laboratory rats ( Rattus norvegicus ) and golden hamsters ( Mesocricetus auratus ) were established by early weaning or by drug treatment. Attempts were made to transfaunate the enteric protozoa of various rodents to protozoa‐free progeny of these colonies. Efforts to transfer the fauna of guinea pigs ( Cavia porcellus ) to rats and hamsters have failed and indicate that this fauna is physiologically isolated as well as morphologically distinct from that of other laboratory rodents. The faunas of rats and hamsters appear to be morphologically identical and freely interchangeable. That certain of the species found in rats and hamsters also occur in the mouse ( Mus musculus ) is indicated by successful transfaunations: mouse to rat and hamster. The suggestion is made that the fauna of the mouse and rat is indigenous but that the fauna of laboratory hamsters may have been acquired since domestication. Hexamastix muris from Neotoma fuscipes and Trichomonas microti from Microtus pennsylvanicus have been transfaunated to rats. Other enteric protozoa from these hosts and also from Marmota monax and Dipodomys merriami have not established themselves in rats and, if morphological differences cannot be found, should be considered biological races of their counterparts in the fauna of laboratory rodents. Species of lumen‐dwelling enteric protozoa should be based upon morphological distinctions and not upon host specificity. The latter, as demonstrated by these experiments, is not rigidly determined.