Ability of a small wild rodent (Peromyscus maniculatus) to host strains of human‐type Giardia duodenalis

Wild deermice (Peromyscus maniculatus) were trapped in a forested area of the Fraser Valley of British Columbia, Canada. Animals were tested for parasites and treated with metronidazole to ensure absence of pre‐existing Giardia infection. Sera were collected from all animals and tested by enzyme‐linked immunosorbent assay (ELISA) to rule out previous infections with Giardia. Cysts from one of two G. duodenalis strains associated with human infections were inoculated into different groups of deermice. Cysts were tested for viability at the time of inoculation. Five different cyst concentrations and appropriate controls were included in each group of thirteen deermice used for each strain. After a suitable incubation period, feces from test and control animals were examined for cysts or trophozoites using both the formol‐ether concentration method and microscopic examination of iron‐hematoxylin stained preparations. Three of the four deermice inoculated with 1 × 10⁵ cysts of an isolate associated with an outbreak of human giardiasis were found to be infected. No infection was detected in any of the deermice inoculated with the Giardia strain isolated from a symptomatic child. The isolate that infected small rodents in these experiments only did so at a higher inoculum and in juvenile deermice. We confirm that wild deermice may act as hosts for some human source strains of the G. duodenalis morphological group.