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Surveys of the prevalence of the intestinal protozoan Giardia spp. in animal populations have relied almost exclusively on the detection of cysts in fecal samples. We have determined the prevalence of Giardia spp. in beaver and muskrat populations in four northeastern states and Minnesota by using both the detection of trophozoites in mucosal scrapings from live-trapped animals at necropsy and the detection of cysts in fecal samples collected from kill-trapped animals. In muskrats the prevalence of Giardia infection was 36.6% by cyst detection in fecal samples (n = 790) from kill-trapped animals and 95.9% in live-trapped muskrats when the intestinal contents were analyzed for the presence of trophozoites (n = 219). Similarly, in beavers, Giardia infection was 9.2% by cyst detection in fecal samples (n = 662) from kill-trapped beavers and 13.7% in live-trapped animals examined for the presence of intestinal trophozoites (n = 302). The detection of trophozoites in mucosal scrapings from live-trapped animals consistently yielded a significantly higher prevalence for both muskrats and beavers than did the method based on detection of cysts in the fecal samples. The prevalence of Giardia infection in juvenile and adult live-trapped muskrats was similar (92.5 and 94.4%, respectively), but the prevalence in juvenile live-trapped beavers (23.2%) was significantly greater than that seen in the adult animals (12.6%). No difference in Giardia prevalence on the basis of sex was seen in either animal species. Regional variation, often statistically significant, was seen in the prevalence of Giardia in beavers in the northeastern states and Minnesota, but was not detected for muskrats.

Giardia cysts isolated from humans, beavers, mice, and muskrats were tested in cross-species transmission experiments for their ability to infect either beavers or muskrats. Giardia cysts, derived from multiple symptomatic human donors and used for inoculation of beavers or muskrats, were shown to be viable by incorporation of fluorogenic dyes, excystation, and their ability to produce infections in the Mongolian gerbil model. Inoculation of beavers with 5 x 10(5) Giardia lamblia cysts resulted in the infection of 75% of the animals (n = 8), as judged by the presence of fecal cysts or intestinal trophozoites at necropsy. The mean prepatent period was 13.1 days. An infective dose experiment, using 5 x 10(1) to 5 x 10(5) viable G. lamblia cysts collected by fluorescence-activated cell sorting, demonstrated that doses of between, less than 50, and less than 500 viable cysts were required to produce infection in beavers. Scanning electron microscopy of beaver small intestine revealed that attachment of G. lamblia trophozoites produced lesions in the microvillous border. Inoculation of muskrats with G. lamblia cysts produced infections when the dose of cysts was equal to or greater than 1.25 x 10(5). The inoculation of beavers with Giardia ondatrae or Giardia muris cysts did not produce any infection; however, the administration to muskrats of Giardia cysts of beaver origin resulted in the infection of 62% of the animals (n = 8), with a prepatent period of 5 days. Our results demonstrated that beavers and muskrats could be infected with Giardia cysts derived from humans, but only by using large numbers of cysts.(ABSTRACT TRUNCATED AT 250 WORDS)