Tularemia is an infectious zoonotic disease caused by one of several subspecies of the bacterium Francisella tularensis. In the spring of 2007, the National Park Service discovered an unusually high number of deceased free-ranging beavers (Castor canadensis) on a lake within Voyageurs National Park, Minnesota. The autolyzed carcasses of five beavers were recovered and submitted to the University of Minnesota Veterinary Diagnostic Laboratory for necropsy. Tularemia caused by F. tularensis subsp. holarctica (type B) was diagnosed in all five animals. The diagnosis was based on postmortem findings of pyogranulomatous splenitis and hepatitis, molecular detection (polymerase chain reaction from spleen samples), and antigen detection (direct fluorescent antigen detection assay from spleen samples). Attempts to isolate F. tularensis from spleen samples using sheep blood agar with cysteine and chocolate agar were unsuccessful, possibly due to the advanced autolysis of the tissues. In the fall of 2007, beavers in the region were captured (and released) for collection of fecal and serum samples. Fecal samples obtained from 94 beavers were negative for F. tularensis by culture. Serum samples collected from 22 beavers were negative for F. tularensis-specific antibodies by microagglutination testing at the Centers for Disease Control. Future studies aimed at increasing the understanding of the impact of tularemia on the beaver population in this region are planned. Individuals working with free-ranging beavers should be mindful of the potential for exposure to F. tularensis.
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