Postmortem Findings in Free-Ranging North American Beavers (<i>Castor canadensis</i>) Reveal Potential Threats to California’s Freshwater Ecosystems

North American beavers (<i>Castor canadensis</i>) are semi-aquatic rodents recognized as keystone species because they increase the diversity of freshwater ecosystems. This study aimed to characterize the mortality and pathological findings in free-ranging beavers in California and, based on these results, identify potential threats to freshwater ecosystems. This study included 18 beavers submitted for postmortem examination at the California Animal Health and Food Safety Laboratory, UC Davis, between 2008 and 2024. Gross and microscopic examinations, and bacteriological, parasitological, immunohistochemical, and molecular techniques, were used as tools to diagnose the cause of death/reason for euthanasia and comorbidities in the beavers. <i>Baylisascaris</i> spp.-associated or -suspected encephalitis was the most prevalent (9/18, 50%) cause of mortality/reason for euthanasia, followed by bacterial infections in six individuals. In these six animals, bacterial bronchopneumonia was diagnosed in two (<i>Staphylococcus aureus</i> and a mix of Gram-negative and -positive bacterial infection) and <i>Listeria monocytogenes</i> encephalitis, bacterial myofascitis (<i>Aeromonas bestiarum</i> and <i>Pasteurella multocida</i>), bacterial encephalitis (<i>Acinetobacter towneri</i>), and tularemia (<i>Francisella tularensis</i>) were diagnosed in one beaver each. Three animals died or were euthanized due to non-infectious causes, including motor vehicle trauma, squamous cell carcinoma, and capture cardiomyopathy. Endoparasitism was the main comorbidity, including granulomatous hepatitis caused by a suspected capillarid species, cerebral <i>toxoplasmosis</i>, <i>Giardia</i> infection, gastric nematodiasis, and cecal trematodiasis. In California, beavers are exposed to various pathogens that represent threats to humans, domestic animals, and wildlife. Since the interspecies transmission of these pathogens occurs in rivers, streams, lakes, and ponds, we suggest that studying beaver health can reflect freshwater ecosystem health. This study also indicates that the translocation of beavers to new areas without consideration and/or mitigation represents a potential risk of pathogen introduction.