Postmortem findings of free-ranging North American beavers (Castor canadensis) reveal potential threats to California freshwater ecosystems

North American beavers (Castor canadensis) are semi-aquatic rodents recognized as keystone species that increase the diversity of freshwater ecosystems. This study aimed to characterize the mortality and pathological findings of free-ranging beavers in California and, based on these results, establish the potential threats to freshwater ecosystems. This study included eighteen beavers submitted for postmortem examination between 2008 and 2024 at the California Animal Health and Food Safety Laboratory, UC Davis. Gross and microscopic examination, bacteriological, parasitological, immunohistochemical, and molecular techniques were used as tools to diagnose the cause of death/reason for euthanasia and comorbidities in the beavers. Encephalitis caused by the larva migrans of Baylisascaris spp. was the most prevalent (9/18) cause of mortality/reason for euthanasia followed by bacterial infections in 6 individuals. In these 6 animals, bacterial bronchopneumonia was diagnosed in two (Staphylococcus aureus and a mix of gram-negative and -positive bacterial infection) and Listeria monocytogenes encephalitis, bacterial myofascitis (Aeromonas bestiarum and Pasteurella multocida), bacterial encephalitis (Acinetobacter towneri), and tularemia (Francisella tularensis) 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 by a suspected capillarid species, cerebral toxoplasmosis, Giardia 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 into new areas without consideration and/or mitigation represents a potential risk for pathogen introduction.