Salmon poisoning disease in grizzly bears with population recovery implications

Abstract

ABSTRACTWildlife restoration programs depend on having animals that have evolved to handle the constraints, including disease, posed by the new environment. Grizzly bears (Ursus arctos horribilis) once thrived while feeding on salmon (Onchorhynchus spp.) and other foods in California, Oregon, Washington, and Idaho, USA. Unlike Canadian and Alaskan salmon, salmon in lower latitudes often carry an encysted fluke (Nanophyetus salmincola) containing an endosymbiont (Neorickettsia spp.) that can produce a deadly disease in ursids and canids called salmon poisoning disease (SPD). Current recovery plans for grizzly bears in the North Cascades of Washington and the mountains of central Idaho, where infected salmon currently occur, call for using bears from several interior populations. We hypothesized that such bears with no history of salmon consumption will be sensitive to SPD. To test that idea, we fed a limited amount of fresh Chinook salmon (O. tshawytscha) harvested from rivers draining the east and west sides of the Cascades to 11 captive grizzly bears in which both parents were from populations with no history of SPD. We detected N. salmincola ova in the feces, and 8 bears exhibited typical SPD symptoms, which included decreased or no appetite, lethargy, vomiting, and diarrhea. We identified N. helminthoeca and Stellantchasmus falcatus (SF) agent (Neorickettsia sp.) in the salmon, but only SF agent occurred in the sick bears. Stellantchasmus falcatus agent is a relatively new arrival in the Pacific Northwest. Because of the severity of the disease and the potential for mortality without treatment, additional studies should be conducted to determine the distribution and prevalence of SF agent in salmon and kokanee (O. nerka) in the recovery area, develop a serum antibody test that could be used to determine exposure of bears to SF agent, and use that test to determine if grizzly bears or American black bears (U. americanus) currently living in the Pacific Northwest have been infected and recovered. If either of the latter have occurred, SPD may not prevent the successful restoration of grizzly bears where SF agent‐infected salmon will be consumed. © 2018 The Wildlife Society.