1Veterinary Specialty Hospital North County, San Marcos, CA, USA; 2Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA, USA; 3Marine Wildlife Veterinary Care and Research Center, California Department of Fish and Wildlife, Santa Cruz, CA, USA; 4Wildlife Health Center, School of Veterinary Medicine, University of California-Davis, Davis, CA, USA; 5Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Stoneville, MS, USA; 6United States Department of Agriculture, Agricultural Research Service, Warmwater Aquaculture Research Unit, Stoneville, MS, USA; 7USDA-ARS, Warmwater Aquaculture Research Unit, Stoneville, MS, USA; 8Aquatic Animal Diseases Laboratory, Aquaculture Division, National Institute of Oceanography and Fisheries, Suez, Egypt; 9College of Veterinary Medicine, University of Georgia, Athens, GA, USA; 10Department of Pathology, Microbiology, and Immunology, School of Veterinary Medicine, University of California-Davis, Davis, CA, USA
Abstract
Southern sea otters (Enhydra lutris nereis) (SSO) were nearly killed off by humans and are now a federally-listed threatened population of ∼3,100 animals. Erysipelothrix rhusiopathiae is believed to be a significant, but under-studied SSO pathogen. Erysipelothrix rhusiopathiae infection is associated with three main forms of disease, local cutaneous, diffuse cutaneous, or septicemic.1 Cetaceans are particularly susceptible to the septicemic form of disease, which is also known to cause fatalities in marine and terrestrial birds and mammals, including humans.2-5 The pathogen can persist for long periods in the aquatic environment, in part by colonizing the mucosal surfaces of fish, marine mammals, invertebrates, and reptiles.6,7 The objectives of this study were to 1) describe pathology associated with Erysipelothrix sp. infection, 2) characterize the genotypic diversity and bacterial sensitivity of SSO Erysipelothrix sp. isolates, and 3) evaluate the virulence of any novel Erysipelothrix species using an in vivo fish model.
Between 1998 and 2021 there were six confirmed cases of Erysipelothrix sp. at the Marine Wildlife Veterinary Care and Research Center (California Department of Fish and Wildlife, Santa Cruz, CA, USA) from >600 necropsied SSO while no isolates were obtained from >120 live-sampled SSOs. Bacterial septicemia was a primary or contributing cause of death in five of the six cases. Other pathology observed in Erysipelothrix sp.-infected SSOs included suppurative lymphadenopathy, fibrinosuppurative arteritis with thrombosis and infarction, bilateral uveitis and endophthalmitis, hypopyon, petechia and ecchymoses, mucosal infarction, and suppurative meningoencephalitis and ventriculitis. Special stains revealed short to long slender gram-positive or gram-variable bacterial rods within lesions, alone or in combination with other opportunistic bacteria, and Erysipelothrix sp. were isolated in pure culture from half of the cases.
Sixteen virulence genes were evaluated for each isolate using multiplex PCR and multi-locus sequence typing analysis was conducted using eight housekeeping genes to generate a maximum likelihood tree comparing SSO isolates to previously isolated samples from marine mammals, terrestrial mammals, and fish. All six SSO isolates had the spaA isotype—three clustered with terrestrial animal spaA E. rhusiopathiae strains via multi-locus sequence typing, and one clustered with marine mammal spaA E. rhusiopathiae strains. These four isolates had identical virulence gene profiles. The two remaining isolates had unique virulence profiles and did not cluster with any known Erysipelothrix sp.; whole genome sequencing revealed a novel Erysipelothrix species and novel E. rhusiopathiae subspecies.
Laboratory injection of tiger barbs (Puntigrus tetrazona) as an in vivo model resulted in infection and mortality for the two novel Erysipelothrix sp. Antimicrobial testing suggests that SSO Erysipelothrix strains are more resistant to conventional antibiotics, including many used to treat infection in cetaceans, than isolates obtained from fish and other animals, thus posing additional risks with respect to human transmission and bacterial spread between animals.9 This is the first description of the pathology, microbial characteristics, and genetics of Erysipelothrix spp. infections in SSOs, a federally-listed threatened species. Methods presented here can facilitate case recognition, help prevent zoonotic transmission, aid characterization of Erysipelothrix sp. isolates, and illustrate assessment of virulence using fish models.
Acknowledgements
The authors gratefully acknowledge the One Health Institute, Karen C. Drayer Wildlife Health Center, and the Students Training in Advanced Research Program at the University of California-Davis for their support of this study. We also thank the California Department of Fish and Wildlife and Marine Wildlife Veterinary Care and Research Center staff for their support for SSO necropsies and microbiology and immense help, especially Katie Greenwald, with organizing and sharing case data, cryopreserved isolated, and histopathology slides. Thank you to the many individuals and agencies that have helped recover sea otter carcasses, including the California Department of Fish and Wildlife, Monterey Bay Aquarium, the U.S. Geological Survey, The Marine Mammal Center, and the U.S. Fish and Wildlife Service. We thank the Tropical Aquaculture Laboratory for providing the tiger barbs.
*Presenting author
+Student presenter
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