Pathology, microbiology, and genetic diversity associated with and novel spp. infections in southern sea otters ().

spp., including , are zoonotic bacterial pathogens that can cause morbidity and mortality in mammals, fish, reptiles, birds, and humans. The southern sea otter (SSO; ) is a federally-listed threatened species for which infectious disease is a major cause of mortality. We estimated the frequency of detection of these opportunistic pathogens in dead SSOs, described pathology associated with infections in SSOs, characterized the genetic diversity and antimicrobial susceptibility of SSO isolates, and evaluated the virulence of two novel isolates from SSOs using an fish model. From 1998 to 2021 spp. were isolated from six of >500 necropsied SSOs. spp. were isolated in pure culture from three cases, while the other three were mixed cultures. Bacterial septicemia was a primary or contributing cause of death in five of the six cases. Other pathology observed included suppurative lymphadenopathy, fibrinosuppurative arteritis with thrombosis and infarction, bilateral uveitis and endophthalmitis, hypopyon, petechia and ecchymoses, mucosal infarction, and suppurative meningoencephalitis and ventriculitis. Short to long slender Gram-positive or Gram-variable bacterial rods were identified within lesions, alone or with other opportunistic bacteria. All six SSO isolates had the genotype-four isolates clustered with strains from various terrestrial and marine animal hosts. Two isolates did not cluster with any known spp.; whole genome sequencing revealed a novel species and a novel subspecies. We propose the names sp. nov. and ssp. nov. respectively. The type strains are UCD-4322-04 and UCD-4724-06, respectively. Experimental injection of tiger barbs () resulted in infection and mortality from the two novel spp. Antimicrobial susceptibility testing of isolates from SSOs shows similar susceptibility profiles to isolates from other terrestrial and aquatic animals. This is the first description of the pathology, microbial characteristics, and genetic diversity of isolates recovered from diseased SSOs. Methods presented here can facilitate case recognition, aid characterization of isolates, and illustrate assessment of virulence using fish models.