Apparent expansion of virulent <i>Vibrio parahaemolyticus</i> in humans and sea otters

Vibriosis is the most important public health threat from seafood consumption and marine recreation. Pathogenic <i>Vibrio</i> spp. employ virulence factors, including hemolysins and secretion systems, frequently detected in human cases, but virulence data in northern and southern sea otters (<i>Enhydra lutris kenyoni</i> and <i>E. l</i>. nereis, respectively) are limited despite their potential as marine bioindicators. Genomic epidemiology was used to characterize virulence factors of <i>Vibrio</i> spp. genomes (<i>n</i> = 570), including <i>V. alginolyticus</i> (<i>n</i> = 55), <i>V. diabolicus</i> (<i>n</i> = 52), non-O1/O139 <i>V. cholerae</i> (<i>n</i> = 163), and <i>V. parahaemolyticus</i> (<i>n</i> = 287) collected in North America (2000–2019). Virulence factors of <i>V. parahaemolyticus</i> were compared between isolation sources: bivalves, environment, humans, and southern and northern sea otters. Hemolysins (<i>tdh</i>, <i>trh</i>) and type III secretion system 2 (T3SS2) gene prevalences were lowest in environmental isolates, while <i>tdh</i> and T3SS2 gene prevalences were higher in human and northern sea otter isolates than those from southern sea otters. A hemolysin allele (<i>trh1</i>) was detected almost exclusively in human and sea otter isolates. Despite <i>V. parahaemolyticus</i> genomic diversity, detected genomic clusters were comprised of highly related and <i>tdh</i>⁺/<i>trh</i>⁺ genomes from nonenvironmental sources including humans and sea otters. Observed pathology in <i>Vibrio</i> spp. positive sea otters frequently included septicemia, enteritis, and moderate-to-severe melena. Co-occurrence of T3SS2 and T6SS1 in <i>V. parahaemolyticus</i> was associated with pathological findings and ampicillin-susceptible genotypes, suggesting a trade-off between virulence and antimicrobial resistance. Based on these findings, <i>V. parahaemolyticus</i> undergoes selection pressures resulting in apparent expansion, i.e. genomic clustering, of <i>tdh</i>⁺/<i>trh</i>⁺ virulent strains infecting humans and sea otters.