During March to August of 2017, hundreds of leopard sharks ( Triakis semifasciata) stranded and died on the shores of San Francisco Bay, California, US. Similar mass stranding events occurred in 1967 and 2011, but analysis of those epizootics was incomplete, and no etiology was confirmed. Our investigation of the 2017 epizootic revealed severe meningoencephalitis in stranded sharks, raising suspicion for infection. We pursued a strategy for unbiased pathogen detection using metagenomic next-generation sequencing followed by orthogonal validation and further screening. We showed that the ciliated protozoan pathogen, Miamiensis avidus, was present in the central nervous system of leopard ( n=12) and other shark species ( n=2) that stranded in San Francisco Bay but was absent in leopard sharks caught elsewhere. This ciliated protozoan has been implicated in devastating outbreaks in teleost marine fish but not in wild elasmobranchs. Our results highlight the benefits of adopting unbiased metagenomic sequencing in the study of wildlife health and disease.
Keywords: Epizootic; San Francisco Bay; leopard shark; meningoencephalitis; metagenomic next-generation sequencing; scuticociliate.