Metagenomic next-generation sequencing reveals Miamiensis avidus (Ciliophora: Scuticociliatida) in the 2017 epizootic of leopard sharks (Triakis semifasciata) in San Francisco Bay, California

Abstract

During March to August of 2017, hundreds of leopard sharks (Triakis semifasciata) stranded and died on the shores of San Francisco Bay, California, USA. Similar mass stranding events occurred in 1967 and 2011, yet analysis of these epizootics was incomplete and no etiology was confirmed. Our investigation of the most recent epizootic revealed severe meningoencephalitis in stranded sharks, raising suspicion for infection. On this basis, we pursued a strategy for unbiased pathogen detection using metagenomic next-generation sequencing followed by orthogonal validation and further screening. We show 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 absent in leopard sharks caught elsewhere. Whereas this protozoan has previously been implicated in devastating outbreaks in teleost marine fish, our findings represent the first report of a ciliated protozoan infection in wild elasmobranchs. This discovery highlights the benefits of adopting unbiased metagenomic sequencing in the study of wildlife health and disease.