Systemic Scuticociliatosis (Philasterides dicentrarchi) in Sharks

Abstract

Scuticociliatosis is an economically important, frequently fatal disease of marine fish in aquaculture, caused by histophagous ciliated protozoa in the subclass Scuticociliatida of the phylum Ciliophora. A rapidly lethal systemic scuticociliate infection is described that affected aquarium-captive zebra sharks ( Stegostoma fasciatum), Port Jackson sharks ( Heterodontus portusjacksoni), and a Japanese horn shark ( Heterodontus japonicus). Animals died unexpectedly or after a brief period of lethargy or behavioral abnormality. Gross findings included necrohemorrhagic hepatitis and increased volumes of celomic fluid. Histologically, 1 or more of a triad of necrotizing hepatitis, necrotizing meningoencephalitis, and thrombosing branchitis were seen in all cases, with necrotizing vasculitis or intravascular fibrinocellular thrombi. Lesions contained variably abundant invading ciliated protozoa. Molecular identification by polymerase chain reaction from formalin-fixed tissues identified these as the scuticociliate Philasterides dicentrarchi (syn. Miamiensis avidus), a novel and potentially emergent pathogen in sharks.