Abstract

Between 15 August 2018 and 4 September 2018 a mortality event involving free ranging smooth dogfish (Mustelus canis) occurred along Coney Island Beach and Brighton Beach in Brooklyn, New York, USA. Four live-stranded sharks were recovered from the beach and were brought to the New York Aquarium for care and possible release. Despite supportive care, all sharks died or were euthanized within 48 hours. Sharks were found on or near the beach by the public. They were minimally responsive when handled and when stimulated to swim had difficulty orientating, had a loss of righting reflex, or would only swim for short distances before sinking to the bottom. Upon further investigation, the New York City Parks Department reported removing five dead dogfish from the beach every morning for the previous two weeks. The New York Department of Environmental Conservation approved a disease investigation. An additional four dead sharks were recovered from the beach for gross necropsy, histopathology, and ancillary testing.

A total of eight smooth dogfish were examined post-mortem. Significant gross and histologic findings were limited to the brain. The olfactory lobe and bulbs were soft, friable, and normal anatomy was difficult to discern. The cerebral spinal fluid (CSF) was diffusely discolored pale pink. The leptomeninges had pinpoint multifocal hemorrhages, with an increase in opacity along the olfactory lobe and cranial cerebrum. Impression cytology of the brain was performed in seven animals. In six of those animals, large numbers of scuticociliates and granulocytic inflammation were apparent. Direct wet-mount cytology of the CSF was performed in five of eight animals. In three of the samples live scuticociliates were seen. Histopathology confirmed the presence of a severe, acute, necrotizing meningoencephalitis due to a scuticociliate protozoan infection in all sharks examined. Excised portions of infected brain from two smooth dogfish were examined under oil immersion (100x objective magnification) as wet mounts using a compound microscope with differential interference contrast optical components, which confirmed that the ciliates comprised a species of Scuticociliatida (Ciliophora).

Most scuticociliates are ubiquitous free-living inhabitants of estuaries and coastal zones¹ but a few are known as opportunistic pathogens of marine ray-finned fishes² and sharks^3,4 in aquaculture⁵ and public aquarium^4,6,7,8. Our findings herein resemble the 2017 mortalities of leopard sharks (Triakis semifasciata; San Francisco Bay, California, USA) that had intense brain infections of Philasterides dicentrarchi (as Miamiensis avidus).³ The phenomenon of scuticociliates causing mortality events in wild shark populations, such as in the current study and San Francisco Bay³, is remarkable in the context of aquatic pathogens among wild fish populations because seldom are such intense infections associated with strandings and species-specific fish kills.

Acknowledgements

The authors thank the New York Aquarium Animal Care staff for their stranding response and dedicated care for the live-stranded sharks. The authors also thank the New York City Parks Department and the New York State Department of Environmental Conservation for their partnership and collaboration in this disease investigation.

* Presenting author

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