Brain-Eating Parasites Responsible for Record Number of Leopard Shark Deaths?

Leopard shark washed up on beach in San Francisco. (Tom Hilton [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons)

Leopard sharks in the hundreds have been washing up on beaches in the San Francisco Bay area since March this year, along with various species of other sharks and rays. The mass die-off has been prompting serious concern from fish pathologists, but Mark Okihiro, senior fish pathologist with the California Department of Fish and Wildlife, believes he has finally identified the cause.

Okihiro is leading the state investigation into the unexpected deaths. He estimates more than 1,000 leopard sharks, 200-500 bat rays, hundreds of striped bass, almost 50 smoothhound sharks, and a few thornback rays, guitarfish, and halibut have died in the bay this year, according to Bay Nature.

From several biopsies of sharks and fish that have been washed up on the beaches, Okihiro discovered “inflammation, bleeding, and lesions in the brain” from being eaten by a nasty little parasite, reported Sport Diver. Okihiro also observed hemorrhaging from the skin near vents and internal organs in some of the sharks.

With the help of a geneticist, Okihiro has identified the parasite as the brain-eating protozoan Miamiensis avidus. The protozoan appears as dark, hairy blotches under the microscope. It is a well-known parasite as it has been responsible for the rapid death of sharks in aquariums that have become infected, according to Sport Diver.

Scanning electron microscope showing the ciliate protozoan M. avidus Medina et al., 2016 [http://revistasinvestigacion.unmsm.edu.pe/index.php/rpb/article/view/12861/11482]