It enters by the nostrils, wiggles upwards through olfactory pathways and plants itself in the brain. Once invaded, victims can't be treated and will most certainly die.

The parasite is called Miamiensis avidus, and it's the pathogen most likely responsible for this year's mass die-offs of several Bay Area fish species.

SEE ALSO: Leopard sharks dying by the hundreds in San Francisco Bay

Mark Okihiro, senior fish pathologist at the California Department of Fish and Wildlife, has been studying the ill-fated fishes for months now, and he recently estimated over 1,000 leopard sharks and 200-500 bat rays have washed ashore in Bay Area waters. After necropsies, Okihiro discovered the creatures' brains were ravaged by an unknown invader.

Okihiro originally attributed the deaths to a fungal pathogen, but the mystery dreepened as dead fish continued to litter beaches. Many of these corpses showed almost no signs of death by fungal invasion, although earlier samples had.

raThe pathologist doubled back on his research efforts, recalling an epizootic of Miamiensis that he'd seen at a white seabass hatchery. The infection he saw decimate the hatchery's juvenile seabass population turned out to be the same pathogen likely killing leopard sharks.

Okihiro's findings read like gruesome science fiction: The shark corpses, some of which were partially mummified upon discovery, showed "severe necrotizing and inflammatory lesions in the olfactory bulbs and lobes of the brain." Lining the creatures' brains and noses were traces of protozoa, single-celled organisms that often exhibit animal-like behaviors.

After blazing through the Bay Area leopard shark population, the protozoa appear to have moved on. According to Okihiro's statement, shark deaths in the San Francisco Bay have dwindled as of late July, so much so that he deemed the epizootic shark strandings largely over – for now.

This year's leopard shark epizootic is a case of repeated history. In 2006 and 2011, scientists also witnessed mass die-offs resultant of fast-spreading pathogens, said Okihiro. These years coincided with wet winters, which drowned the bay in freshwater runoff and lowered ambient water salinity. When spring arrived, and leopard sharks began aggregating by the hundreds for mating and pupping season, the pathogen spread with a vengeance.

Read Michelle Robertson’s latest stories and send her news tips at mrobertson@sfchronicle.com.