Between February and July of this year multiple shark strandings occurred in San Francisco Bay, California, baffling scientists and marine life conservationists. After months of research, a California Fish and Wildlife pathologist has identified a microbial pathogen that may have caused these deaths.

Leopard sharks and bat rays started washing up along the Bay Area coastline earlier this year. More particularly, there were reports of dead sharks, rays, and other fish found on beaches near the Golden Gate Bridge, according to a SFGate report. At the time, experts estimated the death count to be in the thousands, with some presumed to have died underwater and without washing ashore.

Later on, the incident began to affect other marine species. A sevengill shark was found beached near Oakland, California. The mysterious death of the sevengill shark posed a significant problem. According to Sean Van Sommeran of the Pelagic Shark Research Foundation, sevengills live in deeper waters compared to leopard sharks. If they are also getting sick and dying, it could indicate a more widespread and serious infection.

Mark Okihiro, CDFW’s senior fish pathologist, examined the stranded sharks and discovered lesions around their brains. It was surmised that something was entering the sharks’ noses and making their way to their brains and eating them, National Geographic reported. This is said to have caused disorientation among the infected sharks and ultimately leading to their deaths.

Hanna Retallack, an expert focusing on the genetics of infectious diseases at the University of California, San Francisco, studied Okihiro’s leopard shark samples and used a next-generation sequencing technique to look at the DNA and RNA in the cerebrospinal fluid to identify its origin. 99 percent of the genetic material present in the fluid was identified as “shark,” while the remaining one percent matched a fish-killing parasite called Miamiensis avidus.

Miamiensis avidus is a ciliate protozoan known to have caused the deaths of fish in hatcheries and farms, such as olive flounders farmed in Japan and South Korea.

The leopard shark strandings in San Francisco, California is said to have caused by a brain-eating parasite. [Image by barbaraaaa/Thinkstock]

The samples examined included five sharks that died in San Francisco Bay and two others that died in other areas within the same period. Those that died in San Francisco Bay were found to have Miamiensis avidus.

To further verify her initial findings Retallack used another technique called polymerase chain reaction. This time, samples from 13 leopard sharks were used, nine of which were extracted from the Bay Area. The DNA of said parasite was found in all nine sharks, while none of it was found in sharks that died outside the Bay Area.

Although Okihiro is confident they’ve identified the culprit behind the shark die-offs in San Francisco, questions remain as to why and how the Miamiensis avidus is affecting leopard sharks and other fish along the city’s coastline.

Leopard sharks, rays, and other fish were found beached near the Golden Gate Bridge. [Image by Meghan McGrath/Getty Images]

In a previous discussion, Okihiro said he thinks the parasite may have been transmitted by infected fish to leopard sharks, which is known for swimming into shallow waterways to mate and spawn during the spring and summer. Crowding makes it easier to spread diseases, thus contributing to the shark strandings. Another factor also being considered is the increase in freshwater due to heavy rains that hit Northern California this year. It’s also possible that the rain may have flushed toxic inland water into the sea. Both elements weaken sharks’ immune systems, making them more prone to diseases.

Although leopard sharks aren’t categorized as endangered, the incident still causes concern as it could potentially affect more marine life and spread to other areas. There is also particular interest in protecting leopard sharks and maintaining their population as it is considered a “signature species” in San Francisco.

[Featured Image by barbaraaaa/Thinkstock]