Hundreds of California sea lions have been stricken by an often deadly bacterial infection and are washing ashore daily up and down the coast, raising fears among biologists that an already soaring death toll could continue to rise.

Veterinarians at the Marine Mammal Center in the Marin Headlands so far this year have treated 220 sea lions for the malady known as leptospirosis, a pathogen that causes liver failure.

The outbreak, to date, is the second-largest since state officials began keeping track of the infectious disease in 1970, but the numbers are growing. The sick and dying sea lions were found along hundreds of miles of Northern California coastline, including San Francisco Bay, said Shawn Johnson, the mammal center’s director of veterinary science.

“It means that we’re very busy,” said Johnson, who is overseeing the treatment of 30 to 35 sick sea lions. “It doesn’t look like the numbers are declining. We’re still getting an average of six and sometimes as many as 10 animals every day.”

Historically, two-thirds of infected sea lions die from the disease. The ones that the center has rehabilitated are typically released back into the sea.

The leptospirosis bacteria, which can infect people who come in contact with sea lion urine, was first identified in pinniped colonies along the coasts of California, Oregon and Washington nearly 50 years ago. There have been almost yearly leptospirosis outbreaks since 1983, officials said. The largest occurred in 2004, when 306 sea lions were admitted to the center. In 2011, 200 infected animals were treated by veterinarians.

“The magnitude of the outbreaks vary year to year with very large outbreaks occurring every three to five years,” said Katie Prager, a veterinarian with the Lloyd-Smith Laboratory, who, with her colleagues at the UCLA Department of Ecology and Evolutionary Biology, has been collaborating with the Marine Mammal Center for a decade.

Prager believes a combination of factors may be responsible for the increase in cases this year, including changes in herd immunity, sea surface temperatures and sea lion migration patterns.

One confounding factor is that the disease all but vanished between 2013 and 2017, a disappearance that Prager said was “unprecedented.” But, she said, a lot of sea lions that had been exposed, but not killed, by the pathogen may have developed antibodies, essentially immunizing the colony.

Prager also believes El Niño weather patterns might have had something to do with the lack of disease. The unusually warm Pacific Ocean waters caused food shortages, which forced mothers to swim farther than normal to find prey. That, in turn, meant they were not hanging out as much in groups, where disease transmission occurs.

The strange weather conditions of the past few years also caused lower birth rates and the starvation deaths of a great many pups, which, in a kind of ironic twist, means they were saved from contracting leptospirosis.

This year, the sea lion population rebounded and so did the disease. Prager said the majority of leptospirosis cases occur between July and November, which means veterinarians can expect infection rates to continue rising for another month. The researchers are worried that global warming could be changing what, thus far, has been a relatively predictable pattern.

“We don’t know, because of how the climate is changing, whether we can count on the pattern ... to function as it has historically,” Prager said.

The outbreak is, nevertheless, giving scientists a chance to study the causes of the disease and try to improve treatment. Prager plans to take blood and urine samples over the next few weeks from juvenile sea lions at Año Nuevo Island, off the San Mateo County coast, to look for evidence of kidney disease or antibodies, which would show past exposure. The idea is to compare the data with blood and urine samples taken from stranded animals over the past 43 years to determine whether there are patterns.

The infected animals often fold their flippers over their stomachs and are unusually thirsty. They are currently treated with antibiotics, fluids and gastroprotectants for stomach and intestinal ulcers.

“These animals are very sick,” said Johnson, who urged anyone who spots a stranded sea lion to contact the Marine Mammal Center at 415-289-7325.

People and dogs that come in contact with contaminated urine, water or soil can be infected, he said, “so it’s important to take precautions to not interact with these animals.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com. Twitter: @pfimrite