Deadly disease is spreading quickly among California sea lions

On a narrow spit of sand north of Santa Cruz, a young sea lion was lying curled on its side, flippers pressing and releasing against its belly in a slow, painful rhythm.

“That’s a classic sign of lepto,” said Giancarlo Rulli, a volunteer and spokesperson for the Marine Mammal Center, watching the animal’s wrenching self-embrace.

The corkscrew-shaped bacterium Leptospira attacks the kidneys and inflames the gut, leaving sea lions doubled over “like a sick child with a bellyache,” noted Rulli.

Deadly disease in marine mammals

Since late June, nearly 400 marine mammals have been reported stranded or sick along Central Coast beaches. More than two-thirds have died, Rulli said, and many more were likely washed away unseen or perished at sea.

The historically large and prolonged leptospirosis outbreak is compounding a brutal year for seals, sea lions, dolphins, otters, and whales.

These mammals are already affected by toxic algal blooms, shifting prey, ship strikes, and a fresh marine heat blob forming in the eastern Pacific.

Tracking the disease in sea lions

Leptospirosis in California sea lions was first reported along the West Coast in 1970. By the 1980s, the Marine Mammal Center and partners were tracking the disease in detail.

Typically, small, late-summer flare-ups lasting a month or two occurred, punctuated every three to five years by larger outbreaks. In 2011 and 2018, the last two big events, roughly 300 animals were rescued each time.

This year looks different. The outbreak began more than a month earlier than usual and has already surpassed past totals. The death toll appears worse, too. Leptospirosis often kills about two-thirds of those it sickens.

“It’s been a brutal year,” Rulli said. “It’s been hard on the animals, it’s been traumatic for the volunteers, It’s a lot.”

Scientists are still piecing together why. Classic population dynamics – large cohorts of never-exposed young animals passing the pathogen on crowded haul-outs – likely play a role. But other factors may be at work.

“We’re trying to build our understanding of how ocean conditions relate to the occurrence of disease,” said Jamie Lloyd-Smith, an ecologist and evolutionary biologist at UCLA. “It’s a work in progress. And the world is changing quickly underneath our feet.”

The mystery behind the microbe

The Leptospira species affecting sea lions also circulates in terrestrial scavengers – raccoons, skunks, coyotes – raising the possibility of cross-species transmission along beaches.

Whether land mammals introduce new strains to sea lions, or the reverse, isn’t yet known, said UCLA disease ecologist Katie Prager. Nor is the bacterium’s natural reservoir, a key piece Lloyd-Smith’s team is investigating.

What’s clear is the clinical picture: abdominal pain and self-hugging postures, dehydration, kidney injury, gastrointestinal inflammation, and often severe emaciation.

Outcomes hinge on how quickly animals are found, stabilized, and treated with fluids and antibiotics – an increasingly tall order as daily rescue calls stack up.

Rescuers rush to save sea lions

One late September afternoon below the Capitola wharf, two tight clusters of sea lions flopped together on a floating dock, flippers draped over one another, heads resting on neighbors’ bellies. Off to the side, a lone animal kept her distance.

A quick scan revealed another problem: one of the dock sleepers looked skeletal, hip bones and vertebrae cutting through what should have been blubber. Veteran rescuers Jeremy Alcantara and Patrick McDonald knew which animal needed help most.

They moved slowly. A curious gull bobbed nearby. Tourists leaned over the railing. With a practiced swoop, the team netted the emaciated sea lion, hustled her up the ramp, into a crate, and into the back of an air-conditioned van bound for the Marine Mammal Center’s Castroville clinic.

There, staff started antibiotics and fluids. She’s now at the center’s Sausalito hospital. Her name is Woodrow, Rulli said – “but [she] has not been receptive to offers of sustainable ground herring.” For now, she’s stable. The veterinary team will reassess in the days ahead.

The other sea lion rescued that morning, the one found on the Davenport Beach, was too far gone. After transport, staff humanely euthanized the animal.

Marine mammals battle heat and disease

Whether this year’s disease surge connects directly to broader ocean shifts remains an open question. But the backdrop is grim.

A historic domoic acid bloom sickened more than 2,100 animals along Southern California. The San Francisco Bay Area saw a record number of dead gray whales.

From San Diego to Crescent City, reports of humpbacks and grays ensnared in fishing gear soared. And a building marine heat wave threatens to deepen stress on already strained food webs. At the same time, federal support for the data and research that guide response efforts has faced political threats.

“Fortunately, these volunteers don’t give up,” Rulli said. “They’re completely dedicated.”

Disease spread quickly in sea lions

Leptospirosis has cycled through sea lion colonies for decades, but this season’s early start, scope, and lethality set it apart. It may go down as one of the most devastating years on record for California’s marine mammals, or as a warning of a new baseline.

For the rescue network, the work rarely stops. Since April, crews have fielded daily calls about sick sea lions, dolphins, whales, sea turtles, and seabirds.

Some days bring small wins – a net well-thrown, a crate latched, a stable animal taking a cautious bite of fish. Others end in quiet losses, notes on a whiteboard, and a drive back to the next call.

Above all, the season has laid bare two truths: how quickly disease can scythe through a social, coastal species, and how much of the response depends on people willing to answer the phone, haul the nets, and keep trying.

“It’s been a brutal year,” Rulli said again, then added what sounds like both a mantra and a plea: “We keep going.”

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