Sea lions in Monterey Bay battle return of deadly disease

MONTEREY — More than 220 California sea lions, 73 from Monterey County, have been hit by a kidney infection that can be fatal in the second most severe outbreak in recorded history.

The Marine Mammal Center, whose response range spans from Mendocino to San Luis Obispo County, is scrambling to keep up with the influx of patients. Over those 600 miles of coastline, they’ve seen the most cases of infection in Monterey County. In second is Santa Cruz County, where 38 sea lions have been diagnosed with leptospirosis, a disease spread through urine.

Two out of three sea lions who catch leptospirosis do not survive. The factors that contribute to this mysterious disease are being investigated by UCLA biologists, and the Marine Mammal Center in a cooperative research project.

Years when a major leptospirosis outbreak occurs, certain age and gender groups of sea lion populations decline. But Katie Prager, a UCLA researcher who has been studying the environmental and behavioral factors that influence leptospirosis outbreaks for eight years, puts it in perspective this way: “…the sea lion populations have continued to grow at a very healthy rate,” Prager said. “At this point, it’s not a conservation concern.”

Julia O’Hern, operations manager of the Marine Mammal Center’s Monterey Bay Operations in Moss Landing, says this outbreak is especially challenging. Usually pups, and subadults are infected, but this time adults are catching leptospirosis too. “It’s a lot more work moving big sea lions around, and it’s expensive; it takes a lot of antibiotics, it’s hard to administer the antibiotics, and you just go through supplies quickly,” she explained.

She and her team have rescued sick sea lions from every accessible beach in Monterey County she said. San Carlos State Park and Del Monte Beach in the city of Monterey have been hot spots.

Sea lions with leptospirosis are often lethargic, in a fetal position, tucked over, with their flippers folded over their belly, O’Hern said. It takes two to four volunteers to rescue stranded adults.

O’Hern and the Moss Landing team of about 90 volunteers treat the sea lions with a “triage” of fluids, antibiotics, and a fish offering like herring. Once the sea lions are stable enough for a two- hour drive, they transport them to the Sausalito center, so they can receive intensive care.

Research on how leptospirosis originates in sea lion populations and the environmental and behavioral factors that cause widespread outbreaks, like this one, is still emerging.

Leptospirosis cycles through sea lion populations every four to five years and is spread through urine. The last time an outbreak occurred in California was in 2011; from 2013 to 2017 the disease went dormant.

“I was very hopeful, and there was speculation that the maybe disease had burnt out, that it was no longer cycling in the population. Then all of the sudden after four years of being completely gone, it just sprang back, and it came back with a fire… a fury,” said Dr. Shawn Johnson, director of Veterinary Science at the Marine Mammal Center.

Those same years the disease disappeared, “the ocean had a fever,” said Johnson. Warmer-than-average water threw off sea lion migration routes, made it harder for sea lion mothers to find reliable food sources for the family, and was likely the culprit for thousands of sea lion pup strandings.

The effects this warming period had on sea lion populations could be a reason why the current leptospirosis outbreak is particularly severe.

“Since it had been such a long time since the last outbreak, the animals that were born in 2012 and 2013 grew and got really big by 2018. Because they had never been around this disease, they hadn’t been in any outbreaks, they were susceptible [to leptospirosis],” Johnson explained.

Prager, echoed Johnson, and explained that the period of ocean warming may have lowered “herd immunity.” Herd immunity explains how the flu vaccine works — if more individuals in a population are protected against a disease, the overall population will be more resistant to the disease, as well.

Wild animals don’t receive vaccines, but for them, “getting sick with an infection and recovering with antibodies is the equivalent to being vaccinated against a disease,” Prager said. “The herd immunity of the sea lion population would have gone super low during this period where leptospirosis was absent, so when it reenters the population you can see a major outbreak.”

The severity, and size of a leptospirosis outbreak in sea lions, is one diagnostic for ocean health. “Aside from being wonderful, curious, intelligent creatures, sea lions are indicator species. They tell us how the ocean is doing as a whole, how the fish populations are doing; we really just learn so much from them,” O’Hern said.

Residents, and their pets, should keep a safe distance from sea lions. If you see a sea lion you think may be sick, report it to the Marine Mammal Center at: 415-289-SEAL.