Diseased sea lions suddenly showing up from Monterey to Bay Area

San Francisco Chronicle​

08-08-2025

A deadly disease is spiking among California's sea lions — and dog owners taking beach walks should stay on alert, experts say
In July, the Marine Mammal Center's Sausalito hospital checked in more than 100 sea lions — many of them from Monterey and Santa Cruz counties — suffering from the bacterial disease leptospirosis. That number wouldn't be unprecedented later in the year, but center spokesperson Giancarlo Rulli said an outbreak this early represents 'a bit of a phenomenon.'
'We've never seen 100-plus California sea lions come into our hospital with leptospirosis in the month of July before,' Rulli said. 'The open-ended question that we are very much preparing for is that there is no clear indication as to how long this may last.'
The center only treated about 50 sea lions for the disease in the first half of the year, Rulli said, and did not anticipate a surge until late summer or fall. The disease has historically surged every three to five years, but it last spiked off the California coast as recently as fall 2023.
Although the center's goal is to rehabilitate and release sea lions, Rulli said that more than two-thirds typically do not survive the disease, either dying in transport or at the hospital. The disease can cause irreversible kidney damage — and while they're not the only marine mammals that are susceptible, Rulli said sea lions currently represent the 'vast majority' of leptospirosis's victims.
Tightly-tucked flippers and unusual drinking habits are tell-tale signs of diseased sea lions. They're also typically lethargic and might appear to be sleeping on the beach, leading some passersby to approach for a closer look. But Rulli said that beachgoers, especially those with dogs, should give the animals a wide berth.
'This can be fatal to dogs and it often shows up as fever well after the animal has already contracted the bacteria,' Rulli said. 'That could be anything from picking it up on their paws, to the unfortunate circumstance where a dog closely approaches a sea lion impacted by the disease and picks it up from open orifices like their eyes, nose and mouth.'
Rulli urged dog-owners to keep their pets leashed and proactively seek out available leptospirosis vaccines. He's not yet aware of any dogs in the Bay Area dying from the current outbreak. There's also no evidence that the sea lion that bit a child in Monterey last week had any exposure to the disease.
In July, the Marine Mammal Center took in an 'outsized share' of infected sea lions from Santa Cruz and Monterey counties, where 40% of the center's rescues already take place. But the disease has already reached sea lions in Marin, Sonoma, San Mateo and San Francisco counties, Rulli said, in part due to the large expanse of ocean that sea lions are able to cover in a single day.
Experts remain unclear about what's driving the disease's early peak. But Rulli said the outbreak pattern has generally become harder to predict since the 2015 marine heat wave known as 'the Blob.' Every year since 2017, he explained, leptospirosis numbers have trended higher than in the 40 years prior to that anomaly.
Katherine Prager, a UCLA ecologist who studies the disease, stressed the difficulty of proving a clear connection between the Blob and the current outbreak. Observing patterns across long-term data sets could be increasingly difficult in light of ongoing cuts to federal funding, she added — making events like this year's leptospirosis spike harder to explain.
'Predicting any outbreak is difficult, no matter what,' Prager said. 'Oceanographic conditions play into it, but disentangling the complex relationship between oceanographic conditions and the final outbreak can be quite difficult.'