August 11, 2025 - A familiar and troubling scene is unfolding once again at the Marine Mammal Center, as pens fill with sick and dying California sea lions. Veterinarians are seeing the telltale signs of a recurring, deadly kidney disease that spikes every few years along the coast.
The culprit is leptospirosis, a disease caused by the spiral-shaped bacteria Leptospira. Infected sea lions are often seen curled up with their flippers tucked over their bellies in pain, or exhibiting the unusual behavior of drinking water, a sign of severe kidney distress in an animal that typically gets all its hydration from the fish it eats.
Even with urgent treatment involving antibiotics and fluids, the prognosis is grim. According to the Marine Mammal Center, an estimated two-thirds of sea lions admitted with acute leptospirosis do not survive.
While the periodic outbreaks decimate sea lion populations, leptospirosis poses a much wider threat. The disease is described as "a major health burden for humans, domestic animals and wildlife worldwide," with over 500,000 severe human cases reported globally each year.
The bacteria is transmitted through the urine of infected animals. Sea lions can contaminate coastal waters and shorelines, posing a risk to other wildlife, pets, and humans who might come into contact with the contaminated environment.
Cases of leptospirosis in sea lions have been documented annually since the 1980s, but large-scale outbreaks, affecting a hundred or more animals, tend to occur in a predictable but mysterious cycle of every four to five years.
Researchers are still trying to understand why these major outbreaks happen. Collaborators at UCLA suspect a combination of factors may be at play, including changes in the sea lion population's herd immunity, shifts in sea surface temperatures, and evolving migration patterns. Unlocking this mystery is key to predicting future outbreaks and protecting both marine and human health.