Leptospirosis Lethal for Two-Thirds of Infected California Sea Lions

This year’s outbreak of leptospirosis in California’s sea lions has affected more individuals than in previously recorded episodes. Between the end of June and early October, nearly 400 marine mammals were reported stranded or sickened suffering from the zoonotic pathogen.  

—

With two months left in 2025, an outbreak of leptospirosis in California’s sea lion population has already surpassed the numbers recorded in previous years. Since 1980, when comprehensive records were first kept, the rate of bacterial infection appears to spike every three to five years. 

This year, however, the number of cases recorded by September had far outstripped cases typically recorded for an entire year. Between the end of June and early October this year, some 400 marine mammals suffering from leptospirosis were reported stranded or sickened along California’s coast. 

Leptospirosis is caused by a spiral-shaped bacteria. The zoonotic pathogen can infect humans, domestic and wild mammals including seals and sea lions. It affects the kidneys, the organs responsible for filtering toxins and regulating hydration. In marine mammals, dehydration is a particularly problematic consequence as the marine mammals do not have ready access to fresh water and instead get their hydration from food.

The bacteria causes severe abdominal pain in sea lions as it damages their kidneys and inflames their gastrointestinal tracts. The animals are known to continuously press their flippers against their belly – “a classic sign of lepto,” according Giancarlo Rulli, a volunteer and spokesperson with the Marine Mammal Center.

“They hold their stomach just like that. Like a sick child with a bellyache,” he told the LA Times.

The Marine Mammal Center – the largest marine mammal hospital in the world – treats infected sea lions with antibiotics and fluids. But approximately two-thirds of the sea lions diagnosed with the infection do not survive. 

Leptospirosis is believed to spread among California sea lions through their urine either as a result of direct contact or indirectly from contaminated soil or water. The opportunity for infection is heightened at times when they congregate together on beaches and piers. 

Factors that may affect the severity of the outbreak include level of herd immunity, migration patterns and sea surface temperatures. In Europe, both global warming and changes in rainfall patterns have been found to potentially increase the disease burden, with more frequent extreme weather events and floods poised to pose the highest risk for more leptospirosis infections in the future. 

The opportunity to assess 40 years of data has been forestalled by a hold on federal research funding due to cuts in the US federal budget earlier this year and the ongoing government shutdown. 

Leptospirosis is not the only ailment to be affecting the California sea lion population. 

Domoic toxicosis and avian flu have also been consequential. 

Domoic toxicosis is caused by the presence of the single-celled plant Pseudo-nitzchia sometimes referred to as red tide. The toxin is concentrated in the tissues of contaminated crustaceans like crabs and lobsters. Consuming tainted shellfish and fish can cause seizures and heart failure in sea lions. 

The appearance of a new highly contagious variant of avian flu – or H5N1 – is also a cause for concern. It appears that cases no longer require a bird-to-mammal link for transmission. The likelihood that it can be spread directly among marine mammals carries serious implications for the rapidity of transmission.

Illnesses, changes in food availability, casualties from ship strikes and fishing gear entanglements, and warming ocean temperatures have raised concerns over whether ocean conditions are reaching a critical point, with serious implications for the continued survival of sea lions and other marine mammals.