image - sea lion

This sea lion, Grazer, is seen curled up with his flippers folded tightly over his abdomen prior to his rescue by trained Marine Mammal Center responders in Monterey. The posture is known as "lepto pose" and is an indication the sea lion is suffering the effects of the disease. Photo courtesy of Marine Mammal Center

Sea lions sick with leptospirosis have been showing up on California beaches in near record numbers, the Marine Mammal Center reports. As a result, staff and volunteers at the center have been busy in a normally quiet time of the year. 

Leptospirosis is a bacterial disease that was first diagnosed in sea lions in the 1970s, says center Director of Veterinary Science Dr. Shawn Johnson. It can be present in humans and other mammals as well, but a strain that affects marine mammals — and sea lions in particular — is sickening the pinnipeds at a rate not seen since 2004. The center has confirmed more than 250 sea lions have come under its care because of the disease so far this year and the number continues to climb. 

“We’re still rescuing animals every day,” Johnson said recently. “So we could easily become the largest outbreak.” 

The center sees cases of leptospirosis every year but the outbreaks appear to be cyclical with a large one showing up every five years or so. The last notable one was in 2011 and infected nearly 200 sea lions, according to the center. 

While sea lions with the disease are showing up along the entire California coastline, Johnson says there appears to be more in Monterey, Santa Cruz and San Mateo counties. He adds that there are generally more sea lions in this area this time of year so that doesn’t necessarily point to the region as being more susceptible to the disease. 

Of the roughly 220 infected sea lions identified in October, about 30 had been found along the San Mateo County coast, Johnson said, noting that his office receives calls for about five sickened sea lions per day. 

Now, in late November, the center reports that the rate of incoming sick sea lions has slowed. But, with the number of leptospirosis-infected sea lions topping 250, this cycle still comes in as the second-highest outbreak ever recorded.  Sea lions with the disease are often found with their flippers tucked in and their bodies scrunched in response to abdominal pain. The disease affects their kidneys which may prompt them to drink seawater or eat the sand in response to the reflex that’s telling them to drink, Johnson said. 

Normally, marine mammals do not consume water directly as they get all the hydration from the fish they eat. 

As part of their treatment back at the Marin-based center, the sea lions receive fresh water, antibiotics and other support. In spite of this treatment, roughly two-thirds of the sick sea lions will die from their infections, according to the center. 

The cause of the periodic outbreaks is not entirely clear but could be attributed to changes in ocean temperature, migration shifts or a lack of herd immunity. 

The center has partnered with researchers from the University of California, Los Angeles, to better understand the outbreaks. The researchers are collecting blood and urine samples from young sea lions at Año Nuevo to determine whether they have any evidence of kidney disease or antibodies that could point to previous exposure to the disease. 

Scientists are puzzled by the fact that sea lions appear to be more likely to catch the disease than other pinnipeds. 

“Why are sea lions so susceptible to this infection whereas elephant seals and harbor seals aren’t?” Johnson said. “There’s a lot of unanswered questions.” 

Because a sea lion with leptospirosis could potentially pass on its infection to humans, beachgoers are asked to keep their distance. 

Beachgoers who see a sick sea lion are asked to call the Marine Mammal Center rescue line, (415) 289-7325. 

The center is also looking for volunteers to help monitor the beaches for sick animals or to help out at the Marin headquarters. 

For more information on volunteering, visit the Marine Mammal Center website at

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