The California coast is seeing the second largest outbreak of sick sea lions in recorded history. Since January, 19 sea lions infected with leptospirosis, a potentially fatal bacterial infection, have been picked up on Marin County shores. The Sausalito-based Marine Mammal Center has responded to 220 infected sea lions—more than half of all those rescued this year across the state. The center is warning beachgoers to be especially careful to stay away from any stranded sea lions because humans—and dogs—can become infected with the bacteria if they come into contact with contaminated urine, water or soil. In the Point Reyes National Seashore, marine ecologist Sarah Codde said there have been “quite a few reports in last two months about sick sea lions.” Still, she said, the park was not sure if they were infected with leptospirosis or suffering from something else because the Marine Mammal Center had not provided test results. Leptospirosis was first detected in sea lions in California in 1970, and seasonal outbreaks have occurred since the 1980s. In a press release, Dr. Shawn Johnson, the Marine Mammal Center’s director of veterinary science, said major outbreaks of the disease typically occur in a cyclical pattern every four to five years. Researchers believe that many factors, including changes in herd immunity and the temperature of the sea’s surface, lead to a surge in outbreaks, with a majority of cases occurring between July and November. According to Giancarlo Rulli, the center’s marketing and communications associate, Marin’s count of 19 is “in the middle in terms of counties we’ve responded to,” he said. Sonoma has seen 24 infected sea lions; Monterey has seen over 70. Mr. Rulli said the majority of the outbreak has occurred from Monterey northward. One way veterinarians can tell if a sea lion is carrying the leptospirosis bacteria is its desire to drink water—something marine mammals generally do not need to do, since they receive enough moisture from food sources. The kidneys of sea lions infected with leptospirosis stop functioning, however, and thus cannot regulate hydration or filter out toxins. Once diagnosed with the disease, sea lions are given antibiotics, fluids and care for stomach and intestinal ulcers. But even with treatment, roughly two-thirds of infected animals will die. If you see a sea lion stranded on the shore, call the Marine Mammal Center at (415) 289.7325.