Researchers are trying to understand why there is a surge in the number of California sea lions in Monterey Bay stricken by a potentially deadly bacterial infection.

This year researchers report more than 100 animals coming to the center with leptospirosis - a bacterial disease that affects the kidneys and can be deadly if animals are left untreated.

"And we are still in the midst of our year," says Dr. Jeffrey Boehm, executive director at the Marine Mammal Center. "When we tally the numbers up, we're going to see another year like one of those surge years."

Every four to five years since the 1970s, the center has seen a surge in the number of leptospirosis cases in California sea lions. During these years, the center sees a 10-fold increase in the number of infected individuals, to as much as 45 percent of the population, according to Boehm.

Most of the sick animals this year are coming from Monterey Bay and farther south, the center reports.

Staff at the center do their best to nurse infected animals back to health, but the center is also trying to determine when and why the sea lions contracted the disease in the first place.

Leptospirosis, which is caused by spiral-shaped bacteria, was first documented in California sea lions in the early 1970s. Many mammal species, including dogs and humans, carry the bacterium, which can leach into water or soil and survive there for weeks to months. The disease is then

passed to sea lions from exposure to contaminated urine, water or soil.

If left untreated, the disease can lead to kidney damage, meningitis, liver failure and respiratory distress.

Researchers don't know the primary source of the bacteria, Boehm says, but it's one of the critical questions they hope to answer with their study.

In July, a sick sea lion was rescued after spending more than 10 days stranded in a shallow Santa Clara aqueduct near San Tomas Expressway. But she was suffering from a different illness, brain damage caused by exposure to domoic acid, a neurotoxin found in certain algal blooms, and was euthanized.

The Marine Mammal Center is devoted to caring for stranded animals from Mendocino County to San Luis Obispo, including the Monterey Bay. It treats 500 to 600 marine animals a year; roughly half of those are sea lions.

The center advises people and their dogs to stay away from marine mammals they see on the beach and to call the center's 24-hour response line at (415) 289-SEAL should they come across any marine mammal that looks ill.