When Melissa Miller, a veterinarian at the Department of Fish and Game in Santa Cruz, wrote an International Journal of Parasitology paper describing how a cat parasite that causes brain lesions was reaching Central California’s sea otters in 2002, she also put out a press release. Reporters were intrigued. After all, how could Toxoplasma gondii, which usually infects cats, be killing animals at sea?
A journalist heard a possible explanation from a scientist at a nearby aquarium: kitty litter could pass on the parasite to sea otters through sewage emptying into the ocean. Miller was doing a sea otter autopsy and couldn’t be reached for comment. The reporter, on deadline, ran with the story.
“Something about that story seemed to capture popular imagination, and it really took on a life of its own,” says Tim Tinker, a marine biologist who studies sea otters with the United States Geological Survey in Santa Cruz.
There was just one problem. Miller’s study hadn’t found a link between sewage outflows carrying your cat’s litter and sick sea otters. As the main transmission path it made little sense because “sewage doesn’t go directly to the ocean,” but is instead processed before being released, says Oliver Kwok, a microbiologist of the USDA’s Agricultural Research Service in Beltsville, Maryland who studies Toxoplasma gondii.
Miller’s paper actually told a very different story. It showed that water that sweeps across land and into streams was carrying infectious oocysts of Toxoplasma gondii—likely from the feces of outdoor cats—into the ocean, where it reached sea otters (Int J Parasitol, 32:997–1006, 2002).
Over the next several years, Miller and her colleagues pieced together the disease’s path from land to sea. Karen Shapiro, a veterinarian at UC Davis, wanted to see whether healthy or eroded wetlands were better at transmitting these eggs. She released colored microspheres that mimicked the eggs in both environments. Active wetlands filtered out a million times more oocysts than eroded ones, likely because water moves slowly, allowing the eggs to settle. The eggs may also stick to the reeds in wetlands. The implications were clear: erosion of vegetation in coastal sloughs could be worsening the spread of the parasite.
In 2008, the picture got even cloudier—Miller and Tinker found evidence that cast doubt on domestic cats as the main culprits. In one study, eight of nine otters who died of T. gondii were infected by Type X of the parasite, a genetic strain found mostly in bobcats, mountain lions, and other wild felines. Most domestic cats carry a different type, called Type 2, Tinker says. They’ve also noticed “hot spots” of infection around Cambria and San Simeon—relatively pristine environments where few people or house cats live, he says.
Haydee Dabritz, a veterinarian at the California Department of Public Health, found that wild cats had more than double the infection rates of house cats and shed millions more eggs at a time. They also tend to poop near streams, which could easily carry the oocysts to the ocean. Still, there are 80 million house cats and less than 1 million bobcats in the United States, according to Defenders of Wildlife. So it’s hard to imagine domestic cats don’t play some role, she says. “It looks suspicious, but we don’t have the numbers to say one way or the other.”
In the end, T. gondii is just one of several causes of sea otter decline. Though many sea otters exposed to the disease never get sick, it takes root in animals with weakened immune systems caused by starvation or chemicals in their environment. Another cause of death: infected wounds from biting noses during mating, since male otters are particularly aggressive in this region. “The only place where we see noses ripped right off is California,” says Tinker.
T. gondii has been killing fewer otters in recent years, Tinker says, and there was never much evidence even in infected animals that “kitty litter had anything to do with it,” he says. It’s frustrating to have a complicated picture passed off as a simple story, he adds. “We get that a lot at talks. People will ask, ‘Don’t we already know that kitty litter is the whole problem with sea otters and we’ve fixed that?’” he says.