As recently as 1938, California sea otters were believed to be extinct, killed off by 19th century fur hunters. But some of the furry creatures survived, and today more than 2000 populate the central California coast. Now, though, researchers say the otters face a new threat: cats. A new study finds that a deadly single-celled parasite carried by cats is surprisingly prevalent in otters--especially those living near the mouths of rivers or streams.
Toxoplasma gondii is best known for the threat it poses to the fetuses of pregnant women exposed to the protozoan's eggs, or oocysts, when cleaning their pets' litter boxes. Autopsies have revealed that some otters have died from brain infections caused by the parasite.
Puzzled by the appearance of the disease in a sea mammal, a team led by wildlife veterinarian Melissa Miller of the University of California, Davis, tested blood samples collected from 223 live and dead otters. In the July issue of the International Journal of Parasitology, Miller's team reports a staggering rate of Toxoplasma exposure: 42% of the live otters and 62% of the dead ones.
It's possible the parasite has always been in the ocean, but most scientists think it somehow got there from cats--the only known carrier of the oocysts. Toxoplasma oocysts are tough enough to withstand sewage treatment, and they could get into the water from dirty kitty litter flushed down the toilet. Few of the otters in the study lived near sewage outflows, making it hard to assess this possibility. However, the researchers did find a threefold higher risk of exposure for otters living near the mouths of rivers and streams, suggesting that storm-water runoff from fields and lawns frequented by cats could be a source of oocysts. The researchers are now examining shellfish eaten by otters to determine whether they concentrate oocysts. If so, they may pose a threat to humans.
Andy Johnson, manager of the sea otter program at the Monterey Bay Aquarium, praises the study for revealing the extent of the animals' exposure to what appears to be a new pathogen. Miller believes it's too early to offer advice to cat owners, but otter activists disagree. Cats should be kept indoors, says Joshua Cassidy, a wildlife biologist with the Monterey-based Friends of the Sea Otter. And used litter, he says, should go in the trash rather than the toilet.