Parasite in Cats Killing Sea Otters

Hunted to near extinction for their lush fur, sea otters are now protected under the federal Endangered Species Act. Their recovery, however, is being hampered by disease-causing agents spread in runoff. Photo: Christina Johnson

December 2, 2002

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

Offering a partial explanation to a mysterious decline in the southern sea otter population, scientists have established a strong body of circumstantial evidence linking cats to a lethal otter disease.

University of California at Davis professor Patricia Conrad and her doctoral student Melissa Miller, both in the School of Veterinary Medicine, have shown that otters near heavy freshwater flows are three times more likely to have been infected by Toxoplasma gondii - a potentially lethal parasitic protozoan that causes brain infections in otters and brain cysts in humans - than otters from areas where runoff is light.

In a survey of 233 live and dead otters from Santa Barbara to Half Moon Bay, a staggering 76 percent of those near heavy freshwater outflows - storm drains and river mouths - had antibodies to Toxoplasma gondii. There was also a surprisingly high rate of infection in the general otter population. Forty-two percent of live otters had antibodies to the parasite, an almost certain sign of infection. The research was funded by the National Sea Grant College Program and was conducted in collaboration with the California Department of Fish and Game, the California Regional Water Quality Control Board and the U.S. Geological Survey.

The scientists' best guess is that parasite eggs in cat droppings are being washed by sprinklers and rains into coastal-bound storm drains and creeks. Although many different kinds of animals, such as birds and rodents, can serve as intermediate hosts for Toxoplasma gondii, cats are the only animals known to shed the parasite's eggs in their droppings. This cat-parasite link is the reason pregnant women are advised against cleaning cat litter boxes.

University of California sea otter researchers Pat Conrad, left, and Melissa Miller examine a lung scan of a dead sea otter. Photo: University of California.

Otters may be acquiring parasites directly through water contact, or they may be eating infected mussels or other bivalves. Parasite eggs however, have not yet been found in wild bivalves, but they are logically suspect since they filter huge amounts of water feeding on plankton and thus trap pollutants in their tissues. Otters also feed voraciously on shellfish.

Southern sea otters are listed as threatened under the federal Endangered Species Act. Once numbering more than 300,000, there were fewer than 1,500 otters in 1982. For more than a decade, otter numbers rose, hitting a peak in the spring of 1995, when there were an estimated 2,377 individuals. The recovery, for reasons that are still unexplained, appears to have stagnated or slid backward. There were an estimated 2,100 otters off California in the spring of 2002.

While the scientists are not certain how much of this decline can be attributed to Toxoplasma gondii infections, Miller's Sea Grant research suggests that about 60 percent of dead otters in her survey had been infected by the parasite. New research led by a graduate student at the University of California at Davis suggests that many of these otters likely died of toxoplasma encephalitis.

Not just otters but also people are potentially at risk from the parasite. People eat many of the same shellfish as otters. There has also been a documented outbreak of human toxoplasmosis in British Columbia, shown later to have been caused by contaminated drinking water, presumably from cat droppings.

Though a potentially serious human health threat, Toxoplasma gondii is only one of many waterborne protozoans that may be entering beach waters via runoff. A new California Sea Grant study is looking at one of the more worrisome of these, Cryptosporidium, widely regarded as one of the most significant causes of diarrhea in humans.

Leading the project are Rob Atwill, also at the School of Veterinary Medicine at Davis, and Conrad. Taking cues from sea otters, Atwill and Conrad are measuring pathogen levels in bivalves near outfalls of human and agricultural runoff, to track the upstream sources of pollution. Genetic tests are also being used to identify which animal species are the main sources of pathogen pollution. Wildlife, cattle, pets and people can spread Cryptosporidium.

The scientists are also working with dairies along the coast to test the degree to which management practices, such as planting vegetative buffer strips, can reduce pathogen pollution.