The food some California sea otters eat might carry deadly parasites that come from cats and opossums, a recent study suggests.

Scientists from the University of California-Davis and the U.S. Geological Survey say some marine invertebrates on which sea otters feed can act as transport hosts for lethal pathogens that have made their way into the ocean, confirming a theory that has long been reported.

The risk of exposure to the pathogens is higher among sea otters when clams, fat innkeeper worms and marine snails are part of their diet, researchers said.

The sea otter is listed as a threatened species by the federal government. Populations have recently experienced high mortality from infectious diseases among reproductive-age adults. The study could explain the phenomenon.

"Our findings indicate that prey choice in sea otters has very real implications for their health," said Christine Johnson, an epidemiologist at UC-Davis and co-author of the study.

The study, which conducted field research from 2000 to 2006, looked at 120 sea otters living off California's coast. Analysis of their diet and health found that otters most at risk of becoming infected with the parasites Toxoplasma gondii and Sarcocystis neurona are those living in southern Monterey Bay, near Cambria and San Simeon. The parasites are usually found in cats and opossums, respectively.

Researchers say sea otters living in the same area can develop diets different from one another as they compete for food. When they prey on less-desirable foods that can host pathogens, the diet can lead to higher levels of disease.

"Depleted resources and high rates of infectious disease may be acting in concert to limit the recovery of this threatened species," Johnson said. Tim Tinker, a research wildlife biologist with the USGS in Santa Cruz, co-led the study.

"Recovery of the sea otter in California has been especially sluggish at the center portion of its range, where sea otter densities are highest and where most of the reproduction occurs," he said. "Where food resources are limited, individual sea otters tend to become diet specialists."

Some of the specific foods can carry Toxoplasma gondii and Sarcocystis neurona.

If the pathogens enter the ocean, they can attach themselves to invertebrates that do not become infected, but act as transport hosts. Sea otters are infected after they eat pathogen-laced prey.

Andy Johnson, manager of the Sea Otter Research and Conservation Program at the Monterey Bay Aquarium, said scientists have hypothesized for several years that the food sea otters eat could infect them with disease because it carries pathogens found on land.

The findings of the study will allow scientists to look for solutions such as wastewater treatment and storm drain management aimed at keeping pathogens out of the marine environment, he said.

"We will be able to look and say, "Here is the problem, these are the factors that are affecting otters," Johnson said.

The complete results of the study can be found in the current issue of the Proceedings of the National Academy of Sciences.