The otter study may help explain why populations of the frisky animals have not recovered from the 19th-century fur trade, researchers said.

"Before I started this project I didn't think about things like how much cat faeces gets into the environment - how what we dump on our lawns and sidewalks flows into streams to rivers and into the ocean," said Patricia Conrad, a professor of parasitology at the University of California, Davis.

Conrad said 38 percent of live sea otters she tested carried antibodies to Toxoplasma gondii, a single-celled parasite that causes toxoplasmosis.

It was in 52 percent of dead otters, Conrad told a meeting of the American Association for the Advancement of Science.

"What the sea otters are telling us is the problem is far larger than a worry about cat litter. These parasites are in the environment," Conrad said.

Toxoplasmosis does not usually make people or cats sick, but is a known threat to pregnant women, who are cautioned against handling cat litter or cat droppings.

In otters, it gets into their brains. When Conrad studied the bodies of dead otters, they found 17 percent had "very severe lesions" in their brains caused by the protozoa.

While the parasite infects rodents and birds, they are unlikely to spread it. "Of all the animals that get infected with this parasite ... cats are the only host in which the parasite undergoes sexual multiplication and results in the tough, egg-like stage that is passed in the feces," Conrad said.

It may be getting into mussels and oysters, and Conrad's team is testing that idea now.

"Probably the most important thing we can do is manage our cats more responsibly. The most important thing is probably to keep cats indoors," advised Conrad, a cat owner.

SEAL VIRUS

Frances Gulland of the Marine Mammal Center in Sausalito, California, has found that 18 percent of stranded adult California sea lions have cancer. Her team has linked this with a herpes virus called otariine herpesvirus-1, which is related to a virus called HHV-8 known to cause Kaposi sarcoma in people with the AIDS virus.

The seals with cancer also have higher levels of chemicals known as PCBs in their fat, Gulland said. PCBs are pollutants produced by several industries and also linked with human cancer.

In a separate study, Gulland found that diatoms, tiny floating sea creatures, produce a toxin that can kill seals. The toxin, called domoic acid, is found in "blooms" of algae that have become more common off the US Pacific Coast.

Similar algal blooms called red tides are becoming more common off the Gulf Coast and Gregory Bossart of the Harbor Branch Oceanographic Institution in Fort Pierce, Florida, has found in the past that manatees can be sickened by toxins produced by the algae called brevetoxins.

On Saturday he presented evidence these toxins can also affect people, and can be found in sea grasses even without a red tide present.

"The toxin might not make you sick or kill you (but) it may predispose you to other diseases," Bossart told a news conference.

"To me that is significant because these red tides are becoming more common in the Gulf of Mexico."