Sea otters' consumption of food contaminated with deadly pathogens may be slowing their recovery in California, where they're endangered, new research suggests.

A study published in this week's Proceedings of the National Academy of Sciences shows that southern sea otters that feed on small marine snails — an inferior food source compared with the abalone they prefer — were more likely to be infected with Toxoplasma gondii, a parasite shed by cats that can cause deadly encephalitis (brain swelling). Among otters foraging in the areas of San Simeon and Cambria, those feasting on marine snails had a whopping 95 percent chance of developing toxoplasmosis, compared with only 22 percent of those gobbling abalone.

Study co-author Tim Tinker, a research wildlife biologist at the U.S. Geological Service, told that the research confirms that the animals' poor diet – triggered by a limited supply of abalone — was to blame for its slow recovery. When there's plenty of food and fewer otters, the animals tend to feed on the same prey. But when otters become more abundant, they deplete the foods they prefer, including red urchins and abalone. (Disease has also depleted abalone in California waters, he says.)

"The diet diversification that occurs with reduced food availability probably exposes sea otters to more disease pathogens," Tinker says. "The increasingly nutritionally stressed sea otters are exposed to a wider array of parasites, a double-whammy that does not bode well for population growth."

California sea otters were nearly wiped out from fur trading in the Pacific Northwest in the 18th and 19th centuries. There are now about 2,700 of them, significantly fewer than the estimated thousands that were there before the beginning of the fur trade.

Knowing what and where the otters eat, though, may help reduce their exposure to toxoplasmosis, Tinker says. If the parasites are entering the sea via storm drains or sewage treatment outflows, for example, more sewage treatment may cut that risk, he says.

Image of sea otter by Tania Larson/U.S. Geological Survey