By CHRIS DIXON

MONTEREY, Calif. - Three parasites, including one carried by outdoor cats, are being blamed for an outbreak of an nfectious disease that is killing California sea otters, according to researchers at the University of California at Davis and the state Department of Fish and Game.

The most serious parasite is Toxoplasma gondii, which causes a brain infection. Researchers say they believe that Toxoplasma spreads as outdoor cats' feces, laden with millions of parasite eggs, wash into the ocean from streams and drainage ditches.

Another brain parasite, Sarcocystis neurona, is spread similarly, but by opossums. These hardy eggs, or oocysts, are thought to be absorbed by filter-feeding shellfish that are otters' prey.

From 1997 to 2001, Dr. Melissa Miller, a pathologist at Davis, her partners there, the Fish and Game Department, the Morro Bay Foundation and the Central Coast Regional Water Quality Board examined 233 live and dead sea otters.They found startling rates of parasitic infections, particularly Toxoplasma. "Of live, presumably healthy, otters," Dr. Miller said, "42 percent had antibodies to Toxo. Of dead otters, 62 percent had antibodies."

The first signs of the threat were recognized about 1992, when otters, once nearly extinct, rebounded. From 100 along Big Sur, they had expanded their range south to Morro Bay and north to Santa Cruz. Numbering an estimated 2,300 in 1995, sea otters began to compete with fishermen for abalone and other shellfish. But wildlife experts began to see signs that Toxoplasma gondii and Sarcocystis neurona were killing sea otters. The findings were intriguing and troubling, Dr. Miller said, because a disease known only in land mammals had spread to marine mammals. If an otter has antibodies to Toxoplasma, Dr. Miller said, that generally means a chronic, persistent infection. Toxoplasma and Sarcocystis can create cysts in otters' brains. The resulting infections can cause seizures or on their twitching bodies.

In 1995, the United States Geological Survey noted that the population of California sea otters did not appear to be increasing at the expected annual 5 percent rate. Dr. David Jessup, a senior veterinarian at the state Fish and Game Department, said that had otter populations continued to grow apace, they would today total 3,500 and would be off the endangered species list. Last year, however, the population declined, to 2,100. In 2000, Dr. Christine Kreuder, a wildlife veterinarian who is a colleague of Dr. Miller's, started a mortality study as part of her doctorate in epidemiology. With an epidemiologist from the Wildlife Health Center, Dr. Jonna A. Mazet, and other Fish and Game pathologists, they found that 16 percent of all California otter deaths were linked to brain infections caused by Toxoplasma. Sarcocystis neurona killed 7 percent, and a seabird-borne intestinal parasite called Acanthocephalan peritonitis, or thorny-headed worm, killed 16 percent. The decline is alarming, Dr. Jessup said, because "the
otter is a sentinel species." "Their health tells us a lot of important things about the health of the ocean," he added. Dr. Mike Murray, a veterinarian at the Monterey Bay Aquarium, and Dr. Miller attribute the parasite problem in part to the loss of wetlands, which filter many pathogens and pollutants.

"Now runoff comes in and goes straight down a concrete channel to the ocean," Dr. Murray said. The otters, Dr. Kreuder added, are crucial to the health of kelp forests. "Without otters," she said, "sea urchins dine on the kelp and destroy the forest. And the forest just supports hundreds of species. If this level of disease persists, it's hard to imagine how the otter population can recover."

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updated 8/15/03