Toxoplasmosis: Parasite from cat faeces killed four sea otters in California

Four sea otters in California are reported to have died from infection with a rare strain of the parasite Toxoplasma gondii, which is primarily found in wild and domestic cats and commonly transmitted through their faeces. This is the first recorded instance of this severe form of toxoplasmosis in a marine animal, and could mean an unusually virulent strain of the parasite is circulating on land.

“Otters are really good at showing what comes from land to sea,” says Melissa Miller at the California Department of Fish and Wildlife. Their heavy diet of bivalves, which filter water, means runoff contaminated with T. gondii eggs can end up reaching otters.

The parasite commonly causes chronic infections in otters, but it is unusual for infections to rapidly kill adult otters, she says.

Miller and her colleagues performed necropsies on four southern sea otters (Enhydra lutris nereis) found stranded between 2020 and 2022, all during the February and March rainy seasons. Three adult female otters were stranded near Big Sur in San Luis Obispo county in California; one younger male was found in Santa Cruz county around 170 kilometres further north.

The otters were infected with more parasites across more tissue types than usually seen in Toxoplasma infections, and they had severely inflamed fat associated with the presence of parasites. “As soon as I started looking at them under the microscope, I was like, ‘whoa’,” says Miller.

The researchers found that key sequences of the parasite’s DNA were identical to a strain that had previously been reported only in a pair of mountain lions in British Columbia, Canada, in 1995 and a wild pig in the Sierra Nevada mountains in California around 20 years later.

It is unclear whether the male otter was infected in the same area as the others and then swam north, or whether the otters were infected in different places, says Chunlei Su at the University of Tennessee, Knoxville, who wasn’t involved with the work.

Miller says it is also unclear whether the strain – called “COUG” – is as virulent in other animals as it appears to be in otters, though it has been shown to be harmful in mice and she is concerned about other marine wildlife. She adds that humans who handle potentially infected animals should be careful. “Wear a second pair of gloves,” she says.