A rare strain of the parasite Toxoplasma has killed four sea otters along the California coast, raising concerns about a potential public health risk.

“The appearance of this lethal type of Toxoplasma in coastal California is concerning for two main reasons: First, because of potential population health impacts on a threatened species, and second, because this parasite could also affect the health of other animals that are susceptible to Toxoplasma infection,” said study co-author Devinn Sinnott of the University of California, Davis School of Veterinary Medicine.

This rare strain has never been reported in aquatic animals, she and her colleagues said in a university news release.

While toxoplasmosis is common and sometimes deadly in sea otters, this unusual Toxoplasma gondii appears to be especially virulent and capable of rapidly killing healthy adult otters, according to the study.

It likely arrived on the California coast only recently, the study authors noted.

It could pose a public health risk, but no infections with the strain have been reported in humans, the researchers noted.

“Because this parasite can infect humans and other animals, we want others to be aware of our findings, quickly recognize cases if they encounter them and take precautions to prevent infection,” said co-author Melissa Miller of the California Department of Fish and Wildlife.

“We encourage others to take extra precautions if they observe inflamed systemic fat deposits in sea otters or other marine wildlife,” Miller added.

Sea otters are especially vulnerable to Toxoplasma infection because they live near the shoreline and may be exposed to the parasite’s eggs in rainwater runoff. The otters also eat marine invertebrates that can concentrate the parasites, the researchers explained.

The four sea otters studied were stranded between 2020 and 2022. Each had severe inflammation of their body fat, a condition called steatitis.

Microscopic examination of the otters’ tissues revealed high numbers of the parasites throughout each body except the brain, which is typically one of the major organs affected in sea otters with fatal toxoplasmosis.

In all four cases, DNA testing identified a rare strain called COUG, first found in 1995 in Canadian mountain lions after a nearby outbreak among humans.

It’s not known how this strain might affect humans or other animals.

The otters were stranded during periods of high coastal rainfall, so may have been exposed to Toxoplasma eggs through storm runoff.

“I have studied Toxoplasma infections in sea otters for 25 years, and I have never seen such severe lesions or high parasite numbers,” Miller said. “Since Toxoplasma can infect any warm-blooded animal, it could also potentially cause disease in animals and humans that share the same environment or food resources, including mussels, clams, oysters, and crabs that are consumed raw or undercooked.”

Wild and domestic cats can shed Toxoplasma gondii in their feces. This typically doesn’t cause symptoms in healthy humans, but can cause miscarriages and neurological disease, the study authors noted.

The preliminary study findings were published March 22 in the journal Frontiers in Marine Science.

More information

The U.S. Centers for Disease Control and Prevention has more on Toxoplasma gondii.

SOURCE: University of California, Davis, news release, March 22, 2023