Rare Strain of Parasite That Killed California Sea Otters May Threaten Humans, Scientists Say

In this file photo taken in 2004, a sea otter feeds near Valdez, Alaska. David McNew/Getty Images

Scientists warned that a potent new strain of parasite that killed sea otters off the California coast could also threaten the health of other marine wildlife and humans.

The parasite that causes toxoplasmosis (Toxoplasma gondii) is commonly found in sea otters, according to a joint study from the California Department of Fish and Wildlife and the University of California, Davis. But this new strain, which wasn’t known until 2020, appears to be able to cause especially severe infections and rapidly kill healthy adult otters.

Although no infections with the new strain have been reported in humans, scientists are concerned that it might contaminate the marine food chain and elevate human disease risk via shared marine food resources.

“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,” Melissa Miller, the study’s corresponding author and researcher at the California Department of Fish and Wildlife told UC Davis News. “We encourage others to take extra precautions if they observe inflamed systemic fat deposits in sea otters or other marine wildlife.”

Scientists say Toxoplasmosis can also infect migratory birds, livestock, and domestic cats. Their feces transmit it. When infecting humans, the parasite can cause a “fatal neurological disease” in people with weakened immune systems and miscarriage in pregnant women.

However, it remains unclear how this new strain of toxoplasma may affect people, the scientists noted.

Southern sea otters (Enhydra lutris nereis), a species native to marine coastal areas along the central California coastline, are especially vulnerable to Toxoplasma infection. Scientists said this is because they live near coastal cities, where rainwater runoff can carry cat feces containing parasite eggs into nearshore waters. Those animals also eat a lot of mollusks, such as clams, oysters, and snails, which can have the parasites.

The study discusses the death of four sea otters from February 2020 to March 2022. They all had steatitis, or severe inflammation of their body fat, a very unusual symptom in sea otters with toxoplasmosis.

“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 Devinn Sinnott, the study’s co-author and veterinary pathologist at UC Davis.

The findings were published Tuesday in the marine biology journal Frontiers in Marine Science.