How Feral Cats Are Inadvertently Killing California’s Sea Otters


Madison Dapcevich


Madison Dapcevich

Freelance Writer and Fact-Checker

Madison is a freelance science reporter and full-time fact-checker based in the wild Rocky Mountains of western Montana.

Freelance Writer and Fact-Checker

Sea otters photographed in Moss Landing along the central California coast. Trina Wood/UC Davis

Every year, wild sea otters (Enhydra lutris nereis) along California’s southern coast become infected by a parasite only hosted by wild and domestic cats. Building upon decades of study, researchers at the University of California Davis now know how sea otters come into contact with the terrestrial pathogen and why only a handful of them die from it each year.

Publishing their work in the journal Proceedings of the Royal Society B, the study authors write that the answer lies in the parasites’ genes.   


“We’ve shown for the first time that there is a genetic link between specific strains of this parasite and hosts – in this case, sea otters – that actually die from the disease,” study author Karen Shapiro told IFLScience in an interview. “We’ve known for decades that many sea otters and a lot of people, in general, are infected by Toxoplasma, but only a fraction will develop the disease and die from it.”

Toxoplasma gondii is one of the world’s most common parasites infecting feline species. When humans and other animals come into contact with cat feces or undercooked contaminated meat, the parasite can cause a disease known as toxoplasmosis.

But how does it get from cats on land to sea otters in the oceans? Simple: water systems.

Wild and domestic cats are the only known carriers of Toxoplasma, which forms in their feces as tiny egg-like structures called oocysts. As rains wash away feline feces into water systems that drain into the ocean, oocysts make their way across the land and into coastal marine ecosystems.

Otters eat mussels, clams, abalone, and oysters, which means the disease could also spread into local human populations. Trina Wood/UC Davis

“Humans eat what sea otters eat – oysters, mussels, clams, and abalone – and it is very likely that if the sea otters are getting it then so too are people,” she explained.

Researchers compared the pathology data of parasite strains in dead sea otters between 1998 and 2015 and found similarities to those in nearby wild and domestic hosts, highlighting how infectious diseases can spread across ecosystems and wreak havoc on non-native habitats. The study notes that almost three-quarters of southern sea otters become infected with Toxoplasma but only a small fraction will die from it. That’s because atypical strains of about 13 species carry a unique set of six genes that may make the disease fatal.

Sea otters act as a canary in the coal mine for marine organisms because they live near shore, allowing researchers a closer eye into their life. When an otter dies, researchers are able to quickly retrieve its carcass and conduct a necropsy – the animal version of an autopsy – in order to determine what caused its death.

“In contrast, most marine mammals don’t have such unique life history features so when they die, we have no idea why it is as we’re unable to recover a large fraction of their carcasses. The sea otters give us a tool to be able to monitor what is going on in the coastal ecosystem,” explained Shapiro, adding that a large number of oocytes are accumulating in kelp forests home to sea otters, which could ultimately impact humans.


Toxoplasma has a preference for the brain and its neurological tissue. In people and in sea otters, this can develop into encephalitis and ultimately kill the host. The authors provide a number of ways people can limit the spread of Toxoplasma into marine ecosystems, including keeping cats inside and disposing of cat feces in a bag in a trash can rather than outside or in the toilet – wastewater treatment, bleach, and iodine are all ineffective against the parasite.  

Toxoplasma has a preference for neurological tissue in the brain, causing encephalitis in both humans and wild animals. Bryant Austin


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