What’s Killing California’s Sea Otters? House Cats

What’s Killing California’s Sea Otters? House Cats

For a sea otter, a bad infection with the Toxoplasma parasite may feel a bit like drowning.

“The brain is no longer able to function and tell the body how to swim,” said Dr. Karen Shapiro, a veterinarian and pathologist at the University of California, Davis. The parasite, Toxoplasma gondii, enters the otter orally and makes its way to the brain, where it can cause swelling, weakness, seizures, disorientation and death. If the parasite doesn’t kill the otter directly, it can render it more likely to be hit by a boat or eaten by a shark. Among California sea otters, a protected species whose numbers are closely monitored, Toxoplasma infections contribute to the deaths of 8 percent of otters that are found dead, and is the primary cause of death in 3 percent.

Scientists have been working to determine where the Toxoplasma comes from and how to keep it from striking sea otters. They have long viewed one potential culprit with suspicion, and a study published last week identified the offender definitively: house cats.

“This is the ultimate proof that strains that are killing sea otters are coming from domestic cats,” said Dr. Shapiro, a lead author of the study.

The Toxoplasma parasite, a single-celled organism, is able to infect a wide variety of warm-blooded animals, but it needs to colonize cats, either wild or domestic, to reproduce sexually. Once infected, a cat can shed millions of Toxoplasma in its feces, which can contaminate the soil and water if the animal relieves itself outdoors. Toxoplasma gondii is the reason doctors recommend that certain pregnant women avoid litter-box duty. The parasite can harm fetuses and anyone with a compromised immune system, although most people fight off Toxoplasma with few or no symptoms.

Toxoplasma can also kill marine mammals, as scientists who study them have long been aware. It has felled not just sea otters but also dolphins and endangered monk seals in Hawaii. Studies over the past two decades established that rain can wash Toxoplasma from land to sea, where the parasite accumulates in the kelp forests that otters love.

Although Toxoplasma may be best known as a feline parasite, researchers have hesitated to blame domesticated cats for the deaths of sea otters. Even as they issued warnings to keep pet cats indoors, there remained the possibility that wild cats, such as California’s bobcats or mountain lions, could be responsible. But the new study, which analyzed the DNA from 135 sea otters with Toxoplasma infections that died between 1998 and 2015, largely quashes that hypothesis. Dr. Shapiro and her team found that the 12 deadliest otter infections were a perfect genetic match to parasites gathered from feral cats, and a bobcat, living in the hills around the bay where the otters died.

“What they’ve done, which hasn’t so far been done anywhere else, is found both ends of that chain,” said Dr. Wendi Roe, a veterinary pathologist at Massey University in New Zealand, who was not involved in Dr. Shapiro’s study. “Here’s the genotype that is more severe in sea otters, and here it is in this host in proximity to the location that they’re finding those sea otters.”

Not all sea otters that catch Toxoplasma die of it, Dr. Shapiro found. Most of the 135 otters examined by her team showed no evidence of brain damage, an indication that the parasite had not contributed to their deaths. Twelve deaths were determined to have been caused primarily by toxoplasmosis, and all of those otters succumbed to an unusual strain of the parasite, called Type X.

Type X is more common among wild cats than domestic ones, which tend to become infected with Type II, a strain of Toxoplasma that isn’t as deadly to otters. But there are far more domestic cats in the United States than wild ones, leading scientists to think that most of the Toxoplasma in the ocean, including Type X, comes from house cats.

What does all this mean for pet felines? “What we are trying to promote is keeping your cat indoors,” Dr. Shapiro said. “Also, disposing of feces in a bag in the trash, as opposed to flushing it down the toilet, because wastewater plants don’t reliably kill Toxoplasma.”

Many cat owners already know that it is better to keep pet cats indoors to prevent them from killing native birds. The habit benefits sea mammals as well, as house cats typically acquire Toxoplasma by eating infected birds and rodents. Several marine-mammal researchers interviewed emphasized that they kept their cats indoors.

Dr. Stéphane Lair of the University of Montreal said he does allow his cat to go outside, but only on a leash and under supervision. Pet owners, he said, should start thinking of cats as more like dogs — animals that should be contained, even in public.

The stickier question is what to do about feral cats that live and roam freely outdoors. People should not feed feral cats, Dr. Shapiro said. She encouraged finding homes for some of them (and keeping them indoors); spaying and neutering them, and gradually eliminating them from the environment. “We’re not trying to advocate euthanasia for any cat,” she said. “It’s not a problem we’re going to solve overnight.”


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