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.