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Deadly parasite threatens California sea otters

Melissa Miller knew something was off when she began to examine a sea otter that had died in San Simeon, a coastal California town about halfway between San Francisco and Los Angeles, in the winter of 2020. Nearly all of the animal’s body fat was inflamed. “It felt like there were little bumps all through it,” she says—a condition the veterinary pathologist had never seen in her 25 years examining sea otters for the California Department of Fish and Wildlife. She also found unusual lesions in the pancreas and heart.

When Miller looked at the sea otter’s tissues under a microscope, she spotted a familiar foe: Toxoplasma gondii, the parasite that causes toxoplasmosis. But this strain wasn’t acting like the usual Toxoplasma, which typically causes mainly brain and heart inflammation in immune-suppressed animals, and is far less lethal. The San Simeon sea otter, in contrast, had very few parasites in its central nervous system, which suggests it died quickly of an acute infection before the parasite could extensively invade its brain.

Since then, several more sea otters have been found dead on California beaches, all with similar lesions—and all harboring this troubling new strain of toxoplasmosis.

“My concern is pretty high,” says Andrew Johnson, a sea otter conservationist with Defenders of Wildlife, who was not involved in the new study. “Sea otters have so many things that they’re struggling against.” A deadly new strain of parasite is the last thing they need, he says.

Sea otters once ranged across the Pacific Rim, but were hunted nearly to extinction for their pelts in the 18th and 19th centuries. Their decline has larger ecological consequences because they eat urchins that, if their population is unchecked, will devastate kelp—a vital habitat for many marine species.

Ever since the surviving sea otter populations were legally protected, the species has recovered in Alaska. Yet the southern sea otter (Enhydra lutris nereis) remains threatened along the California coast. Its population has remained relatively stable at about 3000 animals, but it faces a number of threats, including white sharks that are becoming more abundant.

Any mammal or bird can be infected by T. gondii. The parasite reproduces sexually in cats, which excrete its eggs in their feces. The durable eggs, called oocysts, can eventually wash into streams (if a wild, feral, or outdoor cat defecates on a stream bank, for example, or if dried feces break down and are blown as dust) and reach the coast. Southern sea otters are commonly exposed through their diet, eating clams and other filter-feeding invertebrates that contain the oocysts.

When Miller started to see more dead sea otters like the one she examined in 2020, she reached out to Toxoplasma experts at the University of California (UC), Davis. Graduate student Devinn Sinnott, a veterinary pathologist, studied the parasite’s genetic markers in the four dead sea otters; they did not match any other known Toxoplasma samples from California wildlife studied by UC Davis parasitologist Karen Shapiro and others.

There was one surprising match: samples taken from two cougars in Canada nearly 30 years ago, the team reports today in Frontiers in Marine Science. The strain was identified there after a 1995 outbreak of Toxoplasma in Victoria, where a drinking water reservoir had been contaminated with the parasite. An investigation found a new strain nearby, dubbed COUG, a nod to the two cougars in which it was detected. Since then, COUG has only been detected once more, in a feral pig in the Sierra Nevada mountains.

How the new strain of Toxoplasma reached the California coast is a mystery. Michael Grigg, a molecular parasitologist at the National Institute of Allergy and Infectious Diseases not involved with the new study, suspects it was spread south by migratory birds, noting that some migratory species, such as snow geese and sandpipers, have been infected with Toxoplasma strains similar to COUG. If an infected bird was eaten by a cat in California, the parasite could establish a foothold.

The distribution of the new strain is unknown. Three cases were found within 25 kilometers of each other in San Luis Obispo County—toward the southern end of the sea otter’s range. An immature male was found at the northern end, near Santa Cruz, but it might have traveled a distance (as young males do) after it was infected. It’s important to know how widespread the new strain is, Miller says, because it could affect decisions on relocation of otters. “Obviously, we want to be extra careful not to move otters to high-risk places.”

“The work in the paper is excellent,” says Pádraig Duignan, a pathologist at the Marine Mammal Center, which treats sea otters, but who was not involved with the current study. “Absolutely top-drawer science.”

The new strain complicates treatment of infected sea otters—drugs used for humans and domesticated animals can help—says Michael Murray, a veterinarian at the Monterey Bay Aquarium, which treats about 10 sea otters with toxoplasmosis per year. “If we have a disease that hits that hard and that fast, it may not live long enough to strand on the beach where we can get our hands on it.”

If the new strain of Toxoplasma kills as quickly as it seems to, animals may die before reproducing, which could have an especially harmful impact on their population. Johnson says one action that can help otters is spaying and neutering cats and managing feral cat populations.

Duignan is also concerned that the problem may be bigger than it appears. More sea otters may have died from COUG without being found on beaches, he notes, or not found soon enough before the carcasses decomposed.

The UC Davis researchers want to learn more about the presence of COUG in the marine environment and hope to test stormwater runoff and shellfish that accumulate the parasite. That could pose a threat to human health, although COUG has never been detected in humans. The immediate focus remains on the sea otters. “Is it going to be a huge problem? I think that remains to be seen,” Johnson says. “I’m just hoping we don’t see more of these cases.”

Source: Science Mag