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Scientists warn of an unusual parasite that killed sea otters off the California coast


Four sea otters that washed up on the California coast have died from an unusual parasite that scientists warn could infect other marine animals and humans.

There are currently no known infections of the Toxoplasma gondii strain among humans, according to a study released Wednesday by the California Department of Fish and Wildlife and UC Davis.

But the microscopic parasite could infect any warm-blooded animal or enter the food chain, according to the study. That includes marine animals such as mussels, clams, oysters and crabs that are eaten raw or undercooked, study corresponding author Melissa Miller of the California Department of Fish and Wildlife said in a written statement.

“I have studied Toxoplasma infections in sea otters for 25 years and have never seen such severe lesions or such high numbers of parasites,” Miller said of the dead otters. “We are reporting our preliminary findings to alert others to this troubling condition.”

Otters are especially susceptible to Toxoplasma infection, the researchers said, because they find food along the shoreline and could be exposed to parasite eggs in stormwater runoff when they feed on marine invertebrates. The parasite is typically found in wild and domestic cats and is shed in their feces, according to the study authors.

The strain of the parasite, called the COUG genotype, was first isolated in the wild from a mountain lion in British Columbia during an investigation of the 1995 human outbreak of the parasite and from a wild boar in eastern California, according to the authors. of the study. The Canadian government reported that 110 acute human infections were identified and none were fatal.

The COUG genotype was present in all four otters, according to the study.

The first otter came ashore in San Simeon, in San Luis Obispo County, in February 2020, according to the study. The adult female otter was still alive, but thin, unresponsive, and whimpering. The other three otters washed ashore already dead from February through March 2022 at Cayucos in San Luis Obispo County and at Natural Bridges State Beach in Santa Cruz County, the researchers said.

Numerous lesions were found throughout the otters’ nervous system, but were missing from the brain, which is typically one of the organs affected by the parasite, according to the study authors.

All four otters showed obvious signs of inflammation in their body fat, Miller said, and under the microscope, their tissue was teeming with parasites, including more rapidly growing and actively multiplying invasive cells.

Otherwise, the otters were healthy adults with no other serious illness, according to the study.

It usually takes time for the parasite to invade and multiply in the brain, but the low number of parasites in the brains of the four otters suggests the infection spread quickly, Miller said.

“We don’t yet know the interval between infection and death for this strain,” Miller said.

Study author Karen Shapiro, of the UC Davis School of Veterinary Medicine, said it was surprising to find the parasite in otters because it had never been recorded in their species.

“So, finding this type (of Toxoplasma) in the marine environment was highly unexpected, and particularly concerning given the rapid and virulent disease it caused in the four sea otters,” Shapiro said in an email.

In humans, the parasite can cause miscarriages and neurological diseases, according to the study.

The type of blubber inflammation associated with toxoplasmosis has been reported in Hawaiian monk seals, according to Devinn Sinnott, a graduate student at the UC Davis School of Veterinary Medicine. That strain has yet to be determined and is an active area of ​​research in the UC Davis lab, Sinnott said in an email.

“We still have a lot to learn,” Sinnott said in a written statement that accompanied the study. “Larger-scale studies are needed to understand the potential impact of COUG Toxoplasma strain infection on sea otter populations, how geographically dispersed it is, how it is introduced into the ocean, and what other animals might be affected.”

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