Parasite From Cats Killing Sea Otters

Analysis by Dr. Karen Shaw Becker

cat parasite toxoplasma gondii

Story at-a-glance -

  • Southern sea otters living off the California coast are dying from Toxoplasma gondii (T. gondii), a parasite hosted by wild and domestic cats
  • Researchers looked into the toxoplasma strains in sea otters stranded along the central California coast, comparing them with T. gondii strains found in nearby wild and domestic cats
  • A genetic link was found, showing that the area’s cats were likely a source of fatal T. gondii infection in the sea otters
  • Infected cats can shed hundreds of millions of egg-like structures called oocysts in their feces, which can contaminate freshwater run-off, accumulate in kelp forests eaten by snails and then, in turn, be eaten by sea otters and lead to infection
  • In Monterey Bay, California, considered to be a high-risk location for T. gondii, up to 70% of southern sea otters are infected with the T. gondii parasite
  • Among a group of sea otter carcasses examined between 1998 and 2001, T. gondii was determined to be the cause of death in 17% of cases and contributed to the deaths of another 12%

Southern sea otters living off the California coast are dying from Toxoplasma gondii (T. gondii), a parasite hosted by wild and domestic cats — and now researchers may know why.

The puzzle of how a parasite hosted by land animals was infecting ocean-dwelling sea otters has been a mystery since the late 1990s,1 but researchers with the University of California (UC) Davis School of Veterinary Medicine and colleagues have identified the specific strains of the parasites harming sea otters and traced them back to cats living nearby.2

Scientists Reveal Path of Parasite From Land to Sea

In Monterey Bay, California, considered to be a high-risk location for T. gondii, up to 70% of southern sea otters are infected with the parasite, although most of them will not die as a result. Among a group of sea otter carcasses examined between 1998 and 2001, T. gondii was determined to be the cause of death in 17% of cases and contributed to the deaths of another 12%.3

T. gondii can use virtually all warm-blooded vertebrates, including humans, as intermediate hosts, but only wild and domestic cats are known to be definitive hosts, meaning the parasite sexually reproduces whiling living in them. Cats can shed hundreds of millions of egg-like structures called oocysts in their feces, which is why pregnant women are often told to avoid contact with cat feces.

Sea otters do not typically prey on mammals and birds that might be contaminated with T. gondii, which is why it was believed that they were infected by ingesting oocysts that accumulated in coastal areas with contaminated freshwater run-off.4 Indeed, the researchers initially found that oocysts could accumulate in kelp forests, which were eaten by snails. The snails, in turn, were eaten by the sea otters, leading to infection.5

In a research published in Proceedings of the Royal Society B, researchers then looked into the toxoplasma strains in sea otters that stranded along the central California coast, comparing them with T. gondii strains found in nearby wild and domestic cats. A genetic link was found, showing that the area’s cats were likely a source of fatal T. gondii infection in the sea otters. The researchers explained:6

“The same atypical T. gondii strains were detected in sea otters with fatal toxoplasmosis and terrestrial felids from watersheds bordering the sea otter range. Our results confirm a land–sea connection for virulent T. gondii genotypes and highlight how faecal contamination can deliver lethal pathogens to coastal waters, leading to detrimental impacts on marine wildlife.”

Oocysts have also been detected in green-lipped mussels from New Zealand that were intended for human consumption, so it’s likely that the sea otters may be exposed via other prey items aside from snails.7 They could also ingest oocysts by grooming.

Three Factors Contribute to Toxoplasma Infection in Sea Otters

According to the UC Davis One Health Institute, which conducted the studies, there are three main factors contributing to sea otters’ infection with the T. gondii parasite. These include:8

  1. Domestic cats infected with T. gondii, which in turn contribute oocysts to coastal watersheds
  2. Destruction of wetlands, which leads to a loss of filtering capacity that would otherwise prevent pathogens in fecal matter from entering the ocean
  3. Water runoff from city cement; impervious surfaces contribute to increased runoff that carries fecal pathogens to the ocean, where sea otters and other marine life are exposed

Given these factors, the UC Davis researchers suggested a number of steps to help alleviate the problem, including keeping cats indoors or, if your cat goes outdoors, putting a litterbox outside as well as inside. “That way feces never actually enters the environment,” they explained.9 It’s also important to pick up your cat’s poop if he does go outdoors, just as you would for dog poop.

Wetland conservation and reconstruction are also important to act as natural borders and filters for ocean waters, while keeping and creating natural spaces, which help to absorb rainwater and prevent runoff, is also helpful.

Southern sea otters, also known as California sea otters, are a threatened species under the Endangered Species Act, so even though T. gondii is not fatal in the majority of animals infected, it still represents another significant threat to these creatures — one that should be reduced as much as possible. The long, complex path that T. gondii travels to get from cats to sea otters should serve as a warning, the researchers noted, calling it just the tip of the iceberg. As the UC Davis One Health Institute asked:10

“If a cat-derived parasite can sicken or kill sea otters, what other pathogens are moving through the environment and into the ocean? And what health effects are they having, now or in the future, on marine animals and people?”

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