Wednesday, 7 March 2012

Toola, Sea Otters, and Toxoplasma gondii

Reports in March 2012 of the death of Toola, a Toxoplasma gondii-infected sea otter who lived out her days at the Monterey Bay Aquarium, reminded me of the threat that T. gondii poses to marine mammals. Toola suffered from neurological damage thought to have been caused by the parasite and required daily anti-seizure medication. Among other things, she was the poster otter for legislation and other efforts to protect marine mammals from various health risks. And she was cute too.

Sea otters frequent the California coast, where they may
become infected with T. gondii. The consequences can be deadly.
Image by Mike Baird, Morro Bay, USA. CC BY 2.0

How do Sea Otters Get Toxoplasma gondii?

My impression has been that the risk of acquiring T. gondii has been rising in marine mammals, and that this is likely to be the result of runoff – oocysts being washed off the land into coastal waters. This made sense to me when considering the number of feral and roaming domestic cats, and the quantity of cat feces that must be carried into coastal waters by runoff (this has actually been studied: “domestic feline faecal deposition in communities adjacent to Estero Bay was conservatively estimated at 107 metric tonnes/year, or 26 kg/ha:” Miller et al.) I was surprised; therefore, to read that the majority of California sea otters tested in the 2008 study reported by Miller et al had a unique strain (dubbed Type X) that is not typically found in domestic cats.

Rather, the paper by Miller et al. reports that Type X T. gondii was found in wild felids (mountain lion, bobcat) and foxes. While foxes might be doing relatively well in urban areas, the number of wild felids is down from what it must have been before humans covered the west coast of North America with concrete and asphalt. So if domestic cats aren’t to blame, why are there more infected marine mammals now than before?

Humans and Mollusks Spread Toxoplasma to Marine Mammals

One answer apparently lies in all that concrete and asphalt. Hardscaping of the coast reduces the amount of runoff that’s absorbed into the ground before it spills into the sea. In addition:

  • Human development has reduced wetlands, which provide natural filtration for runoff.

  • Bivalves such as mussels flourish near storm sewers and have been shown to filter organisms, including T. gondii oocycts out of the water and concentrate them in tissue.

  • Sea otters feed on mussels and other bivalves, consuming at least 76 mussels each day.

Studies done on land mammals have shown that a single oocyst can potentially be the source of chronic toxoplasmosis. Given those odds, its not surprising that Toola, and lots of other California sea otters (and other marine mammals) are infected with T. gondii.

Read the paper:

Miller, M.A., W. A. Miller, P. A. Conrad et al. "Type X Toxoplasma gondii in a wild mussel and terrestrial carnivores from coastal California: New linkages between terrestrial mammals, runoff and toxoplasmosis of sea otters." International Journal for Parasitology: 38(11), 2008

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