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Evidence challenges ‘conventional wisdom’ linking domestic cats to sea otter deaths


“Sea otter health: Challenging a pet hypothesis,” published in the International Journal for Parasitology: Parasites and Wildlife, 2015. Complete article available (open access) online here.


In this article, a researcher from the U.S. Geological Survey (USGS) describes results from a series of studies suggesting that parasites from wildlife — not from free-roaming domestic cats — are the main source of infectious disease observed in California sea otters (an endangered species). In particular, the author provides evidence that wild felids (i.e., bobcats and mountain lions) are more likely to be the source of infection with the common parasite Toxoplasma gondii [1].

Key points

Although it’s true that cats (both domestic and wild felids) are the only known “definitive host” of the T. gondii parasite, different strains are more common in different cat species. Studies along the California coast found that the Type X strain — responsible for a greater proportion of sea otter mortalities than other strains — is more common among wild felids than in domestic cats [2,3].

Sea otter health differences

Additional evidence comes from the comparison of two distinct communities along the coast: Big Sur (sparsely populated, relatively pristine) and Monterey (relatively disturbed, with a high density of humans, domestic cats—including both pets and feral cats, and nearby agriculture).

“the comparison between Big Sur and Monterey suggests that parasites from wildlife spill over into sea otters”

Contrary to expectations, the evidence showed that “when there are differences in health measures, it is mostly the sea otters from waters adjacent to the city [Monterey] that seem to be doing a bit better.” Taken together, these findings led the author to conclude that “the dirty ocean doesn’t make sea otters sick; instead, the comparison between Big Sur and Monterey suggests that parasites from wildlife spill over into sea otters” [1].

Sea otter population data

The results described in this article are further supported by additional research from the U.S. Geological Survey’s most recent sea otter census, which found that “the otter population is likely at its highest level in at least 100 years” [4,5].

Reevaluating the impact of cats on sea otters

If domestic cats were truly a significant threat to sea otter health, we’d expect to see the evidence in census data. Yet sea otter numbers have increased to the point that, were it to hold for three consecutive years, the U.S. Fish and Wildlife Service could consider removing California sea otters from its list of endangered species [4].



  1. Lafferty, K.D. Sea otter health: Challenging a pet hypothesis. International Journal for Parasitology: Parasites and Wildlife 2015, 4, 291–294.

  2. Conrad, P.A.; Miller, M.A.; Kreuder, C.; James, E.R.; Mazet, J.; Dabritz, H.; Jessup, D.A.; Gulland, F.; Grigg, M.E. Transmission of Toxoplasma: Clues from the study of sea otters as sentinels of Toxoplasma gondii flow into the marine environment. International Journal for Parasitology 2005, 35, 1155–1168.

  3. Miller, M.A.; Miller, W.A.; Conrad, P.A.; James, E.R.; Melli, A.C.; Leutenegger, C.M.; Dabritz, H.A.; Packham, A.E.; Paradies, D.; Harris, M.; 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 2008, 38, 1319–1328.

  4. Tinker, M.T.; Hatfield, B.B. California Sea Otter (Enhydra lutris nereis) Census Results, Spring 2016; U.S. Geological Survey Data Series 1018, 2016; p. 10.

  5. Rogers, P. California sea otter population reaches record high number. The Mercury News 2016.


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