Southern sea otters (Enhydra lutris nereis), endangered marine mammals along California’s coastlines, are facing an unexpected threat. The menace comes not from pollution, habitat loss or natural predators, but from a microscopic enemy—Toxoplasma gondii (T. gondii). This protozoan parasite, typically associated with domestic cats, has found its way into marine ecosystems with sometimes deadly consequences for sea otters. Recently, scientists identified transmission of virulent, atypical strains of T. gondii from terrestrial felids to sea otters along the southern California coast, with lethal consequences (1).

Understanding T. gondii and Its Hosts

T. gondii is a versatile parasite that can infect nearly all warm-blooded animals, including humans and marine mammals. However, the T. gondii lifecycle depends upon felids (e.g., domestic cats and their wild relatives) who serve as definitive hosts. It is in their intestines that the parasite completes its sexual reproductive stage. The resulting oocysts are excreted in the animals’ feces. T. gondii oocysts exhibit remarkable resilience, surviving in soil, freshwater and seawater for extended periods. They are even resistant to standard wastewater treatment processes, which means oocysts in cat waste disposed of by flushing will pass through the treatment plant and be discharged into the environment. ​(2,3).

Oocysts can also be washed from soil contaminated with cat waste and carried via storm drains and rivers into the ocean, dispersing them into coastal waters. Once there, the oocysts settle on kelp or in sediments where they can be picked up by marine invertebrates like snails, mussels and clams. Marine mammals such as sea otters become infected when they consume these contaminated invertebrates. Otters can also ingest oocysts during grooming sessions​ (1,3).

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