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).
Sea Otters and T. gondii: A Fatal Encounter
Although surveys of live-caught sea otters show that a large portion of them are infected with T. gondii, for some the infections can become more than just an inconvenience. Postmortem studies of sea otters from 1998 to 2001 indicated toxoplasmosis as a primary or contributing cause of death in approximately 29% of cases (4). In these fatal cases, necropsies found that the parasite often targeted the brain causing meningoencephalitis, severe inflammation of the brain and surrounding tissues. Meningoencephalitis can result in neurological symptoms, impair swimming ability and death (4).
Why do some, but not all, otters infected with T. gondii develop fatal infections? Using genotype analysis, a study by the University of California, Davis, in collaboration with the California Department of Fish and Wildlife, is finally shedding light on this puzzling phenomenon. By genotyping the strains of T. gondii found in necropsied otters they found that the atypical strain, Type X and Type X variants were present in almost 80% of the otters. Unlike other T. gondii genotypes, Type X appears to be particularly virulent in otters, leading to high mortality rates among infected individuals. In their study, all sea otters that died as a direct result of T. gondii infection were infected with Type X or Type X variants. (1,2).
The Virulent Type X Strain: A Killer from Land to Sea
A comparison between the Type X strain of T. gondii infecting sea otters and the strains found in terrestrial hosts like bobcats and feral domestic cats located in coastal California watersheds. These findings confirm a land-sea transmission of virulent T. gondii strains. The same atypical Type X genotype implicated in otter mortality was also detected in wild felids such as bobcats, and occasionally detected in domestic cats. Even occasional occurrence in domestic cats is cause for concern. These animals can act as a reservoir for this virulent strain, and the large populations of feral and domestic cats in urban areas can contribute to environmental contamination through their feces (1,3).
The high virulence of Type X T. gondii poses a unique challenge for otter management since otters infected with this genotype are significantly more likely to die from toxoplasmosis than those infected with other genotypes. The cause of this heightened lethality is not yet fully understood, but researchers wonder if the genetic variations in Type X might make it particularly adept at evading the immune systems of certain hosts (1).
A Broader Ecological Impact
While the impact on sea otters alone is concerning, the spread of T. gondii poses broader risks to marine ecosystems and potentially to public health. The parasite has also been implicated in fatalities among other marine mammals, such as the critically endangered Hawaiian monk seal. T. gondii can affect multiple organs in these animals, potentially compromising their immune systems and making them vulnerable to other diseases. Furthermore, the contamination of the marine food chain with T. gondii poses a serious health risk to those who eat raw or undercooked seafood from infected regions (5).
What Can Be Done? Preventative Measures and Conservation Efforts
The unique land-to-sea transmission of T. gondii underscores the interconnectedness of terrestrial and marine ecosystems and emphasizes the need for integrated conservation strategies. To reduce the risk of T. gondii transmission from land animals to sea otters and other marine wildlife there are several efforts that can be taken.
- Cat Management: Keeping domestic cats indoors can reduce their exposure to T. gondii and eliminate their ability to introduce the oocysts into the environment. Cat owners should also properly contain cat litter and feces and dispose of it in the trash rather than flushing down the toilet.
- Wetland Restoration: Preserving and restoring wetlands can help mitigate the flow of T. gondii oocysts into the ocean. Natural landscapes like wetlands act as filters, trapping pollutants and pathogens before they reach waterways and reducing the exposure risk for marine wildlife.
- Responsible Waste Management: Implementing stormwater management practices to reduce runoff, particularly from urban and agricultural areas. This can decrease the number of T. gondii oocysts, and other contaminants that enter coastal ecosystems. Using permeable pavement and other “green infrastructure” in urban planning can also minimize contaminated runoff.
- Public Education and Awareness: Educating those around us about the consequences of improper cat waste management and runoff pollution can help protect marine life. Programs focused on responsible pet ownership and waste disposal practices could be beneficial for reducing the impact of T. gondii on vulnerable marine populations (3).
Conclusion
The threat posed by T. gondii to sea otters and other marine mammals highlights the delicate balance between interconnected terrestrial and marine ecosystems. Protecting sea otters and other marine mammals from T. gondii requires concerted efforts to address both the terrestrial sources of the parasite and the pathways that carry it to the sea. Cooperation between wildlife conservationists, local governments, pet owners and the public can help protect these remarkable animals from infection with this potentially deadly parasite.
References
- Shapiro. K., et al. (2019) Type X strains of Toxoplasma gondii are virulent for southern sea otters (Enhydra lutris nereis) and present in felids from nearby watersheds. Proc. R. Soc. B. 286, 201913.
- Dubey, J.P., et al. (2020) Recent epidemiological and clinical importance of Toxoplasmosis gondii infections in marine mammals: 2009–2020. Vet. Parasitol. 288, 109296.
- VanWormer, E., et al. (2014) Using molecular epidemiology to track Toxoplasma gondii from terrestrial carnivores to marine hosts: Implications for public health and conservation. PLOS Negl. Trop. Dis. 8, e2852.
- Kreuder, C., et al. (2003) Patterns of mortality in southern sea otters (Enhydra lutris nereis) from 1998–2001. J. Wildl. Dis. 39, 495–509.
- About Toxoplasmosis. CDC Website. Accessed November 11, 2024.
Kelly Grooms
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