Researchers have identified that infection by land-based protozoan parasites such as Toxoplasma and related parasites that come from domestic cats are common causes of illness and death of southern sea otters.
The study, published in the journal, 'Proceedings of the Royal Society B,' marked the first time a genetic link has been clearly established between the Toxoplasma strains in felid hosts and parasites causing fatal disease in marine wildlife.
"This is decades in the making. We now have a significant link between specific types of the parasite and the outcome for fatal toxoplasmosis in sea otters. We are actually able to link deaths in sea otters with wild and feral cats on land," said corresponding author Karen Shapiro, an associate professor with the UC Davis School of Veterinary Medicine and its One Health Institute.
Wild and domestic cats are the only known hosts of Toxoplasma, in which the parasite forms egglike stages, called oocysts, in their faeces. Shapiro led the initial effort to show how oocysts accumulate in kelp forests and are taken up by snails, which are eaten by sea otters.
For this study, the authors characterised Toxoplasma strains for more than 100 stranded southern sea otters examined by the CDFW between 1998 and 2015.
CDFW Veterinary Pathologist Melissa Miller assessed the otters for Toxoplasma as a primary or contributing cause of death.
The scientists compared pathology data with the parasite strains found in sea otters and nearby wild and domestic cats to identify connections between the disease-causing pathogen and its hosts.
The study's results highlight how infectious agents like Toxoplasma can spread from cat faeces on land to the sea, leading to detrimental impacts on marine wildlife.
Southern sea otters are among the most intensely studied marine mammals in California because they are a threatened species and an iconic animal for the state.
They live within just a few hundred meters of the coastline, allowing for the close observation that enables a wealth of scientific data.
Previous research showed that up to 70 per cent of stranded southern sea otters were infected with Toxoplasma, yet the infection becomes fatal for only a fraction of them.
Shapiro noted that Toxoplasma can also affect other wildlife species, but there are more robust data for the otters.
"Toxoplasma is one heavily studied pathogen that we care about, but there are many other viruses and bacteria that are on land and being flushed to the ocean that we probably aren't aware of yet," Shapiro said.
People can help reduce the spread of Toxoplasma by keeping their cats inside and disposing of cat faeces in a bag in the trash, not outdoors or in the toilet because wastewater treatment is not effective in killing oocysts.
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