A recent study that found high proportions of Toxoplasma gondii infection in California sea otters near sites where freshwater runoff empties into the ocean has raised questions about whether cat waste and other pollutants could be contaminating the coastal waters and causing otter deaths.
Over the past 10 years, southern sea otters have been dying at an alarming rate, according to experts, who say emerging diseases, shark predation, pollution, and human interference are threatening the future of this ecologically important species. Researchers from the U.S. Geological Survey National Wildlife Health Center, the University of California-Davis, and the California Department of Fish and Game have teamed up to pinpoint the causes of the decline and have discovered that many sea otters are infected with disease-causing parasites, including T gondii and Sarcocystis neurona.
A recent study headed by UC-Davis and CDFG researchers found that 42 percent of live otters and 62 percent of dead otters were seropositive for T gondii. Test results showed sea otters near large freshwater outflows were almost three times as likely to be infected with T gondii than otters at sites of low freshwater outflows. The study was published in the July issue of the International Journal for Parasitology.
Since felids are the only animals that pass T gondii oocysts into the environment, cats are a likely source of the parasite.
"We think the oocysts from cat waste may get transported to the ocean from fields and yards by surface runoff after storms, or due to landscape irrigation," said Dr. Melissa A. Miller, a co-author of the study, in a statement. "Sea otters generally live near the shoreline, so they would be directly in the path of these biological pollutants if they reach the ocean."
Researchers have not ruled out sewage as a potential source of the parasites, but the study did not find a relationship between sewage exposure and infection in the otters.
Additionally, the bivalves otters feed on may concentrate pathogens and other contaminants in the water. Study co-author Dr. Patricia A. Conrad, a parasitologist, and other collaborators are examining bivalves to determine whether they concentrate T gondii.
"A canary in a coal mine"
The fate of sea otters is important to people, not only because they are charismatic animals, but also because they are an important part of their ecosystems and like the proverbial "canary in a coal mine," they may provide an early warning when dangerous situations arise.
With help from federal and local conservation programs, California sea otters rebounded from the brink of extinction in the early 1900s, when it was estimated that fewer than a hundred otters were left in the state, according to the United States Geological Survey. By 1993, the population had grown to an estimated 2,200 animals.
Maintaining a healthy population of sea otters is important. Otters are considered a "keystone" species because they help maintain the balance in their ecosystem.
In recent years, however, the California sea otter population growth has stalled.
Dr. Miller said that between 150 and 200 otters wash up on beaches, dead, each year. To document the causes of sea otter deaths, the CDFG, UC-Davis, and the National Wildlife Health Center have participated in an intensive sea otter necropsy program for several years. The necropsies have revealed that infectious diseases account for a disproportionate number of otter deaths, sparking studies by a number of agencies to identify the causes.
In the case of T gondii, a number of factors may be contributing to the severity of these infections. These include high levels of exposure to the pathogen, new host-parasite relationships, a genetic bottleneck effect, and immunosupression, according to Dr. Miller.
Presence of T gondii in coastal waters has serious implications because the pathogen can infect many species of mammals, including humans. T gondii infections in pregnant women can cause serious birth defects.
"People are sharing the sea otter's environment in terms of recreation and food," Dr. Miller said.
As the researchers continue to elucidate the sources of the T gondii infections in California sea otters, some experts are urging caution from the veterinary and pet-owning communities.
Dr. Conrad said cat owners should be aware that cat feces might contain T gondii oocysts that can be dangerous to people and to wildlife. She added they should dispose of cat fecal material properly by bagging it and disposing of it at landfills, where precautions are taken to prevent environmental contamination.
"We need to handle cat feces in a responsible way so we don't infect other animals, including marine mammals, or people," Dr. Conrad said. "We have to be responsible as pet owners and veterinarians."