In recent years, wildlife veterinarians have become concerned about the increasing number of southern sea otters dying in California. The current otter population is 10 percent lower than it was in 1995. Disease has been identified as one reason. Two species of protozoa, Toxoplasma gondii and Sarcocystis neurona, have been identified as important causes of fatal brain infections in these otters.
To understand the rise in sea otter deaths, scientists from the Wildlife Health Center in the UC Davis School of Veterinary Medicine joined colleagues from other agencies to investigate potential sources of Toxoplasma contamination.
In a study published in the July issue of the International Journal for Parasitology, the scientists compiled environmental, demographic, spatial and serological data for 223 live and dead California sea otters tested between 1997 and 2001. Using a new diagnostic test developed by UC Davis scientists, the authors found that 42 percent of live otters and 62 percent of dead otters were infected with Toxoplasma. Their study suggests that land-based freshwater runoff is a source of Toxoplasma infection for sea otters.
Cats are the only animals known to shed oocysts, which are the tough, environmentally resistant eggs of Toxoplasma parasites, in their feces. However, the connection between land-loving felines and sea otters puzzles researchers.
Before analyzing their data, lead author Melissa Miller and her co-authors at UC Davis, the California Department of Fish and Game, the Central Coast Regional Water Quality Control Board and the Bay Foundation of Morro Bay theorized that sea otters might be exposed to Toxoplasma through both freshwater runoff and municipal waste (sewage), from cat feces flushed down toilets. Primary and secondary sewage treatment may not kill these oocysts, which are protected by a tough outer shell. The authors found no such association with municipal sewage, but otters sampled near large freshwater outflows, such as streams and rivers, were far more likely to be infected with the Toxoplasma parasites than the general otter population.
"We had thought otters might be exposed to the protozoa from sewage, but this study didn"t confirm that," said Miller, a UC Davis wildlife veterinarian. "However, otters sampled near areas of coastal freshwater runoff were almost three times more likely to test positive for Toxoplasma."
"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," Miller explained. "Sea otters generally live near the shoreline, so they would be directly in the path of these biological pollutants if they reached the ocean. In addition, many of the prey species that otters feed on could potentially concentrate pathogenic bacteria, protozoa and viruses present in contaminated water. In California, surface water runoff is conducted to coastal streams or directly to the sea from lawns, streets and open land via storm drains, ditches and culvert pipes, essentially with no pretreatment."
In addition, some sites along the California coast were found to be "hot spots" for Toxoplasma infection in otters and are currently being studied by the researchers. Co-author Patricia Conrad cautioned that sewage has not been ruled out as a source. "It may be how the sewage is being discharged and dispersed in the ocean," she said. Conrad, a UC Davis parasitologist, is examining the bivalves that sea otters feed on to see if Toxoplasma is concentrating in mussels. "If the bivalves located near a sewage outflow site are able to concentrate oocysts present in sewage, they could pose a high risk." It is unknown whether the otters are ingesting the oocysts in water or by eating contaminated shellfish.
Sea otters are considered a "sentinel species," signaling problems in the marine environment. The high proportion of Toxoplasma infections in the otters is important to people as well because humans are also susceptible to this parasite. Sea otters and humans consume some of the same seafoods, such as shellfish and crabs. Most healthy people can fight off Toxoplasma infection, but it can have serious consequences for the unborn babies of pregnant women and for people with weakened immune systems.
Southern sea otters were hunted to near extinction for their soft fur. In 1977, the U.S. Fish and Wildlife Service designated southern sea otters as a threatened species under the Endangered Species Act. Annual spring counts showed their population grew to as many as 2,377 along the California central coast. But in 1995, the number of sea otters counted peaked and began a downturn. More than 1,000 otters have been found dead along the California coastline over the past seven years. Only 2,139 California sea otters were counted this spring, according to the U.S. Geological Survey. In addition, sea otter mortality appears to be approaching record levels this year, based on the number of stranded animals examined through May 2002.