Researchers said they have uncovered more evidence that links land-based pollution from human activities to diseases in marine mammals. "Wildlife can serve as a source of infection, as we have seen with the avian bird flu, but they can also be sentinels of pathogen pollution – they are often the first victims of these diseases," said Pat Conrad of the University of California, Davis.
"By paying attention to them, it will tell us about our own health and the links between our health and that of the environment."
Conrad and colleagues presented their findings at the American Association for the Advancement of Science annual meeting. She said when she first started studying the death of otters along the California coast, she had no idea her investigation would lead upstream. She said her team began tracing the cause of death to the brain parasite Toxoplasma, and ultimately to cats.
"Before I started this project I didn't think about things like how much cat feces get into the environment," she said, "how what we dump on our lawns and sidewalks flows into streams to rivers and into the ocean."
Her team discovered that otters in areas with heavy freshwater outflow are nearly three times as likely to be infected with Toxoplasma, and because sea otters spend their entire lives near shore, there is a serious implication for humans.
Toxoplasma is the third most common cause of death due to food-borne disease in the United States. Estimates indicate up to 25 percent of the population could be infected with the parasite, although most people rarely show symptoms or are hit only with a flu-like illness.
In pregnant women, however, the infection can lead to miscarriages or developmental problems in the fetus, most often affecting brain function and sight. Toxoplasma also can cause a severe brain disease in people with suppressed immune systems, such as transplant patients or HIV/AIDS victims.
"When otters get toxoplasmosis, they get far sicker than most humans," Conrad said, which actually allows researchers to detect and trace the disease more easily – and calls attention to a flow of pathogens from land to sea that otherwise might have gone unnoticed.
Conrad recommends that cat owners keep their animals inside and dispose of kitty litter by bagging it and sending it to a sanitary landfill. "I know this is tough," she admitted. "I own four cats. I hate cleaning cat boxes, but I know it's in the best interest of the cats, wildlife, and human health."
Todd O'Hara, of the University of Alaska Fairbanks, said land-based pollutants have become so prevalent in the ocean that scientists are using them to trace the diets and travels of marine mammals. This work also sheds light onto potential human-health effects of these contaminants.
"Many things we do in modern, industrialized society produce POPs (persistent organic pollutants): burning of municipal waste, pulp mills, flame retardants, stain repellents," O'Hara said. "There are all sorts of sources."
In laboratory studies and acute exposure scenarios, persistent organic pollutants have been shown to affect brain function, reproductive success and the immune system, but there is little information on the impact of chronic, low-level exposure. Animals such as killer whales and polar bears, whose survival depends on accumulating thick layers of fat as a hedge against long periods when food is scarce, accumulate contaminants in their tissues.
O'Hara works with a research team studying the ecological, biochemical and physiological details contaminant build-up in mammal tissues. For example, levels of organohalogens and mercury are higher in polar bears, seals and other species with fish-based and marine mammal-based diets, while cadmium – a heavy metal associated with bone and renal disease – is higher in organs of mammals with invertebrate-based diets.
"The issue is to understand how pollutants are transferred through the ocean food web to consumers, both humans and animals," said Peggy Krahn of the U.S. National Oceanic and Atmospheric Administration. "Until we understand this, we can't start to say how to mitigate pollutants in the ocean."
Krahn's team traces the flow of contaminants by studying the build-up of pollutants in killer whales. Different levels and timing of chemical use around the globe create distinct chemical signals reflected in the tissues of fish, seals and other killer-whale food. Prey from California waters contains a chemical signature that reflects high use of DDT as an agricultural pesticide prior to the U.S. ban in the 1970s. Likewise, chemical ratios that show high levels of flame retardants such as PBDEs are linked to areas near urban runoff and sewage outfalls.
The signal of localized pollutants is so distinct that it is echoed in the tissues of killer whales, Krahn said. Sub-populations once thought to feed mainly on salmon in Alaska are now suspected to be feeding on shark or tuna in California during the winter. Until recently, information about these animals was based on limited field studies. "This approach allows you to go where mere field observation would fail you," he said. "We're filling in the pieces of a puzzle."
Declines in the killer-whale population of Puget Sound, for instance, might be linked to contaminated fish or increased susceptibility to stresses such as vessel traffic, and adverse health effects of PCBs could explain recent observations of pseudo-hermaphroditic polar bears and bears with decreased immune function. In both cases, however, researchers have not yet obtain sufficient evidence for a conclusive connection.
"As old chemicals are taken out of use, new ones pop up," Krahn said. "Often we don't know if the new substitutes will harm the ecosystem."
Over the past 15 years, 17 percent of dead and stranded sea lions in California have been diagnosed with a urogenital cancer similar to the disease in humans. Researchers found high levels of POPs in these animals and they suspect contaminants, when combined with specific genetic traits – and perhaps a herpes virus – become a key factor in causing the cancer.
"We see the herpes virus in sea lions without cancer, so the virus alone isn't enough to cause cancer in sea lions, which is why we think organochlorines (a class of POPs) play a role either directly in the urogenital tract or in suppression of the immune system," said Frances Gulland of The Marine Mammal Center in Sausalito, Calif.
"These are top-level predators," Gulland said. "Like us, they eat anchovies, squid, salmon and mussels. They are sentinels for human health, because they share our ecosystems and prey. We may see impacts in sea lions before people – they could be an early warning."
