Diseases May be Early Warning Sign for Humans Say Scientists
St. Louis, MO (AAAS Annual Meeting, Press Conference: Saturday, Feb. 18th, 9 am Central Time) - Leading scientists, physicians, and veterinarians are uncovering new links between land-based pollution and diseases in marine mammals, with implications for human health.
“Marine mammals are providing early clues of our unseen impact on the sea,” says Paul Sandifer, Chief Scientist for the new Oceans and Human Health Initiative in the US National Oceanic and Atmospheric Administration (NOAA). “There is mounting evidence that our activities on land are taking a toll on the health of the oceans, and in turn our own well-being.”
“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,” says Pat Conrad from the University of California at 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.”
Cats and Otters: A Deadly Link to Land?
When Pat Conrad first started studying the death of otters on the California coast, she had no idea that her investigation would lead her upstream. Her team traced 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 gets in to the environment - how what we dump on our lawns and sidewalks flows into streams to rivers and into the ocean,” says Conrad.
But with the discovery that otters in areas with heavy freshwater outflow are nearly three times as likely to be infected with Toxoplasma, and the fact that sea otters spend their entire lives grooming, sleeping, eating, and playing in the same nearshore waters where humans swim and surf, this land-sea connection is front and center.
Toxoplasma can also infect humans. It is the third most common cause of death due to food borne disease in the US: estimates indicate that up to 25% of the population may be infected with the parasite, but people rarely show symptoms or are simply hit with a flu-like illness. However, if women are infected when they are pregnant, it can lead to miscarriages or developmental problems in the fetus, most often affecting brain function and sight. Toxoplasma can also cause a severe brain disease in people with suppressed immune systems from transplant surgeries or HIV/AIDS.
“When otters get toxoplasmosis, they get far more sick than most humans,” says Conrad. This allows researchers to more easily detect and trace the disease – and calls attention to the flow of pathogens from land to sea that might otherwise go unnoticed.
Conrad recommends that cat owners keep their animals inside and dispose of kitty litter by bagging it up and sending it to a sanitary landfill. “I know this is tough,” says Conrad. “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.”
Not Just Shellfish Poisoning: The Unseen Effects of Red Tides
Manatee die-offs are revealing new health impacts of algal blooms - for both manatees and humans.
“Recent, frequent red tides off the west coast of Florida correlated with a 54% increase in emergency room admissions for respiratory illnesses, including, pneumonia, asthma attacks, and other respiratory problems,” says Gregory Bossart of the Harbor Branch Oceanographic Institution.
Scientists suspect that these diseases are linked to brevetoxins, compounds produced by specific algal blooms. Neurotoxic shellfish poisoning and other acute effects of consuming brevetoxins are not new to scientists. But Bossart and his colleagues’ work on manatee die-offs associated with red tides shows that inhalation of the toxin is also a primary route of exposure. Recent lab work also suggests that long-term, repeated exposure to brevetoxins suppresses respiratory and immune systems, making manatees and other animals more susceptible to new diseases.
“The insidious effect of chronic exposure is what worries me the most,” says Bossart. “This infers that you could really open a Pandora’s box in these animals, and possibly in humans. Manatees are the 2000 pound canary.”
Bossart is also concerned about new data indicating that fish and other food sources - once thought to die too quickly to pass on brevetoxins – can concentrate and carry the toxin long after an algal bloom. “This is surprising,” he says. “We never suspected that there could be vectors like seagrass or fish.”
Many scientists believe algal blooms are likely caused by multiple stresses, such as nutrient overloading and global warming, acting together. These blooms are on the rise, and many are associated with toxins and bacteria that are harmful to humans. “We need to figure out what is causing this increase,” says Bossart. “It’s in our own best interest.”
Mammals Are What They Eat
Land-based pollutants are 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 – there are all sorts of sources,” says Todd O’Hara of the University of Alaska Fairbanks.
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 like killer whales and polar bears, who share many of our marine foods, accumulate these contaminants in their tissues.
Todd O’Hara works with a research team to study the ecological, biochemical, and physiological details of this contaminant build-up in mammal tissues. One piece of their work focuses on how health risks change with different diets. Levels of organohalogens and mercury are higher in animals like polar bears and seals that have fish 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,” says Peggy Krahn of the NOAA. “Until we understand this, we can’t start to say how to mitigate pollutants in the ocean.”
Krahn and her colleagues trace 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 that are reflected in the tissues of fish, seals, and other killer whale food. Prey from California waters have a chemical signature that reflects high use of DDT as an agricultural pesticide prior to the US ban in the 1970’s. Similarly, chemical ratios that show high levels of flame retardants (PBDEs) are linked to areas near urban runoff and sewage outfalls.
The signal of pollutants is so distinct that it is echoed in the tissues of killer whales. Sub-populations once thought to feed mainly on salmon in Alaska are now suspected to be feeding on high trophic-level fish, such as 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,” says Krahn. “We’re filling in the pieces of a puzzle.”
Some suspect that declines of killer whales in Puget Sound may be linked directly to contaminated fish or increased susceptibility to stresses like vessel traffic. Similarly, adverse health effects of PCBs may explain recent observations of pseudo-hermaphroditic polar bears and bears with decreased immune function, but there is no clear evidence that would prove a cause-and-effect relationship. Scientists hope their research will help society proactively evaluate the effects and trade-offs of important pesticides and other chemicals before they cause problems in the environment.
“As old chemicals are taken out of use, new ones pop up,” says Krahn. “Often we don’t know if the new substitutes will harm the ecosystem.”
“It’s a struggle to find replacements,” says O’Hara. “But wouldn’t it be better to know ahead of time if certain policies or practices might be disastrous? We could talk more about prevention rather than trying to fix the damage.”
Sea Lions, Herpes, and Cancer
In the past 15 years, 17% of dead and stranded sea lions in California have been diagnosed with a urogenital cancer, similar to cancers we see in humans. Researchers found high levels of persistent organic pollutants (POPs) in these animals and believe contaminants, when combined with specific genetic traits and a herpes virus, are 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,” says Frances Gulland of The Marine Mammal Center.
“These are top level predators,” says Gulland. “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.”
Compounding Factors & Climate Change – Alaska as a Laboratory
One of the most challenging aspects of understanding how humans impact ocean health is the fact that so many environmental changes are going on at the same time. Climate change is already having an effect on the spread of disease and pollutants, adding yet another twist to the story.
Jim Berner, an MD with Alaska Native Tribal Health Consortium, works in northern ocean ecosystems, where climate change is having a marked effect. “Alaska is a laboratory for human health issues, particularly those that have to do with climate and zoonotic disease - those that can be transferred from animals to humans. It’s a measurable, visible impact. We’re seeing new health threats in Alaska as a result of climate change.”
Warming temperatures are likely to increase rainfall in the North, leading to increased run-off to ocean systems and melting of ice. These changes affect how contaminants are delivered to the food web, and thus to people. Habitat loss and increases in water temperature also allows both pathogens and disease vectors to move into new areas.
“The rest of the world is likely to see similar changes as we go forward in time. We’re just seeing change more rapidly in Alaska,” says Sandifer.
“What is happening in the Arctic will affect policy at the international and national level,” adds O’Hara. “Native populations have said, this is our food, our way of life, not just our wildlife.”
Getting the rest of the world to pay attention to the health of marine mammals may be the first step in mitigating potential human health impacts. “This is the age of discovery of new diseases and causes of marine mammal deaths,” says Teri Rowles of NOAA. “We learn something new every single month, and we continue to gain insight into what is going on with the ecosystems in which marine mammals live. This information will help us tackle how to ensure the future health of the oceans - and ourselves.”
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