July 1, 2009 -- Cancer patients come in all shapes and sizes, including whale, sea lion and Tasmanian devil.
In these wild animals and others, cancers are significant killers, according to new research, and human pollutants probably play a role. The work suggests that cancer might be an overlooked conservation threat.
A closer examination at the causes and pathways involved might also give scientists insight into our own health.
"At the end of the day, there isn't much difference between people and animals and the kinds of diseases they can develop," said Denise McAloose, chief pathologist at the Wildlife Conservation Society in New York City. "We're trying to create a connection for people to wrap their heads around and say, 'Hm, maybe some things we find in animals can help us learn about human cancers, too.'"
So far, most research on animal cancers has looked at just one cancer in one animal or species at a time. To see what, if anything, cancer in wildlife could tell us about the health of the environment and ourselves, McAloose and a colleague collected a wider variety of case studies.
The Tasmanian Devil emerged as one of the most dire examples. The animal, which looks sort of like a dog-sized rat, is the world's largest carnivorous marsupial. It is also the top predator in Tasmania, an island off the southeastern coast of Australia.
In the mid-1990s, Tasmanian Devils developed a rare type of cancer called devil facial tumor disease, which spreads through direct contact from fighting, biting, and scratching. The disease has already killed more than half of the population, and scientists fear that the animals might go extinct because of the disease.
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It would be the first time that a contagious cancer caused the extinction of a species. And extinction of Tasmania's top predator, McAloose said, would throw off the biodiversity and health of the entire ecosystem.
"This really raises the bar to a very high level in the conservation and health communities," she said. Her study appeared in the journal Nature Reviews Cancer. "The whole scientific community should be interested in understanding why they are getting these cancers."
There is no human equivalent for devil facial tumor disease, but many wildlife cancers have parallels in our world. Lymphomas in chicken, fibropapillomatosis (which involves skin tumors) in green turtles, and genital tumors in dolphins and sea lions are all cancers that are caused by viruses, in the same way that the HPV virus transmits cervical cancer in women.
In fact, scientists originally discovered that viruses could transmit cancer in animals before they found the same to be true in people.
In many cases, animal cancers appear more often in places that are more polluted. In beluga whales that live near aluminum smelting facilities in the St. Lawrence River estuary in Quebec, for example, cancer is the second leading cause of death. Fish that live in industrial areas have high levels of skin and liver cancers. And in experiments, brown bullhead catfish that were exposed to polycyclic aromatic hydrocarbons, one of the main pollutants in these areas, rates of skin tumors increased by 38 percent.
Many of these cancers have emerged in just the last decade or two, said Richard Montali, a veterinary pathologist specializing in zoo animals and wildlife at Johns Hopkins University in Baltimore. Wild animals, he said, might serve as a sentinel for toxic substances in the environment that might also threaten people.
"We're all living cells whether you're a lower animal or a human," Montali said. "Wherever there are animal cells, there will be cancers."
Studying cancer in wildlife isn't easy, which helps explain why so little work has been done in the field.
"It's not as simple as making a doctor's appointment and going down the street for a check-up," McAloose said. "We have to actually get to these animals. That can be challenging."
Still, she said, making an effort to monitor and track these diseases will be essential for maintaining the health of animals, people, and the Earth.
Related Links:
Discovery News Blog: Born Animal
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