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What we can learn from cancer in the animal kingdom

May 28th, 2007, 02:25 PM
What we can learn from cancer in the animal kingdom


Special to Globe and Mail Update

May 28, 2007 at 12:39 AM EDT

This month, the American Cancer Society's journal, Cancer, published a database identifying 216 chemicals that are known to cause breast cancer in animals. Many of the same chemicals are also present in consumer products, food contaminants, air pollutants, and in our places of work.

Until recently it was widely believed that cancer was caused mostly by our lifestyle and dietary choices, with a little bit of hereditary bad luck thrown in. It is not only humans, however, who are getting cancer. The evidence of cancer in the animal realm is one of many factors that are fuelling a sea change in public thinking about the causes of cancer.

Beluga whales have lived in the world's northern waters for millions of years, eating octopus, crabs, and fish. In the St. Lawrence estuary, however, the belugas are now getting cancer, and yet they don't drink, smoke, eat junk food or lie in the midday sun. Their death rate from cancer (1 in 4) is the same as it is among Canada's human population. They are also having trouble reproducing as many humans are.

When scientists look for cancer among belugas that swim in the open Arctic waters, however, they find none. So what's happening? The beluga autopsies reveal high levels of polycyclic aromatic hydrocarbons, a known carcinogen that most likely comes from an upstream aluminum smelter.

It's not just the whales. In Washington, D.C., four blocks from the White House, the Registry of Tumors in Lower Animals has almost 4,000 specimens of cancer in fish, amphibians, reptiles, and invertebrates that have been collected by the Smithsonian and the National Cancer Institute.

Epidemics of liver cancer have been found in 16 species of fish in 25 different polluted locations, both fresh and salt-water. The same tumours have been found in bottom-feeding fish in industrialized and urbanized areas along Canada's Atlantic and Pacific coasts. In Canada's non-polluted waters, cancer in fish is almost non-existent.

And then there is the disturbing evidence of cancer in dogs. A 1989 study of more than 8,000 dogs showed that canine bladder cancer was associated with their living in industrialized countries, mimicking the distribution of bladder cancer among humans.

Between 1975 and 1995 the incidence of bladder cancer in dogs examined at veterinary teaching schools in North America increased six-fold. Scottish terriers, Shetland sheepdogs, wirehaired fox terriers and West Highland white terriers had a higher risk than mixed breeds, suggesting a genetic susceptibility to cancer among the terriers, but not a reason for the increase.

When researchers interviewed the owners of Scottish terriers with bladder cancer, they found that dogs whose owners had used phenoxy acid herbicides on their lawns were four to seven times more likely to have cancer than dogs whose owners had not.

The "cancer in dogs" studies reveal the multi-factorial nature of cancer. Bladder cancer in dogs is linked to the use of insecticidal flea and tick dips, but more so if the dogs were obese, and lived near another source of pesticides. Dietary protection is important, too. In the terrier study, the researchers found that when the Scotties ate green leafy vegetables three times a week, there was a 90% reduction in their risk of cancer.

Birds very rarely get cancer, but in recent years herring gulls in the polluted waters of the Great Lakes have been getting cancer, demonstrating the same multi-step model of carcinogenesis that is apparent among humans. Research shows clearly that the waters contain chemical pollutants that are known carcinogens.

We humans are being exposed to the same burden of toxic contaminants that is producing cancer in animals and fish. What makes us think we are immune to their effect, or that our cancers are caused by lifestyle choices?

Until Canada develops a strong and urgent commitment to remove the pollutants from our waters, air and food, and from the drugs, cosmetics and cleaning products that people use, we may be fooling ourselves if we expect the incidence of cancer to fall.

Go on searching for a pharmaceutical cure, by all means. But it also makes sense to observe what is happening to the animals and fish and make urgent moves to prevent cancer both in animal and human populations, by removing the carcinogens that are known to cause cancer.

Guy Dauncey is co-chair of Prevent Cancer Now. Michael Gilbertson has worked with the federal government monitoring and regulating persistent toxic substances for 34 years.