Cancer sleuths say canines are friends indeed

Monday, July 3rd 2000, 12:00 am
By: News On 6

Some researchers get to spend all day working on pet projects.

They are a small but dedicated group of scientists, scraping together funding here and there, who believe that some secrets to diseases of man lie in man's best friend.

Of all the world's animals, none has lived closely alongside humans longer than the dog. For more than 12,000 years, dogs have shared people's homes, jobs and, with enough whining, their dinners. They even share some of the same diseases. Only dogs and men, for example, develop prostate cancer with any regularity.

"They breathe the same air. They walk on the same dirt," said Col. William Inskeep of the Armed Forces Institute of Pathology.

Dr. Inskeep and his colleagues are studying illnesses in dogs deployed to the Persian Gulf in 1990 and 1991. Researchers have also looked at dogs to learn more about exposure to secondhand smoke and electromagnetic power lines, and about bone cancer, prostate cancer, lymphoma and many other malignancies.

The handful of scientists interested in this field, many of them confessed dog lovers, say they realize that many of their colleagues think canine epidemiology is barking up the wrong tree.

"If you want to generalize to humans, study humans," said Jennifer Kelsey of Stanford University School of Medicine. But, she said, "under certain circumstances the dog studies could shed light on human disease."

If nothing else, researchers say, the data could help veterinary medicine, improving the well-being of millions of pet dogs and the owners who love them. "We owe it to the dog to find out how we can prevent cancer in them," Dr. Kelsey said.

But along the way, the study of dogs might also help prevent cancer in people. Take, for example, the way dogs are helping guide the study of bladder cancer in their human companions.

In the 1980s, research suggested that bladder cancer rates in dogs might be higher in industrial areas. Research has also linked bladder cancer, particularly among obese animals, to exposure to certain compounds in insecticides.

In Colorado, researchers are comparing 200 dogs with bladder cancer to 200 dogs without cancer to examine the risk of exposure to certain compounds in chlorinated water. It is the first study of its kind funded by the National Center for Environmental Health, which is part of the U.S. Centers for Disease Control and Prevention.

"Our mission is public health with regard to human health," said the CDC's Lorrie Backer. However, she added, dogs might be a simpler way to study some aspects of disease. Suppose that these chlorination byproducts did pose some risk to human health, she said. A person might not develop cancer for decades.

"Dogs have shorter life spans," she said. "A dog might develop bladder cancer after seven years."

Dogs' compressed life spans aren't the only reason they might be good models for studying disease, said John Reif of the Department of Environmental Health at Colorado State University, who heads the bladder cancer project. Looking at dogs might quiet the background noise that interferes with study results. Human epidemiological data can be vastly complicated because people relocate, eat a dizzying variety of foods and put a lot of foreign substances in their bodies.

Dogs have simpler lives. "You pretty much know where the dog is spending the vast majority of its time," Dr. Reif said. "The dog is not commuting to work. The number of residences where a dog has spent its time is limited." Dogs also don't drink alcohol, and eat mostly the same thing every day.

A simple way to study whether a chemical might cause cancer is to expose rats in a laboratory. But by turning to pet dogs, Dr. Reif said, "you're doing studies in a natural environment under exactly the conditions that you're interested in."

For example, Dr. Reif and his colleagues have reported in the American Journal of Epidemiology that when exposed to environmental tobacco smoke, dogs with short noses have an increased risk of lung cancer, while breeds with long noses have an increased risk of nasal cancer.

Pet epidemiology studies also "never subject animals to noxious exposures, thereby representing an attractive alternative to laboratory animal approaches," researchers from the Robert Wood Johnson Medical School in Piscataway, N.J., wrote in 1997 in the journal Environmental Health Perspectives.

And sometimes, the conditions under scrutiny can't be re-created in a laboratory anyway. During war "these dogs are the only other mammal in the theater of operation," Dr. Inskeep said. He wants to see whether dogs sent to the Persian Gulf War died from any unusual illnesses. When a dog deployed to the Gulf War dies, veterinarians perform autopsies and take samples of every major organ. Pathologists – who do not know where the dog has served – then examine the tissues looking for signs of disease.

Samples from the war dogs are being compared with those from other military dogs of similar breeds that served elsewhere in 1990 and 1991. There are no study results yet, though, because six of the 118 dogs who went to war are still alive. One of them is a 14-year-old German shepherd now living at Lackland Air Force Base in San Antonio.

In previous generations, researchers examined the medical conditions of dogs that served in Vietnam, and discovered inexplicably high rates of testicular cancer. Ten years ago, in the Journal of the National Cancer Institute, researchers reported that the testicular cancer rate in military dogs was high enough to suggest that, among men, "military service in Vietnam be considered a risk factor for testicular cancer."

Most of these dog studies have focused on cancer, looking for common risk factors between dogs and people.

"The cancers that dogs develop are more completely understood," said David Waters, a veterinarian at the Comparative Oncology Program at Purdue University in West Lafayette, Ind. "They fall into categories that make them more similar to cancers in people."

And even when dogs have cancers all their own, researchers can find clues to disease, Dr. Waters believes. For instance, large-breed dogs are more susceptible to bone cancer, for reasons that aren't understood. Learning why might reveal secrets about bone cancer. Then there are purebred cocker spaniels, who rarely get testicular cancer.

"To me, that is extremely intriguing. Why isn't the cocker spaniel susceptible?" Dr. Waters said. These differences in disease patterns, he said, "can be quite instructive."

Researchers say they lack some basic knowledge that might help the understanding of cancer. The kinds of cancers that affect humans are counted, ranked and statistically scrutinized, but not those that affect dogs.

Veterinary schools keep tabs on what kinds of malignancies they see, but those numbers may not reflect the general dog population. Most dog owners live too far away to take a sick pet to a veterinary school. Or, faced with a dog's cancer, an owner will often choose pain control and euthanasia over actual cancer treatment, so the case will never appear in a veterinary school database. "We know the most common ones [cancers] referred to veterinary schools, but they are the ones for which we have treatment," said Stanford's Dr. Kelsey, who is in the school's department of health research and policy.

Epidemiologists like Dr. Kelsey have learned a great deal from studying the kinds of cancers that strike people. So if dogs, who so closely share daily life with people, don't get certain cancers, what in their biology protects them? And for the cancers they do share, what forces are at work in both species?

"I think we have a gold mine," Purdue's Dr. Waters said of the data he and his colleagues are gathering. "I think we have a wonderful opportunity to have an impact on our understanding of cancer."