When Janet Riordan returned home from a European vacation in January, she expected a storm of tail wagging and barking from her 7-year-old golden retriever, Reggie. The moment she saw him, she knew something was wrong.
“He came to me in my arms and appeared to be sobbing. I had never seen an animal behave like that,” said Riordan, who lives in Mequon, a suburb of Milwaukee, Wis.
A veterinarian confirmed her fears: Reggie had an aggressive form of lymphoma, a cancer of the white blood cells.
Riordan knew the toll that lymphoma could take; four years earlier her father died of it.
“It was devastating,” Riordan said. “I never thought I would lose my dad and my dog to the same disease.”
Pet owners share their homes, their exercise habits and sometimes even their food with their four-legged companions. And increasingly, they are sharing the same diseases: Dogs and cats suffer from obesity, diabetes, heart disease, cancer and asthma, just like humans.
Now researchers are examining the role that pollutants and other environmental factors play in these dual diseases. Doctors and veterinarians have begun to work together to identify common risk factors, such as pesticides, air pollutants, cigarette smoke and household chemicals.
“Because our pets share our environments, they are exposed to many of the same pollutants as us,” said Melissa Paoloni, a veterinary oncologist at the National Cancer Institute in Maryland.
Pets, like many young children, often have higher exposures to lawn and garden pesticides and to household chemicals that can accumulate in dust or on carpets.
Scientific research is beginning to reveal some links between their environment and their health. Lawn care chemicals may increase the risk of canine lymphoma and bladder cancer. Cats exposed to flame retardants have a higher rate of thyroid disease, according to one study. And researchers are launching the Golden Retriever Lifetime Study, the largest project ever to tackle disease prevention and treatment in dogs.
“People are beginning to realize the untapped resource that companion animals present for research in human health,” said Rodney Page, director of the Colorado State University’s Animal Cancer Center.
Studies in pets can never replace studies in humans, but they can present corroborating evidence. Linking pollutants to human health effects can prove controversial, “but if we can find the same links in dogs or cats, that can have a powerful effect,” said John Reif, a Colorado State University veterinarian and epidemiologist. “It’s one more piece of evidence that the link is a real one.”
Riordan will never know what caused Reggie’s lymphoma. Golden retrievers generally have a high rate of cancer, most likely for genetic reasons. But some research suggests that environmental chemicals may play a role in the development of lymphoma in dogs.
Researchers at the University of Massachusetts and the Tufts University Cummings School of Veterinary Medicine questioned the owners of more than 700 dogs about use of pesticides. Roughly one-third of the dogs had been diagnosed with canine malignant lymphoma, while the other two-thirds had either benign tumors or were undergoing non-cancer surgeries.
Dogs whose owners reported use of professionally applied lawn pesticides were 70 percent more likely to have lymphoma, according to the study published in the journal Environmental Research in January.
Dogs also were at higher risk of lymphoma if their owners used self-applied insect-growth regulators on their yards, such as Nylar, Precor and Gentrol, which control cockroaches, fleas and other pests. However, dogs exposed to flea powders, sprays and on-spot treatments were no more likely to develop lymphoma than those whose owners did not use them,
In addition, Scottish terriers exposed to certain herbicides, including the common weed killer 2,4-D, were more than four times likely to develop bladder cancer than those whose yards were untreated, according to a 2004 study by Purdue University veterinarians.
Results of other studies have been mixed, with some showing an increased lymphoma risk in pets exposed to lawn chemicals and others finding no link.
Malignant lymphoma in dogs closely resembles non-Hodgkin’s lymphoma. More than 60,000 Americans a year are diagnosed with the disease, making it the sixth most common cancer in the United States.
“The close interaction and shared household environments of dogs and their human owners provides a unique opportunity for evaluating how herbicide and pesticide exposure may contribute to human non-Hodgkin’s lymphoma,” the study authors wrote.
Pesticides may increase the risk of the disease in people, too. Last year, Danish researchers found that people with high levels of DDT and other organochlorine pesticides were more likely to develop non-Hodgkin’s lymphoma years later.
“Clearly dogs are not humans, but physiologically speaking, they are very similar,” said Lisa Barber, a veterinary oncologist at Tufts University and study author.
“The most heartbreaking thing is their short lifespan. It’s also what makes them useful models for human disease,” she said. Because dogs live accelerated lives compared with humans, researchers can gather information on a lifetime of exposure much more quickly than in people.
Canaries in the coal mine
Using animals as sentinels for human health is not a new concept. More than 100 years ago, miners took caged canaries into coal mines to warn them of toxic gases.
In the 1950s, thousands of people in Japan died or suffered serious effects from eating mercury-poisoned fish from Minamata Bay. Locals had first noticed strange neurological symptoms in cats, which they described as dancing in the streets before collapsing and dying.
Pets also played an important role in drawing a link between asbestos and mesothelioma. In the 1980s, researchers found high levels of asbestos fibers in the lungs of pet dogs diagnosed with the lung disease. The finding helped increase understanding of the threats that asbestos posed to people, said Reif from Colorado State.
More recently, researchers have found that ozone, the main ingredient of smog, may contribute to asthma in cats, and household tobacco smoke may be a risk factor for nose, throat and lung cancers in dogs.
A rise in hyperthyroidism in cats also has been linked to brominated flame retardants, which are used in upholstery and electronics and contaminate dust and canned cat foods. Cats with overactive thyroids — which can lead to weight loss, increased appetite, hyperactivity and death — had higher blood levels of the chemicals, according to one small study led by Environmental Protection Agency scientists.
Because of their meticulous grooming habits, cats may ingest a lot of dust. The link to hyperthyroidism in felines “should be alarming to parents of crawling toddlers who explore their environments by putting everything in their mouths,” said Donna Mensching, veterinary medical director of the Washington Poison Center in Seattle.
Toddlers with high exposure to the flame retardants have lower IQs, according to one study. The chemicals also have been linked to altered thyroid hormones in pregnant women, which might harm a baby’s brain development.
How genes and pollutants add up
Looking at the way environmental pollutants might interact with genetics in animal breeds susceptible to certain diseases may benefit human health as well.
“We know something about their breed history and susceptibility to certain diseases, which may make it easier to tease out gene-environment interactions,” said Dr. Robert A. Hiatt, an epidemiologist at the University of San Francisco and a former family physician.
The functions of certain genes are very similar in dogs and humans, according to Hiatt. “What we learn from pets may also be applicable to humans,” he said.
One of many questions the Golden Retriever Lifetime Study aims to address is how environmental chemicals may interact with genes in a breed that is susceptible to health problems. An estimated 60 percent of golden retrievers die from cancer, according to the Morris Animal Foundation, a nonprofit group that is funding the study. As a breed, they may be genetically susceptible, regardless of what chemicals they may have been exposed to.
The nationwide study will enroll 3,000 young golden retrievers and follow them through their entire lives. Page, one of the lead investigators, likens it to the Nurse’s Health Study, one of the longest running women’s health studies in the country.
“The opportunity will be quite seminal and transformative in terms of exposure science, because it will offer a new set of data with which to evaluate similarities with human exposure data,” Page said.
This research also may help experts develop treatments for diseases.
“We can cure anything in a mouse, but so many times new drugs fail miserably when taken straight from lab animals to human trials,” said Heather Wilson-Robles, a veterinary oncologist at Texas A&M University.
Dogs and cats develop diseases spontaneously for many of the same reasons people do, which means experts can predict from pets how a new drug may act in humans. “Mouse models are really important in the development of new treatments, but we are skipping a step when we take a drug from lab animals to humans without first looking to our veterinary patients,” Wilson-Robles said.
In Reggie’s case, Riordan and her vet looked first to human studies to form a treatment plan. He received chemotherapy and experimental high-dose vitamin C injections, a treatment that Riordan had uncovered while researching options for her father. “We thought if it worked in humans, it might work for dogs,” she said.
In February, less than two months after being diagnosed with canine lymphoma, Reggie died.
Riordan wasn’t aware of the link between lawn-care products and lymphoma in dogs, but, she said, “we were always really careful about chemicals. We don’t use pesticides in our yard or a lot of chemicals in the house.”
While Riordan hopes researchers may one day be able to prevent dogs like Reggie from getting cancer, she knows tragedy comes with pet ownership. “We love them so much that even if they don’t die of cancer, they will ultimately break our hearts,” she said.
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