On the trail of canine cancer
By Jane Brackman, PhD
In Dog Diseases Treated by Homeopathy (first published in 1863, before chemotherapy, radiation, biopsies and blood panels), author James Moore advised concerned dog owners on treating cancer: “An operation cannot remove the cancer, but it can remove the tumor, which causes much suffering; the knife is, therefore, merely palliative in its effect. Still, the disease, even then, is likely to return at a period more or less remote.”
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“Cancer” is the broad term for a complex cluster of more than a hundred diseases. Although there are many causes, each type of cancer starts with alterations in genes that tell cells how to function, which triggers accelerated and uncontrolled cell growth. The defective signal may hide in abnormal genes inherited from parents, or germinate when normal genes are exposed to harmful environmental influences.
Some breeds are predisposed to certain types of cancer, and in those cases, a strong inherited genetic component is suspected. It is thought that a small number of genes account for cancer risk, but, although they are directly related to the development of individual cancers, rarely is a single gene the sole cause.
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Goldens as Case Studies
The high incidence of cancer in Golden Retrievers appears to be a relatively recent phenomenon. Although the breed was neither over- nor under-represented in a 1988 health study conducted by the University of Pennsylvania School of Veterinary Medicine (Prymak et al. 1988), a health report published 10 years later by the Golden Retriever Club of America (GRCA) identified cancer as the cause of death in 61.4 percent of their dogs (Glickman et al. 1999).
Interestingly, cancer risk in European-bred Goldens appears to be significantly lower. A 2010 study put the mortality figure at 38.8 percent (Dobson 2012, Adams et al. 2010). Although much higher than average, the incidence is substantially lower than that found in North American Goldens.
Goldens in Europe and the U.S. may look similar, but there are enough DNA differences to separate the dogs into two distinct populations corresponding to their geographic regions. Gene pools on both continents are large, so breeding between the two populations is rare.
When studied in the lab, genomic differences suggest that risk for some types of cancer is related to recent genetic mutations in North American Golden Retrievers. And this could be good news: genetic differences between European and North American Golden Retrievers may be key to understanding the etiology of canine cancer overall.
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Animal epidemiology studies (which deal with the incidence, distribution and possible control of diseases) are few in number, largely because they face so many obstacles. Among them are limited funding, participating private practices using non-standardized methods to record data, pedigrees that are difficult to confirm, and grieving owners who aren’t eager to authorize post-mortem tests. Some cancers are silent killers, so a sudden death may not be attributed to cancer. And, due to attrition, studies are difficult to adequately enroll and don’t last long enough to develop statistically significant patterns.
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Based on observations summarized in questionnaires, researchers hope to identify potentially modifiable risk factors that may account for the high incidence of cancer and other diseases in Golden Retrievers and, eventually, in all dogs.
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Although other breeds are at higher risk for cancer, Golden Retrievers were chosen because their population exceeds that of most other breeds by a large margin: the larger the sample size, the more accurate the data. Additionally, because these versatile dogs perform multiple jobs—from assistance and search-and-rescue work to field, performance and companion tasks—they are subject to a broad range of environmental exposures.
Rodney Page, veterinary oncologist and director of Colorado State University’ Flint Animal Cancer Center and the project’s principal investigator, describes some of the factors underlying the study. “Within the estimates that we currently have, we know that 50 percent of canine cancers are curable with surgery and other treatments. Our study is addressing the ones you can’t cut out: the 15 percent of mast cell tumors that aren’t operable; lymphoma, a whole-body cancer; osteosarcoma and hemangiosarcoma, because they spread rapidly before they’re discovered. These four types of cancer cause approximately 80 percent of cancer deaths in Golden Retriever dogs.” These fatal cancers begin to increase in incidence around five to six years and peak about age nine or ten. However, they may also develop in dogs age two and younger.
More than 2,050 veterinarians are providing health care for enrolled dogs. Annually, they conduct physical exams and report findings online; collect samples of blood, urine, feces, hair and toenail clippings and send them to participating laboratories; submit health information from additional heath visits; collect tumor tissue samples when applicable; and provide owners with information and guidance to help them make a choice about necropsy (an autopsy performed on an animal) after the dog dies.
Participating veterinarian Stephanie Ensley of Bentonville, Ark., who enrolled her own Golden in the program, elaborated on the study’s intent. “The information we’ll gather looks at areas of potential exposure by air, contact and feeding. Owners are expected to provide information as detailed as chemicals used in the home, yard and on the dog, and drinking water sources, to name just a few. When all this information is put together and analyzed, we’ll have an opportunity to find commonalities that may be related to cancer and other diseases. The more data available, the more opportunity to find a connection. On the flip side, we’ll also find commonalities in dogs who live to be 15 and over.”
Among the study’s other expected outcomes are insights into how dog-breeding practices and lifestyle choices might be modified to reduce the prevalence of myriad diseases, and the creation of a repository of biological samples that may be made available to researchers working to develop genetic tests.
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