No one really knows if canine and feline cancer rates are increasing. Cancer is not a reportable disease in pets; there are no databases of cancer cases covering large enough populations in sufficient detail to make reliable statements about overall cancer rates. I often see claims in the media—from veterinarians as well as lay people—that canine and feline cancer rates are rising, and we are experiencing an “epidemic.” Usually, this is a prelude to further claims about the causes of this so-called epidemic. Finally, a solution is typically offered, involving either eschewing the purported causes of cancer or employing a diet, supplement, “detox” program, or some other product or method recommended (and sold) by whomever is making the claims. One recent example of this phenomenon is the video series called “The Truth About PET Cancer,”1 which includes many statements such as these: “You look back 50 years ago, where some will say that the cancer rate may have been one in 100 dogs. Today, according to PhDs, the dog has the highest rate of cancer of any mammal on the planet. Literally, from last year, them saying one in two, to this year, one in 1.65 dogs will succumb to cancer... and one in three cats.” “There is an epidemic of cancer. There’s a lot more cancer.” “In my experience, we are seeing cancer in younger and younger animals.” “The cancer rate in pets parallels the cancer rate in humans. We’re all getting cancer.” “We do see, commonly, dogs under 18 months of age with terminal cancer. What used to be a disease of the old is now unfortunately a disease of the young.” In this video series, the cause most often cited for the supposed epidemic is commercial pet food, though vaccines, parasite preventatives, environmental toxins, and many other purported carcinogens also are mentioned. The proposed solutions mostly involve feeding dogs and cats raw, homemade, and ketogenic diets, though avoiding the “toxins” mentioned above is also recommended, as is the use of herbs, acupuncture, essential oils, energy medicine, and many other alternative treatments. So is there any evidence behind claims that cancer is becoming more common and affecting younger animals than in the past? There are plenty of personal experiences and anecdotes offered to support this idea, but it has long been clear that our subjective impressions and clinical experience are terribly inaccurate when it comes to assessing incidence or prevalence.2-4 What do the data say for humans? In human medicine, there is pretty good epidemiologic surveillance of cancer. Cancer is a reportable disease, and long-term data have been collected on the occurrence of many kinds of neoplasia. These data show that while cancer is certainly a major cause of morbidity and mortality, the claim cancer rates are rising across the board in humans is clearly wrong. Overall cancer incidence in the United States is 439.2 cases per 100,000 people per year based on data from 2011 to 2015.5 This general figure isn’t really very useful, as it leaves out all the important details, including who gets what type of cancer and the risk factors for each. Male dog cancer incidence rates estimated across the study period (1985 to 2002) for all cancers and selected cancer sites by calendar period. Vertical bands represent the 95 percent confidence intervals of the estimated incidence rates. Cancer risk is associated with age, genetic risk factors, specific lifestyle and environmental risks, chance events, and other variables we can’t always assess accurately. Individual cancer risk isn’t predicted very well from overall numbers that don’t take into account particular risk factors. Despite these caveats, the best available evidence from the National Cancer Institute shows an overall decline in cancer incidence and mortality.5 Incidence may be declining, stable, or increasing for some cancers and some populations, and the issue is complex, but on balance, overall cancer rates and deaths from cancer in people are not increasing. What about pets? Is cancer truly increasing in dogs and cats? The answer, unfortunately, is no one really knows for sure. Cancer is not a reportable disease in pets, and there are no databases of cancer cases covering large enough populations in sufficient detail to make reliable statements about overall cancer rates. There have been a few studies attempting to assess the rates of cancer in dogs and cats. These have used different methods and different populations, and they have been conducted at different times, so we cannot make direct comparisons among them. Additionally, we rarely have accurate information about the age, sex, and breed distributions of pet populations, which are key variables. There also is dramatic variation in how much effort is put into finding and identifying different types of cancer in dogs and cats. There are no formal, organized screening programs; many dogs and cats receive little or no medical care; and owners often choose not to pursue a specific diagnosis when their pets develop a growth or become ill. Therefore, the real rates of different cancers, and any change over time, is very difficult to know with any certainty. The studies that have been conducted do show some patterns that likely reflect underlying cancer biology. Age, for example, is always a significant risk factor, with more cancers seen in older animals. The claim that cancer is becoming “a disease of the young” is certainly inconsistent with the available evidence. Some breeds also have higher rates of specific cancers, and this indicates genetic risk factors are important. Other individual risk factors are relevant as well, such as sex and geographic location. Females, for example, have much higher rates of mammary cancer in populations where dogs are frequently not spayed, but this common cancer virtually disappears from populations where early neutering is the norm. Trends over time are rarely reported and are difficult to interpret due to changes in study methods, our ability to detect cancer, and the willingness of people to seek diagnosis and treatment for their animal companions. Canine, feline cancer studies Here are some of the few studies done on cancer incidence in dogs and cats: Dorn, 19686: 381.2 per 100,000 dogs over three years, 155.8 per 100,000 cats over three years McVean, 19787: 507 per 100,000 dogs, 412 per 100,000 cats Reid-Smith, 20008: 852 per 100,000 dog - years, 319 per 100,000 cat - years Dobson, 20029: 747.9 per 100,000 dogs/year Merlo, 200810: 99.3 per 100,000 dog - years (males), 272.1 per 100,000 dog - years (females) Vascellari, 200911: 143 per 100,000 dogs/year, 63 per 100,000 cats/year The variation here is significant. This reflects the different study methods, time periods, locations, and many other variables in these studies. In general, the rates in dogs are similar to the much more reliable overall numbers reported for humans, while the rates in cats appear to be consistently lower. Female dog cancer incidence rates estimated across the study period (1985 to 2002) for all cancers and selected cancer sites by calendar period. Vertical bands represent the 95 percent confidence intervals of the estimated incidence rates. However, the devil is in the details. In nearly all studies, mammary cancer is the most common cancer, so females have higher rates of cancer than males. In populations where most females are neutered, however, mammary cancer virtually disappears, and these numbers don’t reflect even this single, clear risk factor. There is virtually no data to assess changes in cancer incidence over time. One study (Merlo 2008)10 used a database with nearly 20 years of data to attempt to make some assessment of trends over time (See charts). These data do show an overall increase in cancer incidence for male dogs, though most of this appears to be significant only in the most recent period. For females, cancer incidence has fluctuated over time, most likely due to changes in mammary cancer rates caused by changes in neutering practices. This single, tenuous data set does not support claims of rampant global increases in all cancer. What’s to be done? Without a doubt, pet owners naturally want to do everything possible to prevent cancer in their animal companions. It is unfortunate this normal desire is manipulated by misinformation and unjustified claims. Until we have large longitudinal databases and formal cancer surveillance mechanisms, we should avoid confident, sweeping statements about cancer rates based on personal opinion and observation. With the evidence we do have, it is more productive to focus on specific, modifiable risk factors and evidence-based methods of treating cancer when it does occur. References The Truth About PET Cancer. Bolinger T. 2015-2018. TTAC Publishing, LLC. Available at: https://thetruthaboutpetcancer.com Rosling H. Rosling O. Rosling Ronnlund A. (2018) Factfulness: Ten reasons why we’re wrong about the world and why things are better than you think. New York, NY. Flatiron Press. Gilovich, T. (1993). How We Know What Isn't' So: The Fallibility of Human Reason in Everyday Life. New York: The Free Press. Kida, T. (2006). Don’t Believe Everything You Think: The 6 Basic Mistakes We Make in Thinking. New York: Prometheus Books. Cronin KA. Lake AJ. Scott S. et al. (2018), Annual Report to the Nation on the Status of Cancer, part I: National cancer statistics. Cancer, 124: 2785-2800. doi:10.1002/cncr.31551 Dorn CR. Taylor DON. Schneider R. et al. Survey of Animal Neoplasms in Alameda and Contra Costa Counties, California. II. Cancer Morbidity in Dogs and Cats From Alameda County. JNCI: Journal of the National Cancer Institute. 1968;40(2):307–318. https://doi.org/10.1093/jnci/40.2.307 MacVean DW. Monlux AW. Anderson PS Jr. et al. Frequency of canine and feline tumors in a defined population. Vet Pathol. 1978;15(6):700-15. Reid-Smith RJ, Bonnett BN, Martin SW. et al The incidence of neoplasia in the canine and feline patient populations of private veterinary practices in southern Ontario. Proceedings of the 9th International Symposium on Veterinary Epidemiology and Economics. 2000. Available at www.sciquest.org.nz Dobson JM. Samuel S. Milstein H. et al. Canine neoplasia in the UK: estimates of incidence rates from a population of insured dogs. J Small Anim Pract. 2002;43(6):240-6. Merlo DF. Rossi L. Pellegrino C. et al. Cancer incidence in pet dogs: findings of the Animal Tumor Registry of Genoa, Italy. J Vet Intern Med. 2008;22(4):976-84. doi: 10.1111/j.1939-1676.2008.0133.x. Vascellari M. Baioni E. Ru G. et al. Animal tumour registry of two provinces in northern Italy: incidence of spontaneous tumours in dogs and cats. BMC Veterinary Research. 2009;5:39 https://doi.org/10.1186/1746-6148-5-39 Brennen McKenzie, MA, MSc, VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.
