October 2, 2018
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:
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
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.
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.
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.
Here are some of the few studies done on cancer incidence in dogs and cats:
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.
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.
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.
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.
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