Cancer in Humans and Dogs-Are We Victims of Our Success?

Cancer is one of the most dreaded diseases pet owners and veterinarians must contend with. While data are frustratingly sparse, there is no doubt that cancer is common in dogs, with estimates ranging from 25% to 40% of dogs likely to have a cancer diagnosis at some time in their life. 

That time, of course, is nearly always in middle age or later. Cancer is a classic disease of aging. While young dogs, and humans, do sometimes get cancer, the vast majority of cases occur in older individuals. These charts show the frequency of benign tumors and cancers in male and female dogs by age. Cancer is quite rare until about 6-7 years, and then it rapidly becomes much more common. (Interestingly, cancer rates actually decrease in the very oldest dogs. Some of this is just a function of there being few dogs that reach this age at all, but there appear to also be some biological reasons why individuals who reach extreme old age are actually less likely to get cancer than they were when they were younger [ and here]).

Comparison of cancer deaths in humans and dogs show a nearly identical pattern, with a dramatic increase in cancer risk around the period that can loosely be considered “middle-aged,” and then some decline in extremely old populations. 

Of course, cancer is not a single disease, but a general type of disease that can manifest in many different places and ways. Because of this variety, there is no single “cause” of cancer, nor likely to be any single “cure.” However, a limited set of things have to happen for cancer to develop and cause illness. These include-

  • Spontaneous or inherited mutations in genes that support uncontrolled cell growth or inhibit the body’s natural defenses against it
  • Changes in the regulation of gene activity (aka epigenetic changes)
  • A permissive cellular environment that allows uncontrolled cell growth and spread of abnormal cells

Most often, multiple things have to go wrong for cancer to develop and progress to cause actual disease. Several layers of controls on cell growth must fail, and multiple mechanisms for identifying and eliminating abnormal cells must malfunction. Events that could lead to cancer are occurring in all of us all the time, but without the “perfect storm” of multiple failures, we never know it. This perfect storm becomes more likely as we age because of the accumulation of failures in the systems that prevent the development and survival of cancer cells. 

Because carcinogenesis (the development of cancer) is so complex, dangerous oversimplifications are common. It is undeniable, for example, that some environmental risk factors can increase the chances of cancer. Smoking is one of the clearest examples, leading to a dramatic increase in risk of several types of cancer. However, even in this case, it is mistakenly simplistic to say that “smoking causes cancer” without acknowledging that this is a crude shorthand for “smoking increases the risk of cancer.” 

Most people who smoke start doing so in their teens of twenties, yet most don’t develop tobacco-associated cancers until they are in their fifties or sixties. Some heavy smokers never get cancer at all. There is more to the story than just this one risk factor. Age, genetics, and other environmental factors also play a role.

While it is absolutely true that smoking increases the risk of developing cancer, and no one should ever do it, an overly simplistic view about how this works has become part of an unfortunate tendency to overestimate the importance of “toxins” and “chemicals” and visible risk factors that are perceived as “unnatural” while ignoring the equal or greater importance of unavoidable factors such as the passage of time or genetic predisposition. The laudable desire to minimize environmental risk factors becomes the extreme paranoia that sees all human products and activities as “unnatural,” and therefore harmful, and views cancer as a consequence of poor choices rather than an inherent risk of being alive. 

I have discussed this extreme view, and its use to peddle quack remedies and dubious lifestyle advice before (see my post The Truth Behind “The Truth about Pet Cancer” and my review of The Forever Dog). The idea that we and our dogs are living in a “toxic” world that is causing rampant cancer, and that we could avoid it if we would just feed the “right” foods or supplements and avoid the “wrong” diets, medicine, vaccines, or whatever, is simplistic nonsense.

Fear sells, and cancer is legitimately terrifying. Sadly, proponents of unscientific strategies for preventing cancer will often use our fear as a tool, claiming that there is an “epidemic” of cancer, in dogs and humans, and that we are more at risk than ever before. This is almost certainly untrue in humans. The occurrence of some cancers is increasing (e.g. breast cancer in women) while other cancers are becoming less common (e.g. small cell lung cancer in men, thanks to a decline in the popularity of smoking, and cervical cancer, thanks to the HPV vaccine). Overall, the occurrence of cancer seems to be declining, and the death rates from cancer are absolutely improving, due to constantly improving prevention, detection, and treatment. Unfortunately, the data are too limited to support any strong conclusion either way in dogs.

While the concept of a “cancer epidemic” is hyperbole, it is fair to ask if cancer is more common today than it was in the past, even if we do not buy into fairytales about the past as a health “Golden Age” in which humans and dogs lived longer, healthier lives (exactly the opposite of the truth). There is, as it turns out, some reason to believe that cancer is more common in both species than it was in the past.

Again, the situation is complex, and the types of cancer seen have changed over time due to changes in various risk factors. Stomach cancer in humans, for example, declined by 90% during the twentieth century, as refrigeration replaced salting and smoking as a means of preserving foods. This is a clear example that the myth of a healthier pre-industrial lifestyle is nonsense. 

However, many other types of cancer did become much more common during the twentieth century in people. This is often attributed to an increase in environmental risk factors, including purported “toxins.” While this is true in some cases (smoking and some industrial pollutants being clear examples), it overlooks one of the most important reasons why cancer in humans increased during this time, and also a likely reason why cancer probably increased in dogs as well. The simple fact that people, and probably dogs, are living longer.

Cancer is an age-associated disease, and the longer individuals live, the more likely they are to develop it. A population with more older people, or dogs, is going to have more cases of cancer. And there is no doubt that human populations throughout the world are getting older as people are living longer. 

Human life expectancy has increased dramatically, thanks to the fruits of scientific discovery. Food production, nutrition, vaccination and healthcare, sanitation, accident prevention, and many other changes have reduced or eliminated numerous causes of death, especially in children, and led to a population with a much higher proportion of old people. 

The chart below is a little unusual, but it shows the proportion of the world’s population in each age group for the years 1950-2018 (and then estimates from 2018 to 2100). Even in this short period, from the middle of the twentieth century, the proportion of people in middle-age and older has exploded.

As I’ve discussed before, it seems likely that owned dogs have experienced a similar increase in lifespan, and in the proportion of old dogs in the population, but there isn’t much hard data available.

Apart from the likely increase in cancer rates due to an increase in the number of old individuals, a recent paper has proposed another possible explanation. Not only are humans, and probably dogs, living longer, but we have extended our lifespan dramatically over such a short time that evolution has been unable to adjust our biology to account for this, leaving us vulnerable to age-related diseases that some even longer-lived species are protected against. 

Sarver AL, Makielski KM, DePauw TA, Schulte AJ, Modiano JF. Increased risk of cancer in dogs and humans: a consequence of recent extension of lifespan beyond evolutionarily-determined limitations? Aging Cancer. 2022 Mar;3(1):3-19

It turns out that if a species occupies a niche that favors a longer life, and the lifespan of individuals in that species increases gradually over evolutionary time, then genetic adaptions develop which delay the onset of age-related diseases. Elephants, whales, sharks, parrots, and the unusually long-lived rodent known as the naked mole rat, all have long lifespans in nature, and they have all developed a variety of genetic mechanisms protecting them against cancer throughout their lifespan. 

While humans are a relatively long-lived as mammals go, the “natural” life expectancy for most of our species’ history, prior to the development of science and science-based technologies, was about 30-40 years. This is as long as evolutionary mechanisms protecting against cancer would be expected to have developed to work since there was no evolutionary need for protection past the age when the large majority of people died. 

This nature lifespan happens to line up quite neatly with the age at which cancer rates increase dramatically in humans today. Fewer than 5% of cancer cases occur in people under 35 years old, and fewer than 10% in people under 45. The vast majority of cancer occurs in humans older than the natural expected lifespan.

Similarly, wild and feral dogs tend to have a lifespan of less than 5 years. This is likely similar to the natural lifespan of dogs before modern pet ownership, with improvements in nutrition and medical care, became common. Owned dogs now routinely live at least 8-10 years in large breeds and commonly into their mid-to-late teens in smaller breeds. As in humans, fewer than 10% of canine cancer cases occur in dogs under 5 years of age. Most cases occur in dogs older than most dogs would have lived before the very recent past. 

Evolution could not have provided protection against cancer throughout the lifespan in humans and dogs, as it has in elephants and sharks, because such changes require thousands to tens of thousands of years and many generations to develop. Such protections only exist through the expected natural lifespan, which was the same for most of our species’ history and much shorter than our typical lifespan today.

The simple way of putting this idea is that we and our dogs are victims of our success! We have extended our lifespans “unnaturally” far faster than evolution could have done, so there has been no time to develop the kind of protections against age-related diseases such as cancer that exist in species that naturally live a long time. As the authors of the paper explain it,

A major component of the elevated cancer risk seen in domestic dogs and in humans when compared to other animals is due to the shattering of the life-expectancy barrier that was evolutionarily determined: essentially “living longer than nature intended.” 

The excess of cancers seen in modern dogs (and in humans), and the patterns of their association with age, would strongly suggest that there has been neither sufficient time nor selective pressure to allow for evolution of adaptive mechanisms to reduce the risk of cancer that comes with the creation of aged cellular environments, increased exposures, and the accumulation of somatic mutational events in these populations.

In dogs and humans, recent dramatic alterations in healthcare and social structures have allowed increasing numbers of individuals in both species to far exceed their species-adapted longevities (by two to four times) without allowing the time necessary for compensatory natural selection. In other words, the cancer-protective mechanisms that restrain risk at comparable levels to other species for their adapted lifespan are incapable of providing cancer protection over this recent, drastic, and widespread increase in longevity.

While this may not be the whole explanation for differences in cancer rates between species, it is a plausible hypothesis with reasonable support. The good news is that we may be able to compensate for the lack of evolved cancer protection in humans and dogs despite having dramatically extended our lifespan through a variety of strategies. Certainly, identifying and avoiding environmental risk factors should be part of this effort. But we cannot focus exclusively on the “toxic environment” view, because it is incomplete. Our understanding of the underlying mechanisms of aging has expanded dramatically, and these mechanisms provide many targets for lifestyle interventions and medications that can extend healthy lifespan and reduce age-associated diseases like cancer. 

In one sense, technology and science have created the problem, though not in the way that shrill critics of modern life believe. We have defeated many causes of illness and death that afflicted us and our canine friends for millennia. This has left behind others that were never a threat when we all died young of the causes we have now tamed. But we can use the same processes of scientific discovery to understand the new threats and tame them as well, giving us and our dogs as much happy, healthy time together as possible!

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One Response to Cancer in Humans and Dogs-Are We Victims of Our Success?

  1. Pingback: ? Cáncer en humanos y perros: ¿somos víctimas de nuestro éxito? |

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