I’m asked this question several times per week, and I wish I knew how to answer directly, confidently, and accurately. I’ve addressed this subject in a previous article on this site, but I wanted to take the time to delve into some of the more complicated issues related to this controversial topic.
Epidemiology is the branch of science studying the causes and effects of disease on the health and outcome of specific populations. In epidemiology, several lines of evidence together are required to infer causation. Causation can be very difficult to distinguish from what would be considered a simple association or correlation. This is because events can occur simultaneously as a result of random chance, bias, or confounding variables.
Etiology is a word describing the actual cause of a disease or pathology. To say a particular variable “causes” cancer would require performing an accurately designed research study, which is a very daunting task in veterinary medicine because our inability to control for other variables also could influence outcome.
Take for example the fact many owners pride themselves on rescuing their pets from shelters. These are pets where very little, if any, information exists about their health care prior to adoption. How then can we determine causality for adopted pets, when so little is known about the risk factors they may have incurred during their “previous lives”?
An example of a known etiological factor causing a predisposition to cancer occurs in cats infected with either the Feline Leukemia Virus (FeLV) or Feline Immunodeficiency Virus (FIV). Cats infected with FeLV are 60 times more likely to develop lymphoma/leukemia compared with healthy non-infected cats. Cats infected with FIV are five times more likely to develop the same cancers. Cats co-infected with both FeLV and FIV are 80 times more likely to develop lymphoma than non-infected cats.
FeLV infection was the most common cause of blood borne cancers in cats during the 1960-1980s. During that time, approximately two-thirds of cats with lymphoma were co-infected with FeLV. Cats tended to be young (4-6 years) and disease was found more predictably in certain anatomic locations (e.g., mediastinal lymphoma).
With the development of better screening tests to eradicate or isolate infected cats, as well as commercially available FeLV vaccines, the number of FeLV positive cats decreased dramatically after the late 1980s. However, cats still developed lymphoma, and the overall prevalence of this cancer actually increased over time. The disease appears to be shifting to other anatomical locations, namely the gastrointestinal tract. What then is responsible for causing lymphoma in cats now?
There are a handful of research studies available that examine the causes of cancer in pets.
To my knowledge, commercial diets, vaccination (other than for sarcoma developments as listed below), tap water, shampoo, or cat litter have not been accurately studied and proven to cause cancer in pets. But there is a large body of information on the Internet suggesting each of these is a known, etiological cause of tumors in dogs and cats.
There are three “take home” areas I would like to highlight to summarize what we know (which is honestly far less than what we don’t know) when it comes to proving how cancer arises in animals.
- Environmental exposures — The three biggest culprits studied included pollution, environmental tobacco smoke (ETS) and pesticides. The definition of a pesticide is any substance used for destroying insects or other organisms harmful to cultivated plants or to animals (e.g, topical flea/tick medications).
- a. There is evidence supporting an association between exposure to ETS and lymphoma and nasal tumors in dogs and lymphoma in cats
- b. Exposure to pesticides containing dichlorophenocyacetic acid (2,4-D) is associated with increased risk of lymphoma in dogs, however data is conflicting
- c. Dogs living in urban areas are at increased risk for developing lymphoma
- Neuter status — Hormones can act to promote tumor development or inhibit cancer, depending on tumor type. Female dogs are less likely to develop mammary tumors when they are spayed early in life, presumably due to lack of exposure of mammary tissue to ovarian derived reproductive hormones. However, neutering may actually cause an increased risk of developing prostate cancer in male dogs, indicating a possible protective effect of hormones in such cases. Neutering may also increase risk of developing osteosarcoma and transitional cell carcinoma of the urinary bladder in dogs, regardless of gender.
- The administration of injections (not only vaccinations) can cause injection site sarcomas in cats, but the injection alone is not sufficient to create tumors — more and more evidence points to an inherent susceptibility to tumor development that is “set into motion” in response to the injection.
I understand how frustrating it is for an owner to discover their pet has cancer, and being the academically minded person I am, I would want to know the same things they do. How did this happen? Did I do something to cause this? What can I do to prevent it from happening to another pet?
People do not come to see a veterinary oncologist because they are “bad pet owners.” On the contrary, I meet some of the most dedicated and educated pet parents around. And this makes it even more challenging to not be able to tell them why their pets became sick.
The best I can do is offer the options for treatment and focus on the “here and now.” Together, we can only control what we have now, and I try my best to dispel the myths and misconceptions, while putting forth accurate information for people to think about and understand. In the meantime, I’ll use my cliché answer.
“Pets get cancer because of a combination of genetic influences, environmental factors, and just plain bad luck…”
Dr. Joanne Intile