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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

What Causes Cancer in Pets?

Hearing the news that your pet has been diagnosed with cancer can be both devastating and terrifying at the same time. It is natural to have many questions about exactly what the diagnosis means, what might happen to your pet as the cancer progresses, and what options you have for treating the disease.


One of the most common questions I am asked by owners during an initial appointment is, "What caused my pet’s cancer?" I can definitely appreciate why this is an important piece of information they would want to understand. Unfortunately, this is a very difficult question to answer accurately, as in nearly all cases cancer is typically caused by a combination of genetic and environmental influences, many of which may have occurred years before the diagnosis was made.


The fact that certain types of cancers occur more often in particular breeds of dogs and cats lends much evidence to the concept of a genetic cause for the disease. We do know that the genetic mutations that cause cancer can occur in the reproductive cells of male and female animals, and these mutations can be passed on to puppies and kittens, giving rise to a heritable predisposition to different types of tumors. Most cancers, however, arise from mutations that occur to genes during a dog’s or cat’s lifetime that were not present at birth. These mutations can result from internal factors, such as exposure to naturally occurring hormones, or external factors, such as environmental tobacco smoke, chemicals, or even sunlight.


In people we know that up to one-third of all tumors are related to environmental and lifestyle factors. In veterinary oncology, we have discovered that nutrition, hormones, viruses, and carcinogens such as smoke, pesticides, UV light, asbestos, waste incinerators, polluted sites, radioactive waste, and canned cat foods can increase the risk of cancer in pets.


Some examples of known causes of cancer in companion animals include:


Increased risk of mammary cancer in un-spayed female dogs and cats.

  • Dogs spayed before experiencing their first heat cycle have a 0.5% chance of developing mammary cancer during their lifetime. This increases to 8% if they are spayed after they have experienced one heat cycle, and 26% if spayed after they have experienced two heat cycles.
  • Cats spayed before six months of age are seven times less likely to develop mammary tumors than cats spayed after six months of age.
  • It is thought that the hormones that are released during heat cycles cause mutations within the mammary tissue, leading to the development of tumors.


There is a possible association between environmental tobacco smoke exposure and development of oral cancer in cats.

  • The hypothesis is that the carcinogens present in cigarette smoke will be passively deposited on the fur of the cats, and when cats groom themselves, they inadvertently ingest these particles, which can lead to tumor development within the oral cavity.


There is an association between Feline leukemia virus (FeLV) and Feline immunodeficiency virus (FIV) and development of lymphoma in cats.

  • FeLV and FIV are retroviruses that affect cats, and can cause a variety of clinical signs in infected animals. Many cats that test positive for either virus as kittens may not show any clinical signs for several years. These viruses are known to cause cancers in cats. Cats that test positive for FeLV are 60 times more likely to develop lymphoma than cats that test negative for this virus, and cats that are FIV positive are five times more likely to develop lymphoma. Cats that test positive for both viruses concurrently are 80 times more likely to develop lymphoma.


Studies have shown conflicting information regarding the risk of exposure to herbicides and/or pesticides and the development of cancer in pets. For example, some studies have shown an increased risk for the development of lymphoma, which is a cancer of white blood cells, while other studies have refuted the risk. Because the results are inconclusive I generally recommend that owners should strive to minimize their pets’ exposure to these chemicals and discuss any concerns they may have with their primary care veterinarian.


It is important to remember that it is often difficult to prove "cause and effect" when it comes to cancer. This is true for even well designed research studies designed to look at those exact parameters, so one has to be careful when researching this topic and not over interpret the available information. There are so many potential interactions between genes and environment influences that could lead to the development of a tumor, and ultimately, we may never be able to know exactly what caused the cancer in the first place.


Although I can appreciate why an owner would want to try and understand how it is their pet developed cancer, what I often try to have owners focus on is, now that we have the diagnosis, how we can move forward with a plan to treat it so that we can provide the best possible quality of life for as long as possible for their pet? Keeping the emphasis on the present tense is what allows owners to continue to maintain their wonderful bond with their pets during the duration of their cancer treatment and beyond.


Dr. Joanne Intile


Image: Photosani / Via Shutterstock

Comments  8

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  • Bad News
    10/03/2012 11:43am

    I think it's human nature to want to know "why" with any type of bad news. I'd guess that human oncologists get the same questions.

    I think that so many times people assume a critter's cancer diagnosis is a death sentence, especially since treatment is rarely for a cure, but for quality of life.

    Yes, it's devastating. However, depending on the type of cancer, treatment can provide a good quality of life. My Winston Alexander (RIP) had lymphocytic lymphoma and was pretty darned happy for over two years after diagnosis. Sure, it was expensive and took a lot of time with medications, but completely worth it.

    On the other side of the coin, my Winifred Louise (RIP) had adenocarcinoma. It was only 10 days from diagnosis to a belly full of tumors that took her quality of life.

    Knowing "why" would have been nice, but it wouldn't have changed anything.

  • 10/08/2012 09:17pm

    I totally agree, my dog was fit and healthy and within 1 week I was being told she would be lucky to survive the next few hours. 8 litres of fluid to flush the calcium out of her blood and 4 chemo treatments later she is back to how she was. She has lymphoma so we will be lucky to have her for another year but quality of life is EXCELLENT. Yes it is expensive (£2000 so far and only 4 out of 13 treatments done plus biopsy etc) so thank god for insurance!!. Single mum so no way otherwise. Chemo is not severe in a dog as vets are only concerned with quality of life, not survival. I would recommend anyone to try it. Remember you can stop whenever you want if it doesn't go well.

  • Old wives' tale
    10/03/2012 02:42pm

    According to a June 2012 article in the Journal of Small Animal Practice, the theory that early neutering reduces the risk of mammary cancer is an old wives' tale.

    See: The effect of neutering on the risk of mammary tumours in dogs – a systematic review. W. Beauvais, J. M. Cardwell and D. C. Brodbelt. J.Sm.Anim.Pract, June 2012; 53(6): 314-322.

    "Due to the limited evidence available and the risk of bias in the published results, the evidence that neutering reduces the risk of mammary neoplasia, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations."

  • I don't think so
    10/03/2012 05:09pm

    environmental and predisposed your best answer to why pets have cancer? Why don't we take a look at what our pets eat, how many unnecassary vaccinations they get over their lifetimes, and how young we spay and neuter our pets.

  • 10/03/2012 08:52pm

    As a veterinarian we are always looking for evidenced based medicine. Unfortunately in veterinary medicine it is hard to design and carryout well designed randomized studies with sufficient numbers to have meaningful results. Thus I take some issue with realsymphony post as dispensing or claiming blogs or personal opinion as fact is very dangerous. We all worry abt what we expose our bodies and our pets to. And I think most vets would agree that factors beyond genetics play a role (I.e environmental) in disease and cancer. However I would caution readers as they read realsymphony post. If we really want to learn more abt what causes cancer we should be looking for published research not personal opinion.

    It should also be noted that the link abt Purdue and vaccine studies has some basis. Vaccines have been associated with sarcoma formation in cats. However it is not just vaccines... it is any injection (subcutaneous fluids, microchips, etc).

  • but there are studies
    10/22/2012 09:26pm

    Yea, I gotta go with the idea that diet is very important and a cancer preventative. Not to mention the fact that once one has cancer diet is still very important. Not too many studies done on animals and diet but lots for humans and diet.That is good enough for me.
    Would you feed your children a strict diet of canned food and cereal? So why are processed foods for pets so acceptable?
    Animal by products for pets do contain some nasty things.Rendered fat, by product meal and even euthanized pets can be added to make pet food and it is perfectly legal.
    Don't take my word for it, Dr Kuhly wrote about it in one of the blogs right here on petmd.

  • PDBE and TCPP,
    03/12/2014 10:21pm


    2 well know fire retardant that cause cancer in human and have minimal effect to prevent fire.
    However governments seem to be reluctant to removed those chemical in our furniture.

    A test on blood show a level of the chemical in blood stream.

    As animal love to be on our couches bed would this be a major reason for cancer in pets?

  • Pet Foods May be a Factor
    05/15/2015 01:31am

    Many commercial pet foods are lacking in vegetable compounds that we know from our own research may have cancer preventative properties. If you do your research you will find that super premium foods contain a portion of foods like cabbage, berries, spinach, and other types of antioxidant rich vegetables or fruits. Even if you don't buy such a food you can always add in a veggie mash to their kibble to compensate for that. I personally add a berry powder to the dog's wet food and those berries have many other health benefits beside this.

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