What Causes Cancer in Pets?

Via Shutterstockby Dr. Joanne Intile, DVM, DACVIM | petMD.com


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.

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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


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