Cat being examined by a veterinarian

As a veterinary cancer specialist, I talk to distraught pet owners every day who are in shock and grief over cancer diagnoses for their beloved pets. They are looking for answers. What does this diagnosis mean? When did the tumor start? How long will my pet live? How will the treatment make my pet feel? But the million dollar question is “Why does my pet have this tumor?”

Most of the time, I have no idea why a dog or cat develops a particular tumor. Occasionally, I see a cat infected with feline leukemia virus — a known cause of feline lymphoma. We also know genetics play a role in whether some pets get cancer, because some tumors run in breeds. For example, Pugs are prone to mast cell tumors and hemangiosarcomas are common in Golden Retrievers. Reproductive status matters as well. Female dogs who are not spayed before their first heat (or those not spayed at all) have a much higher rate of breast cancer. Body size also plays a role in the development of bone cancer, since the typical dog with this disease weighs more than 70 pounds. But these examples describe only a few of my patients.

The Answer to a Vexing Question

Because I don’t know why most of my patients get cancer, and out of desperation to answer this question for owners, I started telling them, “It’s just bad luck.” Well, it turns out I may have been right.

A recent article in the prestigious journal Science and reported on by The New York Times blames cancer on random mutations arising during DNA replication.

The research behind the article, authored by two scientists at Johns Hopkins University in Baltimore, started with the observation that certain body parts develop cancer at different rates. For example, the colon develops cancer more commonly than the esophagus, even though these structures are both part of the same digestive “tube” in your body. Using seriously high-powered math, they determined most cancers develop because of a random process.

Here’s how they arrived at that conclusion: Cancer is driven by stem cells — cells which have the ability to self-renew. Necessary cell renewal keeps us healthy, while unchecked stem cell growth becomes cancer. The number of times healthy stem cells divide in a lifetime is a known fact for a number of different organs in the body. When the scientists compared the number of stem cell divisions to the rate of cancer for those organs, they found that the more times the stem cells divided, the greater the cancer rate. In other words, cells that divide more frequently have a greater chance of a DNA coding error that can lead to cancer.

The results of this human study reinforce two important truths for me as a veterinary oncologist. The first truth helps me remove some of the guilt pet families suffer when their pets are diagnosed with cancer. Now, for many families, I can assure them their pets’ diseases are likely caused by events over which they have no control. They did not feed their pets the wrong food, or exercise it too much or too little, or unknowingly expose it to cancer-causing pesticides. Once pet families can get past blaming themselves for their pets’ tumors, we can get to work trying to make their pets better.

Prevention and Early Detection

The second truth I found in the Science article is the importance of cancer screening for pets. Because the random genetic events leading to cancer cannot be predicted, pet families must do all they can to limit environmental exposure to cancer promoters, and they must work with their veterinarians to detect cancer early. Pet families should familiarize themselves with the common signs of cancer in pets and report them to their veterinarians immediately.

So, just to point out a few proactive things you can do, if you identify a lump and your veterinarian suggests testing the lump, don’t delay! A “watched” lump almost never gets smaller. Since light-coated dogs and cats are at risk of developing sun-induced cancers, sunbathing should be off their list of activities in an effort to help prevent squamous cell carcinoma of the skin. Talk with your veterinarian about the benefits of spaying your female puppy, and keep your cats indoors to help prevent exposure to the feline leukemia virus and the feline immunodeficiency virus to decrease their cancer risk.

As a practicing veterinarian, I need my veterinary scientist colleagues who conduct research to discover better methods of early cancer detection and treatment for both dogs and cats. With watchful pet families and veterinarians armed with good detection and treatment options, we can overcome cancer’s random actions.

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