A Different Animal: Cancer in Pets and People
Cancer can strike every organ in the body, and each different type of cancer carries a different prognosis and requires a different treatment. That’s because, although we tend to lump all cancers into the same basket, each one is a separate disease. That is true when we look at cancers within the same species (such as all human cancers or all canine or feline cancers) or when we start comparing the same kind of cancer across species (such as skin cancer in humans and canines). In this article, we take a brief look at how cancers that are important in people usually manifest differently in our pets.
The Human Side
According to the National Cancer Institute (NCI), a group that tracks cancer diagnoses and outcomes among humans, the top five cancers in people are breast cancer in women; prostate cancer in men; and then lung, colorectal and skin cancer in both genders. That probably doesn’t surprise you, since your doctor is likely always talking to you about various screening tests, such as the mammogram or colonoscopy, or about the risks of cigarette smoking or not wearing sunscreen. Organizations like the NCI record and publish information about human cancer rates in the United States to help assess the impact of new treatments and prevention strategies.
No PSAs for Persians
There is no national organization that tallies the occurrence of cancer in pets like the NCI does for humans, but veterinarians know that the top cancers you should worry about in your pet are very different from those you should worry about for yourself. You won’t hear me recommending, for example, a screening colonoscopy for your Curly-Coated Retriever or a PSA (prostate specific antigen) test for your Persian. That’s because colorectal and prostate cancers are very uncommon in pets. Since pets don’t smoke, lung cancer is also uncommon. But there are other cancers that are common in both pets and people. Let’s take a look at the top four among those that we veterinary oncologists worry about.
Just as in women, breast (mammary gland) cancer is a commonly diagnosed cancer in dogs and cats — if you live in a region where spaying and neutering is not part of a routine pet preventive health care program. Removal of a dog’s ovaries before her first heat cycle during a spay procedure dramatically decreases the occurrence of breast cancer. That is why one common recommendation from veterinarians is to spay your puppy at about 6 months. Spaying also reduces the occurrence of breast cancer in cats. Breast cancer is the most common tumor found in female dogs and the third most common cancer in cats. In humans, it is one of the top three cancers in women, but obviously, preventive measures differ. In humans, we normally use screening mammograms as an early detection tool.
Lymphoma: Not a Good Prognosis
In cats, one in three cancer diagnoses is lymphoma, a cancer of a portion of the immune system known as the lymph system. It is most often diagnosed in the feline gastrointestinal tract, but any organ can develop the cancer. Dogs, too, can develop lymphoma. For example, both of former President George W. Bush’s Scottish Terriers, Barney and Miss Beazley, died from the malignancy.
Despite the incurable nature of lymphoma in cats and dogs, chemotherapy is the mainstay of treatment. Chemotherapy can offer pets who are suffering from the disease an improved quality and quantity of life. The typical dog receiving chemotherapy for lymphoma lives an extra year, which is a long time in dog years. In comparison, some forms of human lymphoma are curable, while others are not.
On the Surface
Although skin cancer is common in both pets and people, how it manifests as a disease is different. I rarely see skin cancer in pets caused by overexposure to the sun. The most common skin cancer I see in my daily oncology practice is mast cell tumor (MCT), mostly in dogs and occasionally in cats. Physician oncologists, however, rarely see that tumor in their practices. Most humans have never heard of a mast cell, but nearly all of us have felt their negative effects, since those are the cells responsible for allergic reactions ranging from from those to bee stings to hay fever. A mast cell tumor in pets occurs in the skin or just below the skin and is one of the tumors pet owners can identify just by petting their animals.
Mast cell tumors may just be a bump in the skin that looks like any other benign fatty tumor, but more commonly they are pink to red and may be scabby. A simple in-office test known as a fine needle aspirate can help in the early identification of an MCT. A veterinary pathologist will evaluate the aspirate and, if necessary, recommend surgical removal of any mast cell tumor. Surgery is often all that is needed to cure your dog of the tumor, but in some severe forms, chemotherapy and radiation therapy are also needed to control the malignancy.
Melanoma can be a concern in pets, but it is rare compared to its occurrence in people. Unlike the skin spots that alert human patients and physicians to its presence, melanoma in dogs is usually found in the mouth or nail bed. In cats, the most frequent location is in the iris, the colored part of the eye.
Bone Cancer: the Same in Dogs and Kids
Osteosarcoma, or bone cancer, is a disease in pets that shares some similarities to the disease in people. Osteosarcoma in dogs serves as a model for the disease in children due to its similar behavior in both species. It is diagnosed more commonly in dogs than in children, and the good news is that research to improve treatments for dogs with the disease has been translated into new and better treatments for children, showing yet again how our enduring friendship with dogs is beneficial.
For example, ongoing investigation into naturally occurring canine osteosarcoma in dog patients has allowed for the testing of surgical methods that are readily translated to pediatric osteosarcoma — something that can’t be done with rodent models of the disease. The most notable contribution of dogs to the treatment of human osteosarcoma is the pioneering of limb-sparing surgery to remove tumors without removing entire limbs. The NCI has funded that groundbreaking research.
Next on the horizon for osteosarcoma is the investigation of genetic abnormalities leading to the disease and the development of molecularly targeted therapies against those abnormalities.
In dogs, an early sign of osteosarcoma is limping due to pain resulting from the destruction of the bone by the tumor. Although difficult to discuss with fear-fraught pet families, amputation of the leg can dramatically improve quality of life for dogs afflicted by the disease.
Chemotherapy slows the spread of osteosarcoma, and lucky dogs may experience a year or more of good quality of life.
But Some Things Remain the Same
Though we can see that cancers in pets and people differ in many ways, basic common sense when it comes to prevention and early detection in both species is the same. If you, the diligent pet owner, are concerned about your pet’s cancer risk, see your veterinarian for a complete examination and a conversation about minimizing risks. For more on detecting cancer in your best friend, review the Ten Warning Signs of Cancer in Pets.