Cancer has to be the most feared diagnosis in all of medicine, one that sends patients and their families on a bewildering journey through statistics, treatment options, and life-or-death decisions that have to be made right now. Cancer has become so widespread that the care and treatment of its human patients is one of the world’s largest industries. Now cancer affects a significant percentage of veterinary patients as well.
Most medical dictionaries define cancer as a disease resulting from an abnormal and uncontrolled division of cells that invade and destroy surrounding tissue. In most cases, this cell division creates malignant growths called tumors. Cancer cells often migrate via the blood or lymph, resulting in the development of additional tumors throughout the body.
Cancer has no known cause, but its risk factors include genetics, diet, hormone imbalances, exposure to radiation, viruses, vaccinations, and environmental toxins such as lawn chemicals, flea and tick dips, asbestos, and tobacco smoke.
In the 1960s, about four out of every 1,000 dogs were diagnosed with cancer in the United States each year. At that time, the most common canine cancers involved the breast in females, the testes in males, and connective tissue, skin, lymph nodes, mouth, throat, and bones in both genders.
In 1997, a Morris Animal Foundation survey found that cancer was the leading cause of non-accidental death in America’s dogs. Today nearly half of dogs over age 10 die of cancer.
The similarities between canine and human cancers are striking, but there are differences. For example, dogs have 35 times as much skin cancer as humans, 4 times as many breast tumors, 8 times as much bone cancer, and twice the incidence of leukemia. Humans have 7 times as much lung cancer as dogs and 13 times as much cancer of the stomach and intestines.
Running in the family
In a 1997 Swedish study involving 222,000 dogs, the breeds at highest risk for cancer included Boxers, Giant Schnauzers, and Bernese Mountain Dogs (all of whom had a mortality rate due to cancer of over 30 percent), Irish Wolfhounds, Cocker Spaniels, and Doberman Pinschers (over 20 percent), and Pomeranians, Newfoundlands, German Shepherd Dogs, Saint Bernards, Great Danes, Greyhounds, and Basset Hounds (over 10 percent of deaths due to cancer).
English scientists published a study in 1999 that found that in the United Kingdom, Afghan Hounds, Irish Wolfhounds, Standard Poodles, and Rottweilers had the highest incidence of cancer, while Airedales, Beagles, Dachshunds, Irish Setters, Jack Russell Terriers, Rough Collies, and Yorkshire Terriers had a relatively low risk of dying from cancer.
In a 2003 Danish Kennel Club study, researchers investigated the age and cause of death for nearly 3,000 dogs and found that cancer affected 14.5 percent of the dogs studied. Bernese Mountain Dogs, 34.4 percent of whom died of cancer, had the highest cancer rate in Denmark.
Accurate cancer statistics for America’s dogs are hard to come by, but studies published by epidemiologists provide estimates that appear in the following descriptions. Today the most common type of cancer in American dogs is skin cancer, followed by mammary cancer and lymphosarcoma.
The early warning signs of cancer in dogs are similar to human warning signs publicized by the American Cancer Society. These include any abnormal swelling (especially a swelling that continues to grow), sores that don’t heal, weight loss, bleeding or discharge from any body opening, a reluctance to move or exercise, a loss of stamina, or difficulty breathing, urinating, or defecating.
Any sort of lameness in an older dog, especially large breeds, should be investigated as a potential cancer case. Even minor or subtle symptoms, such as sleeping more than usual, refusing to play, or having less interest in social interaction, can be warning signs.
Types of cancers
There isn’t room to describe every cancer that affects America’s dogs, but the following alphabetical list describes some common diagnoses. Becoming familiar with the descriptions below will help you make sense of these and other canine cancers.
• Bladder cancer: Bladder and ureteral cancers are most common in older dogs. While some studies have shown a higher risk in females and other studies found no gender differences, there may be a higher risk in neutered dogs of both sexes.
Bladder tumors have been associated with the use of flea and tick dips, flea and tick shampoos, or exposure to aromatic hydrocarbons such as paraaminobiphenyl, paranitroliphenyl, and betanapthylamine. The authors of one study suggest that it is not the active ingredients in flea and tick products that cause bladder cancer but rather “inert” or “carrier” ingredients such as benzene, toluene, xylene, and petroleum distillates, all of which are known carcinogens and which often make up 95 percent of the total product. They are used as solvents for the active ingredients.
A Purdue University study published in 2004 found that Scottish Terriers exposed to lawn chemicals have an increased incidence of bladder cancer. Scottish Terriers were chosen for the study because they develop bladder cancer 20 times more often than other breeds, but dogs of any breed can develop the disease.
Other bladder cancer risk factors include obesity and living in a marshy area.
• Hemangiosarcoma: Originating in the endothelium (the lining of the spleen and blood vessels), hemangiosarcoma forms highly malignant tumors that develop throughout the body, especially in the spleen, liver, and heart.
German Shepherd Dogs, Golden Retrievers, Boxers, and English Setters are at higher than average risk, and the disease is most common in middle-aged or older dogs of medium to large size. In many cases, symptoms are noticed only after the disease has progressed to an advanced stage.
Initial symptoms include bleeding (especially nosebleeds), weakness, pale mucous membranes in the mouth and eyes, panting, and abdominal swelling. Death often occurs quickly, within one to four months of diagnosis. Many dogs with this disease die suddenly without manifesting clinical symptoms.
Spayed females are four times more likely to develop vascular tumors (cardiac hemangiosarcomas) than intact females; neutered males are also at higher risk of hemangiosarcoma than intact males.
• Histiocytosis: The most common cancer found in Bernese Mountain Dogs, histiocytosis is rare in other breeds, although it can occur in Rottweilers, Golden Retrievers, and Flat-Coated Retrievers. Its symptoms include depression, fatigue, lethargy, loss of appetite, and weight loss. Malignant histiocytosis progresses rapidly and has usually metastasized by the time symptoms develop. Most patients die within two to four months of diagnosis. Systemic histiocytosis creates skin abnormalities on the face and legs.
Most patients are middle-aged or older. Histiocytosis that spreads to the lungs can interfere with breathing, and anemia is another common symptom.
Histiocytomas are benign tumors that usually appear on the head of dogs under three years of age. They are not considered a health risk.
• Leukemia: Leukemia, or chronic lymphocytic leukemia (CLL), usually affects older dogs and involves the rapid reproduction of mature lymphocytes throughout the body, including the bone marrow. Because elevated circulating lymphocyte counts are easily identified in complete blood panel tests, CLL is often discovered when the blood is tested for other reasons.
Chronic lymphocytic leukemia tends to progress slowly and is often not treated until the circulating lymphocyte count increases to very high levels or the dog becomes lethargic, CLL’s main symptom.
The condition can progress to a lymphoblastic crisis, also called lymphoblastic leukemia, which is a more aggressive form of the disease, comparable to advanced stage lymphosarcoma. With conventional treatment, most dogs with lymphoblastic leukemia survive for about a year.
• Lung cancer: While unusual in dogs, lung cancer does occur, and the number of cases diagnosed each year appears to be increasing. However, this may be the result of improved diagnostic techniques rather than an increasing number of cases.
According to some research, short-nosed breeds exposed to secondhand smoke have twice as much risk of lung cancer than long-nosed breeds. (Conversely, long-nosed breeds living with smokers have an increased risk of nasal cancer.) Exposure to asbestos can increase the risk of cancer of the lining of the lungs (mesothelioma), and dogs with this type of cancer are likely to live with owners whose work or hobbies exposed them to asbestos.
• Lymphosarcoma (Lymphoma): The third most-common cancer in dogs, lymphosarcoma (also known as lymphoma) affects lymphocytes (a type of white blood cell) and tissue of the lymph nodes, spleen, liver, gastrointestinal tract, and bone marrow.
Although lymphosarcoma strikes dogs of all ages, most patients are over age five, with males and females at equal risk. Boxers, German Shepherd Dogs, Doberman Pinschers, Golden Retrievers, Scotties, West Highland White Terriers, and Pointers may be most vulnerable to this disease.
There are five classifications of lymphosarcoma, depending on the tumor’s primary location.
The most common type involves external lymph nodes. It is also the most likely to be overlooked because many dogs have only mild symptoms such as fatigue or decreased appetite. More obvious symptoms include weight loss, vomiting, diarrhea, excessive thirst or urination, weakness, or difficulty breathing. In some cases, the only signs are enlarged lymph nodes under the neck, behind the knees, or in front of the shoulders.
The other classifications are gastrointestinal (symptoms include vomiting, diarrhea, weight loss, and loss of appetite), mediastinal (affecting the chest, creating breathing problems and excessive thirst and urination), cutaneous (affecting the skin, which can be dry, flaky, scaly, irritated, and itchy), and bone marrow (producing anemia, infections, and bleeding).
Because lymphosarcoma spreads quickly, its diagnosis involves biopsies, aspiration of affected tissue, blood tests, urinalysis, and a search for tumors throughout the body using X-rays, sonograms, or other methods.
• Mammary cancer: The most common cancer in female dogs is breast or mammary cancer. According to some studies, mammary tumors are more common in purebred dogs than in mixed-breed dogs of the same age, and they are far more common in dogs that are intact or were not spayed until after age two and a half years. Spaying offers maximum protection to dogs spayed before their first heat cycle and almost as much protection to those spayed before their second season. Obesity is a risk factor for mammary cancer, and the breasts most likely to be affected are those farthest from the head.
Approximately half of dogs with mammary gland tumors have more than one. These tumors tend to develop between the age 6 and 10 years.
Mammary tumors vary by size, texture, and condition. They may contain fluid or be ulcerated or inflamed. None of these symptoms reveals whether a tumor is malignant, and in dogs that have not been spayed, about half the tumors tested are benign.
Lymph node involvement increases the risk of cancer spreading to the lungs or other organs.
Survival rates are higher for dogs with small rather than large tumors and for dogs whose tumors have not metastasized.
• Osteosarcoma: Highly aggressive and fast growing, osteosarcoma affects more than 8,000 American dogs every year and causes an estimated 85 percent of all canine bone tumors.
The illness has been diagnosed in six-month-old puppies, but it is most common in older Great Danes, Golden Retrievers, German Shepherd Dogs, Great Pyrenees, Saint Bernards, Newfoundlands, Bernese Mountain Dogs, Irish Wolfhounds, Rottweilers, Labrador Retrievers, Doberman Pinschers, Weimaraners, Boxers, and other large-breed dogs. It is almost 500 times more likely to affect dogs weighing over 35 kilograms (about 80 pounds) than those weighing less than 10 kilograms (about 23 pounds), and males are at greater risk than females.
Any stress on weight-bearing legs is a risk factor, including previous fractures and infections. Bone tumors are most likely to affect the legs but can also occur in the skull, ribs, vertebrae, or pelvis.
Osteosarcoma is twice as common in spayed females and neutered males as in their intact counterparts.
After producing tumors that weaken bones, osteosarcoma spreads throughout the body. Its main symptoms – lameness, intermittent pain, leg swelling, and fractures at the tumor site – may be mistaken for arthritis or other chronic conditions until the disease is advanced. As pain increases, behavioral symptoms such as irritability, aggression, and a reluctance to exercise become more obvious.
Without treatment, most dogs with osteosarcoma die within two months of diagnosis, and only 20 percent survive for two years. Limb amputation is commonly performed to provide pain relief, but it does not usually cure the disease or prevent its metastasis. The most common cause of death is the spread of cancer to the lungs.
• Prostate cancer: In humans, prostate cancer is a common but slow-growing cancer that affects older men. In dogs (the only other species to have significant amounts of prostate cancer) the disease is fast-growing, aggressive, and likely to spread to lymph nodes, lungs, and bones. In one study, one out of every 150 male dogs age eight and older was found to have prostate cancer. In most cases, prostate cancer is diagnosed in its advanced stages.
• Skin cancer: The skin is the most prevalent tumor location in dogs, comprising an estimated 58 percent of all canine cancers. Most skin cancer tumors contain mast cells, squamous cells, or melanin-pigmented cells. These tumors are usually soft or solid raised, nodular masses. If malignant (many are benign), treatment depends on their stage or grade.
Mast cell tumors, also called mastocytomas or mast cell sarcomas, are the most frequently diagnosed cancers in dogs. They are most common in middle-aged Boxers, Pugs, Rhodesian Ridgebacks, Boston Terriers, Schnauzers, Beagles, Labrador Retrievers, Dachshunds, Fox Terriers, English Bulldogs, Staffordshire Terriers, and mixed-breed dogs.
Squamous cell carcinomas are common in lightly pigmented dogs such as Beagles, Dalmatians, Whippets, and white English Bull Terriers. Nail bed squamous cell carcinomas tend to occur in black-coated large-breed dogs.
Melanomas are usually solitary black tumors. Melanomas of the mouth and nail bed are usually malignant.
• Testicular cancer: Human males tend to develop only one type of testicular cancer (seminomas) while intact dogs can develop any of three different types (Sertoli cell tumors, seminomas, and interstitial cell tumors).
Canine risk factors include undescended testicles, which remain in the body cavity instead of migrating to the scrotum, as well as inguinal hernias. Neutering prevents the development of testicular cancer. Breeds associated with testicular tumors include Samoyeds, Cocker Spaniels, Beagles, and English Bulldogs.
Links between cancer and environmental toxins have long been suspected, and during the Vietnam War, working dogs exposed to parasitic infections, chemicals used to treat those infections, and agricultural chemicals such as herbicides developed increased levels of testicular cancer.
More on the way
In the coming months, we will explore conventional, complementary/ alternative, and support therapies for canine cancer. Next month, we’ll discuss what works and what doesn’t in conventional cancer treatment – and how much it costs.
Also With This Article
Click here to view “Cancer Treatments for Dogs”
-Author of The Encyclopedia of Natural Pet Care, Natural Remedies for Dogs & Cats, and other books, CJ Puotinen lives in New York with her husband, Labrador Retriever, and red tabby cat.