Cancer in Dogs: What Are Signs, Most Common Ones and Worst Affected Breeds?

Just like in humans, cancer can be devastating for dogs. Cancer is the leading cause of death in dogs, with the American Veterinary Medical Association estimating that almost half of canines over the age of 10 will develop the disease.

Prior studies have shown that certain dog breeds are at a higher lifetime risk of developing cancer. Common breeds in the United States at an above-average lifetime risk of cancer include Bernese mountain dogs, boxers, French bulldogs, golden retrievers, Great Danes, mastiffs, rottweilers, flat-coated retrievers, German shepherds and Labrador retrievers.

The question of why some breeds experience higher rates of cancer is a difficult one to answer definitively.

"It is challenging to point to one factor that makes some dog breeds more prone to cancer than others," Jill Rafalko, a researcher with PetDx—a biotechnology company located at the University of California San Diego's Center for Novel Therapeutics—and Andi Flory, the firm's chief medical officer, told Newsweek.

"Like cancer in people, the cause is multifactorial, including both genetic and environmental influences. Certain breeds may harbor genetic variants that increase their risk of developing certain cancers," Rafalko and Flory said.

Most common cancers in dogs and symptoms

The most common cancers in dogs, as defined by the American Animal Hospital Association, are anal sac adenocarcinoma, hemangiosarcoma, lymphoma, malignant melanoma, mammary gland carcinoma, mast cell tumor, soft tissue sarcoma, and osteosarcoma.

Below are some of the typical signs and symptoms in each case.

• Anal sac adenocarcinoma: Discomfort in the area around the anus, anal discharge, scooting across the floor, licking in the anal area, difficulty pooping, constipation.
• Lymphoma: Enlarged lymph nodes, decreased appetite, swelling on the face or limbs, increased thirst and urination.
• Malignant melanoma: Enlarged lymph nodes, abnormal concentrations of the pigment melanin in parts of the body.
• Mammary gland carcinoma: Firm swelling of the mammary glands, lethargy, decreased appetite and weight loss, weakness, coughing.
• Mast cell tumor: A new skin mass, a known mass that has changed in size or color, an unexplained allergic reaction that can potentially be severe, swelling and redness in the affected area.
• Soft tissue sarcoma: Large, diffuse mass of soft tissue generally in the skeletal muscle.
• Osteosarcoma: Signs of lameness, soft tissue swelling in the limbs, limb asymmetry, decreased appetite, pain in the limbs, elevated heart rate, dehydration, neurologic signs.

Study on average age of cancer diagnosis in dogs

Rafalko, Flory and their PetDx colleagues have just published a study in the peer-reviewed, open-access journal PLOS ONE, which investigated the average age of cancer diagnosis in dogs. One of the aims of the study was to determine the age at which individual dogs should start cancer screening.

"Cancer is a big problem in dogs—in fact, it's the most common cause of death in dogs and, unfortunately, often diagnosed late in the course of disease, when treatment options are limited," they said.

"Until recently, cancer screening in dogs has primarily relied upon a physical exam and routine lab tests at a dog's annual or semiannual wellness visit to detect the disease, however, this paradigm is unable to detect cancer early in most dogs."

In 2021, the PetDx lab developed a new "liquid biopsy" test called OncoK9 using next-generation sequencing (NGS) that is able to detect 30 different types of cancer with a simple draw of blood, enabling cancer screening and early cancer detection in dogs.

"With the widespread availability of NGS-based liquid biopsy testing and the knowledge that certain breeds of dogs are diagnosed with cancer earlier than others, it was important to determine the age at which individual dogs should start cancer screening to try to catch the disease early—ideally when there may be more treatment options, and treatments may be more successful," the scientists said.

In the PLOS ONE study, the PetDx researchers examined more than 3,000 dogs with cancer to work out the typical age at which the disease was diagnosed.

They also analyzed the typical age of cancer diagnosis based on the dogs' breed, weight, sex and disease type.

The scientists found that across all dogs in the study, the typical age at cancer diagnosis was 8.8 years, while they also identified other trends.

"On average, larger dogs were diagnosed with cancer at earlier ages than smaller dogs, and certain breeds were diagnosed with cancer at younger ages than others, Rafalko and Flory said. "With this information, we were able to make evidence-based recommendations for when to start cancer screening in individual dogs, based on their breed or weight.

"We know that cancer develops over time, so it is reasonable to start screening for cancer two years before the typical age at which cancer is diagnosed. In short, this means that all dogs should begin cancer screening at age 7, but some large dogs and dogs belonging to specific breeds may benefit from starting screening as early as age 4."

Breeds that can benefit from starting cancer screening at ages younger than age 7 include boxers, Great Danes and mastiffs (start screening at 4 years old), Bernese mountain dogs (start screening at 5 years old), and golden retrievers, French bulldogs and rottweilers (start screening at six years old).

At 11.5 years, the bichon frise had the oldest median diagnosis age. Meanwhile, female dogs were typically diagnosed at older ages than males, and dogs that were neutered were diagnosed later than intact dogs.

Pet owners and veterinarians can use the OncoK9 Cancer SAFE (Screening Age For Early detection) site to find a recommendation for the most appropriate age to begin cancer screening in an individual dog, including mixed-breed dogs.

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