A pet’s cancer diagnosis can be devastating, and we recognize how this disease can affect pets and their families. Cancer, which is essentially unrestricted cell growth, is generally a disease of aging pets, but many cancers can manifest earlier in life. Pet survival time and quality of life can often be increased if the cancer is caught early, so families should closely monitor their pets for any abnormalities, especially as they age. Read on to learn more about four common cancers of pets, and the clinical signs you may observe. 

#1: Osteosarcoma

This aggressive cancer of the bone most frequently affects large- and giant-breed dogs, but can also occur in small animals. Osteosarcoma is a painful, malignant cancer that accounts for approximately 95% of all bone tumors in dogs and cats. Since this cancer usually affects a long bone in a limb, amputation is typically the treatment of choice. Unfortunately, by the time most dogs are diagnosed, osteosarcoma has already spread to other tissues, such as the lungs, so most affected pets require a combination of chemotherapy and surgery. With the appropriate treatment, many dogs have a good quality of life up to one year after diagnosis, whereas cats may live four years or longer. 

Clinical signs of osteosarcoma in pets can include:

  • Varying degrees of lameness
  • Swelling along one limb
  • Recent aggression when the affected limb is touched
  • Decreased willingness to walk or play
  • Decreased appetite
  • Lethargy or apathy

#2: Lymphoma

Lymphoma is a cancer of a blood cell known as a lymphocyte. Also known as lymphosarcoma, this type of cancer is more common in cats than dogs. The gastrointestinal tract, lymph nodes, and a chest structure known as the mediastinum are most commonly affected in cats, but this cancer can involve a variety of tissues in dogs and cats. The average age of diagnosis in cats is between 2 and 6 years old. Feline leukemia virus is a known cause in cats, but a vaccine has dramatically decreased incidences of this virus and the cancer. Canine and feline lymphoma is treatable, and many pets achieve remission with chemotherapy, but prognosis depends greatly on the cancer grade, tissues affected, and the pet’s leukemia virus status. 

Clinical signs of lymphoma in pets can include:

  • Enlarged lymph nodes in the neck, underarm, and inguinal regions, and behind the knees
  • Vomiting or diarrhea
  • Decreased appetite
  • Weight loss
  • Muscle loss
  • Lethargy

#3: Mast cell tumors

Mast cells, which contain small granules called histamine, are natural immune-system components. Since these cells function throughout the body, they can cause cancer in many different organ systems, with skin manifestation most common in dogs and cats. Tumors vary grossly in appearance, making any abnormal skin growth potentially worrisome. The prognosis for mast cell disease depends on the tumor grade, location, and spread of disease. Surgical removal is the mainstay treatment for these locally invasive tumors.

Clinical signs of mast cell tumors in pets can include:

  • Skin tumors that range in appearance from flat, raised, reddened, ulcerated, soft, firm, or rounded, or no apparent shape
  • Many tumors that alternately grow and shrink 


A cancer of cells that line the blood vessels, hemangiosarcoma is a malignant disease that can occur in any dog breed, but is common in German shepherds and golden retrievers. This cancer frequently originates in the spleen, liver, heart, or skin. Dogs who suffer from splenic hemangiosarcoma often show no signs until the tumor ruptures, causing internal bleeding and rapid onset of clinical signs. If caught early enough, this cancer can be treated with surgery and chemotherapy, depending on tumor location, but many pets are not diagnosed until the disease is already life-threatening, leading to a poor prognosis. 

Clinical signs of hemangiosarcoma in pets can include:

  • Collapse
  • Pale gums
  • Bleeding
  • Bloated or painful abdomen
  • Weight loss
  • Lethargy
  • Inappetence

Have you noticed any of these signs in your pet? Are you curious to learn more about pet cancers? Contact our veterinary team to set up an appointment. Be aware that if your pet collapses, has pale gums, or is actively bleeding, you should go to your nearest veterinary emergency center immediately.