Lethargic cat
In a recent article, I compared the common types of cancer in pets with those found in people. My next step, in this article, is to write about how pet owners might recognize signs of cancer in pets, with a special focus on the common types. Some of these signs, such as weight loss and bad breath, may be indicative of cancer or they may signify other health problems. Regardless, they should always prompt a discussion with your veterinarian.

Here are the ten signs that top my concern list as a veterinary oncologist:

  1. Bleeding or discharge from any place on the body, such as the mouth, eyes or nose, or in the urine
  1. Change in urination or defecation habits
  1. Sores that do not heal
  1. Bad smell from the mouth or body
  1. Difficulty chewing or swallowing
  1. Loss of energy; reluctance to exercise
  1. Loss of appetite
  1. Weight loss
  1. Swellings or lumps that enlarge
  1. Lameness or stiffness
Let’s look at how these signs present with some of the more common cancers that affect cats and dogs.

Breast Cancer

Possible signs: Swellings or lumps that enlarge; sores that do not heal.

In both dogs and cats, breast cancer can be detected by the pet owner during a relaxing session of tummy rubbing and scratching. Breast cancer starts as tiny, pinhead-size lumps anywhere along the chain of mammary glands found on the underside of the chest and abdomen of your male or female dog or cat (although it is rare in males). Once the tumors reach the size of raisins, they can often be felt as somewhat soft to firm lumps or masses. Any lumps or masses in the mammary area should be evaluated by your veterinarian.

Cat on being weighed on scale


Possible signs: In cats, weight loss, poor appetite, vomiting, diarrhea or blood in the stool. In dogs, rapidly enlarging lymph nodes. Occasionally, increased water consumption and urine output.

The most common form of lymphoma differs between dogs and cats. In cats, insidious weight loss is the hallmark of lymphoma, which occurs most commonly in the feline gastrointestinal tract. I know from personal experience and from scientific research that pet owners sometimes have a hard time assessing a plumper-than-normal or a skinnier-than-usual dog or cat. Weight loss and appetite loss are indicative of many diseases other than cancer, though, so it’s important to notice any changes. Next time you visit your veterinarian, ask her to help you assess the “body condition score” of your pet. Ideal body conditions generally score about a three on a scale of one to five or a four or five on a scale of one to nine.

One good way to keep tabs on your pet’s weight is to stay in close contact with your veterinarian, who keeps detailed records about your pet’s weight as part of her wellness examinations. Most veterinary offices have a readily accessible scale and would welcome your pet for a quick weigh-in anytime you are concerned about potential weight loss. If lymphoma affects your cat’s stomach, you may see vomiting. If it affects her intestines, you may notice diarrhea or blood in the stool. Some cats with lymphoma may also have a poor appetite or stop eating altogether.

From my veterinary perspective, the most common clinical sign of lymphoma in dogs is swollen lymph nodes. The easiest lymph nodes for owners to see and feel are just beneath the skin under the chin, in front of the shoulders and behind the knees. In a normal, healthy dog, lymph nodes are not detectable by the average owner. Lymph nodes affected by lymphoma, however, are an example of rapidly enlarging lumps that should be immediately evaluated by a veterinarian. Another key sign, although it is seen in less than half of dogs with lymphoma, is a metabolic change that results in an increase in both water consumption and urine output. Again, keep in mind that increased drinking and urinating can also be signs of diseases other than cancer and always warrant a visit to your veterinarian.

Skin Cancer

Possible signs: Lumps or bumps that enlarge, sores that do not heal, limping and/or bleeding or broken toenails.

In dogs, the most common type of malignant skin cancer is a mast cell tumor. These tumors are superficial lumps that can be painful. They often swell, frequently bleed and then scab over, only to bleed again a few days later. They should not be squeezed by the owner, as squeezing can make them swell even more.

Similar signs occur in cats with the most common type of feline skin cancer — squamous cell carcinoma. This tumor may cause skin ulcers that bleed and scab, especially in the lightly haired skin around the eyes and nose and on the ear tips.

Unlike in humans, melanoma is typically benign in dogs and cats. It can occur in the mouth in dogs, however, and when it occurs in this location it is often highly malignant (see the Oral Cancers section below). The other location where melanoma can be malignant is at the junction between a dog’s claw and toe. If you see swelling, bleeding, an unexpected broken toenail or limping caused by a mass at the claw-toe junction, it may indicate a serious problem in your dog. Your dog should be evaluated by your veterinarian, who may recommend a biopsy.

Melanoma is somewhat unique among cancers in that it spans the spectrum from benign when found in the haired skin of pets to deadly when it occurs in the toes or mouths of dogs.

Dog panting

Oral Cancer

Signs of oral cancers include bad breath, blood in the saliva, decreased appetite, and difficulty in chewing or swallowing.

Many different tumors occur in the mouths of dogs and cats, but all have similar clinical signs. The most common in dogs is melanoma, while squamous cell carcinoma commonly occurs in cats.

Dogs with melanoma of the oral cavity may experience blood in the saliva, difficulty chewing and swallowing, or a decreased appetite. Dog owners frequently first notice heavy-duty hound halitosis, or bad breath. Cats with squamous cell carcinoma may exhibit similar signs. Because these tumors often block the tear ducts, a cat owner might also notice an increase in eye discharge in just one of their pet’s eyes or a funny look to their cat’s face because of the facial swelling associated with these tumors.

There are many types of cancers that occur in the oral cavity in both dogs and cats, and some of them can be quite aggressive. Anytime you note the above signs, or a lump or bump in your pet’s mouth, you should consult your veterinarian.

Osteaosarcoma (Bone Cancer)

Possible signs: Lameness and reluctance to put weight on a particular leg; painful hard lump or swelling.

The most frequently diagnosed tumor of the bone in both dogs and cats is osteosarcoma, or bone cancer. The clinical signs of any bone tumor include lameness and reluctance to put weight on a particular leg because the tumor makes it painful to walk on. If the tumor occurs in just the right location, you may be able to feel a hard lump or swelling on the bone, although be advised that these lumps can be extremely painful to the touch. An X-ray and biopsy will be necessary to confirm the diagnosis.

Be an Observant Owner

Though I’ve covered some of the more common signs of the most prevalent cancers, the important message here is to realize that many types of cancer have similar signs. Some of these signs can also be indicative of serious diseases other than cancer. When you are interacting with your pet daily, look for the signs I have described. If you see something of concern, have your pet evaluated by your family veterinarian. Even something as nonspecific as a general loss of energy or an unwillingness to exercise can be a warning that something is wrong. Always remember that an early diagnosis can sometimes help improve the chances of treatment success, whether your pet has cancer or any other serious disease.

More on Vetstreet.com:
  • 10 Questions to Ask If Your Pet Gets the Big Cancer Diagnosis