7 Subtle Signs of Cancer in Pets That Most Pet Parents Overlook

Sarah Wooten, DVM
By Sarah Wooten, DVM. Reviewed by Jennifer Coates, DVM on Jan. 29, 2024
white dog and tan cat curled up together

With cancer in dogs and cats, early detection and treatment are critical. By detecting cancer early, you can work with your veterinarian to increase the chance of cure or remission.

But noticing the subtle signs of cancer early enough can be difficult if you don’t know what you are looking for, and sometimes the signs can be so subtle that pet parents may mistake them as a normal part of aging.

Talk to your veterinarian if you notice any of the following subtle signs of cancer in your pet. If the cause is cancer, you can catch it early and start a treatment plan.

1. Weight Loss

Unexplained weight loss in dogs and cats—especially when a pet isn’t on a calorie-restricted diet—can be a sign of cancer. While some cancers can cause quick and dramatic weight loss that is hard to miss, it’s more common that the weight loss is gradual over time. It can be so gradual that, initially, it goes unnoticed by the pet parent.

Sometimes, weight loss isn’t even noticed until the pet is taken to the veterinarian and the change in weight only becomes obvious when compared to historical weight measurements.

If a pet is losing weight even though they’re eating normally, then the top diseases to consider are diabetes mellitus, conditions that affect the digestive tract’s ability to digest and absorb nutrients, hyperthyroidism (more common in cats), and cancer.

2. Lumps and Bumps

While lumps, bumps, and other skin changes are easier to notice on short-haired dogs or cats, many times they go unnoticed on long-haired pets.

Pet parents will also often postpone a veterinary visit if a lump is small; however, you cannot determine if a skin lump is cancerous based on the size. Even the smallest skin lumps can be cancerous.

Tumors of the mammary chain—also known as breast cancer—are often missed by pet owners. Female dogs and cats most commonly develop breast cancer when they were not spayed early in life.

Dogs and cats are also subject to cancers of the gastrointestinal system, vascular system, liver, kidneys, urinary bladder, endocrine glands, immune system, lymphatic system, and reproductive organs. These cancers may cause fluid to accumulate or masses to form in the belly. If your pet is large or overweight, you may not notice any changes until they become life-threatening.

Another place where lumps and bumps can be hidden is in the mouth. Dogs and cats can get oral tumors on the gums, palate, or tongue. Tumors under the tongue are very hard to find—unless you are looking for them!

3. Changes in Coat

A normal, healthy pet has a glossy, full coat. Changes in the coat can indicate cancer, such as:

  • Hair loss

  • Brittle or dry hair

  • Excessive dandruff or scaling

  • Skin infections that don’t respond well to treatment

  • Excessive shedding

Skin cancers can directly affect a pet’s coat, but so can cancer elsewhere in the body. For example, cancers of the endocrine system, such as tumors on the pituitary, thyroid, or adrenal glands, can all cause changes in your pet’s coat.

Overgrooming, no grooming, or excessive licking at a body part can also be subtle signs of cancer in both dogs and cats. If a body part is painful due to cancer, a dog may excessively lick that area, which can cause brown lick stains and hair loss.

If a cat feels sick or is in pain, they may not groom enough or at all, which can lead to a matted, unkempt coat. Alternatively, cats are also known to overgroom and pull out hair in response to stress or pain.

4. Changes in Appetite

An increased or decreased appetite can be subtle signs of cancer in dogs and cats. If a pet is feeling yucky or is in pain, then they may not want to eat. Conversely, other cancers can cause a pet to eat more than normal. Some cancers consume a lot of calories, which will increase a pet’s appetite (if they are feeling well enough to eat).

Certain types of aggressive, malignant cancers can cause a pet to eat normal or more than normal amounts of food and still lose weight. And benign tumors of the adrenal or pituitary glands can cause a condition called Cushing’s disease that leads to hormonal changes and an increased appetite.

5. Changes in Urination or Bowel Movements

Changes in your pet’s bathroom schedule are worthy of note when it comes to early detection. Different types of cancer can cause changes in your pet’s potty habits, from an increased need for potty time to constipation.

For example:

  • Cancer of the gastrointestinal system can cause diarrhea and/or constipation.

  • Cancer of the adrenal gland, pituitary gland, thyroid gland, liver, or kidneys can increase thirst in dogs and cats and lead to an increased need to urinate.

  • Cancer of the urinary system can increase the urge to pee but sometimes obstruct the urethra, which makes it difficult to urinate. So, pets with urinary cancer may have to pee frequently, have accidents inside the house, or seem to strain when urinating.

6. Changes in Behavior

A change in behavior can also be a subtle sign of brain cancer, metabolic changes associated with cancer, or cancer pain. This includes:

  • A lack of energy or lack of interest in the things that used to bring your pet joy

  • Accidents in the house

  • Changes in sleep patterns

  • Pacing

  • Increased aggression or clinginess

Seizures or tremors can also be a sign of cancer, and unless you witness them, they can be missed by pet parents. Some signs that may indicate a pet has experienced a seizure include loss of balance, twitching, excessive drooling, and temporary blindness.

7. Coughing

Cancer can cause dogs and cats to cough, so any dog or cat that develops a persistent cough needs to be seen by a veterinarian. A persistent cough can be a potential sign of a tumor pressing on an airway, fluid in or around the lungs, or lung tumors.

Persistent coughing can also be a sign of other pet health issues, so if your pet has a cough they cannot seem to kick, it’s always best to take them to their veterinarian.

How You Can Help Detect Cancer in Pets Early

Veterinarians are trained to notice any abnormalities in your dog or cat, and a veterinary examination is your best weapon against cancer.

However, most pets only see their veterinarian once or twice a year. So, it’s helpful if you play an active role in monitoring your pet’s health as well. As a pet parent, you see your pet every day, which means you can keep a look out for these potential subtle signs of cancer.

To do at-home checks, you can run your hands over your pet to feel for any lumps or bumps and visually inspect your pet. Don’t forget to inspect the nipples and surrounding tissues in female dogs and cats for any changes, lumps, or bumps.

Feel your pet’s rib cage for any signs of weight gain or loss, or better yet, weigh them monthly and keep a log so you can notice trends early. Look in your pet’s mouth and check their teeth and gums. Hold a treat out, and let your pet lick the treat while you visually inspect the top and bottom of the tongue.

Pet guardians know their pets best, and by conducting these monthly at-home inspections, you may be more likely to catch subtle signs of cancer early enough to make a difference.

Featured Image: iStock.com/Neniia Lanti

Sarah Wooten, DVM


Sarah Wooten, DVM


Dr. Sarah Wooten graduated from UC Davis School of Veterinary Medicine in 2002. A member of the American Society of Veterinary Journalists,...

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