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Cancer is one of the leading causes of death in dogs and cats, but early detection can make all the difference. Unfortunately, many symptoms of cancer in pets can be subtle or mistaken for normal aging, minor illnesses, or behavioral changes. That’s why it’s critical for pet parents to know what to look for—especially the less obvious signs that may signal something more serious beneath the surface.
Catching cancer early often means more treatment options and a better prognosis. The following 15 signs are commonly overlooked but may point to underlying cancer in your dog or cat. If your pet exhibits any of these changes, it’s worth a conversation with your veterinarian.
- Persistent Lameness or Limping
While limping may result from a sprain or arthritis, it can also indicate bone cancer—especially if it worsens over time or doesn’t respond to pain relief. In dogs, particularly large breeds, osteosarcoma is a common and aggressive bone cancer. Don’t dismiss chronic lameness as normal wear and tear; it could be your pet’s way of signaling something far more serious. - Unexplained Weight Loss
If your pet is shedding pounds without a change in diet or activity, cancer could be to blame. Certain tumors increase metabolic activity or suppress appetite. Weight loss is one of the most common warning signs of cancer in both dogs and cats and should always be taken seriously, especially when it’s rapid or continuous. - Swollen Lymph Nodes
Lymphoma often presents with enlarged lymph nodes—especially under the jaw, behind the knees, or in the groin area. You may notice firm, painless lumps in these areas during regular petting or grooming. Though not all swollen nodes are cancerous, they are a major red flag that requires prompt veterinary evaluation. - Bad Breath or Bleeding From the Mouth
Persistent bad breath, oral bleeding, or difficulty eating can point to oral cancers—particularly in older pets. Unlike routine dental issues, oral tumors may not improve with cleaning and often continue to worsen. Look for drooling, blood-stained toys, or sudden changes in chewing behavior. - Non-Healing Sores or Skin Changes
If your pet has a sore that doesn’t heal or a lesion that keeps growing, it could be a sign of skin cancer or another internal issue manifesting externally. Skin cancers like mast cell tumors often appear as raised lumps, and they may change shape or color quickly. Early diagnosis is essential to prevent spreading. - Unusual Odors
Foul or unusual odors coming from your pet’s mouth, ears, or other parts of the body—especially when combined with swelling or discharge—can be a sign of underlying cancer. Tumors in the mouth or anal region, for instance, often produce a distinctive and unpleasant smell not linked to hygiene.
- Sudden Behavioral Changes
A once-social dog who becomes withdrawn, or a cuddly cat who hides constantly, might be in pain. While many conditions can cause behavior shifts, cancer-related discomfort is often mistaken for moodiness or aging. Pay attention to sudden fear, irritability, or a reluctance to be touched in certain areas. - Difficulty Breathing or Coughing
Persistent coughing, wheezing, or labored breathing can signal lung cancer, heart tumors, or cancer that has metastasized. While infections and allergies are more common causes, any unexplained breathing issues—especially if they linger—should be checked by a vet. - Abdominal Swelling or Bloating
A swollen or distended belly could indicate a tumor, internal bleeding, or fluid buildup from cancer. Hemangiosarcoma, a type of cancer that often affects the spleen, may cause sudden bloating and collapse. If your pet looks rounder without weight gain or becomes sensitive when touched, seek immediate care. - Loss of Appetite
Refusing meals for more than a day or two is never normal for a healthy pet. If your dog or cat seems uninterested in food or has to be coaxed to eat, it may be due to nausea, oral pain, or general malaise—common symptoms of several types of cancer. 
Frequent Vomiting or Diarrhea
While occasional stomach upset is common in pets, frequent vomiting or diarrhea could suggest gastrointestinal tumors. These may be accompanied by bloody stools, dark tarry stools (a sign of internal bleeding), or weight loss. Don’t dismiss these symptoms as a passing bug—get them checked.- Increased Thirst or Urination
Drinking more water than usual or having accidents indoors may be signs of hormone-related cancers, such as adrenal or pituitary tumors. It could also be related to paraneoplastic syndromes where tumors affect hormone levels, leading to kidney strain or diabetes-like symptoms. - Trouble Swallowing or Eating
If your pet suddenly seems to struggle with eating, chews unusually, or avoids hard food, it may be due to a tumor in the mouth, throat, or esophagus. Watch for pawing at the mouth, gagging, or dropping food—especially if this behavior is new. 
Seizures or Neurological Symptoms
Sudden seizures, unsteadiness, disorientation, or behavioral confusion can indicate brain tumors. These symptoms often appear suddenly and may worsen over time. If your pet starts walking in circles, pressing their head against walls, or staring blankly, call your vet immediately.- Bleeding or Discharge From Unusual Places
Any unexplained bleeding or discharge—whether from the eyes, nose, mouth, anus, or genitals—can be a sign of tumors affecting internal organs or soft tissue. While not all bleeding is cancer, persistent or repeated episodes should never be ignored.
Even the most loving and observant pet parents can miss subtle signs of cancer. The key is to stay alert to any change in your pet’s habits, appearance, or mood—and to trust your instincts if something seems “off.” When it comes to cancer, early detection can be lifesaving. Regular vet checkups and knowing what to watch for are your best defense.
