Cancer Warning Signs in Cats and Dogs: A UK Owner's Guide

A useful framing

Cancer in pets is a wide category. Some cancers are local and can be entirely cured by surgery (many skin tumours, mammary tumours caught early). Some are aggressive but treatable with surgery or chemotherapy (lymphoma, mast cell tumours). Some progress slowly with no treatment (some older-cat thyroid cancers). And some are aggressive and limit treatment options (osteosarcoma in large dogs, hemangiosarcoma).

What matters is catching it early enough that the treatment options are wide. The signs that should prompt a vet check are mostly subtle changes — things easy to attribute to ageing, weather, or temperament. The owners who catch cancers earliest are the ones who pay attention to small shifts.

This guide isn't medical advice for any specific pet — if you notice any of the signs below in your animal, the answer is always "book a vet check." Your vet will tell you whether the concern is real and what to do about it.

The general warning signs to know

Many of these are non-specific (they could mean many things), but the combination, persistence, or progression of them in an older pet warrants a vet visit. The following are widely cited by UK and international veterinary oncology organisations:

• A new lump or bump — anywhere on the body. Don't wait to see if it grows; have it checked. Most lumps are benign, but the only reliable way to know is for the vet to sample it (a quick fine needle aspirate is usually painless and inexpensive).
• A lump that changes — grows, changes colour, ulcerates, bleeds, or starts itching
• Sores that don't heal — a wound that's been there for weeks without progress
• Weight loss without an obvious cause — particularly in older pets, particularly when appetite is unchanged or increased
• Loss of appetite — persistently for more than a few days
• Increased thirst or urination — can indicate many conditions including some cancers
• Persistent vomiting or diarrhoea beyond a normal upset
• Difficulty eating or swallowing — dropping food, pawing at the mouth, weight loss in conjunction
• Persistent lameness — particularly in larger breeds; bone cancer (osteosarcoma) often presents this way
• Difficulty breathing — can indicate cancers in the chest or pressing on airways
• Bleeding from any orifice without obvious cause
• Swollen abdomen, particularly if rapid in onset
• Lethargy or reduced exercise tolerance — a previously active pet noticeably slowing down
• Bad breath — can indicate oral or dental tumours, particularly in cats
• Behaviour changes — withdrawal, irritability, hiding, stopping grooming

None of these signs mean cancer. Many other conditions cause them. But each is a reason to see a vet rather than wait.

What diagnosis involves

If cancer is suspected, the workup typically progresses through:

1. Clinical examination — thorough physical assessment, checking lymph nodes, palpating the abdomen, listening to the heart and lungs.
2. Sampling the suspected lesion — fine needle aspirate (cells under microscope), or biopsy (tissue sample, usually requires sedation or anaesthesia). Sampling is often the single highest-value step.
3. Staging — if cancer is confirmed, working out where it is and isn't. Includes blood and urine tests, chest X-rays, abdominal ultrasound, and sometimes advanced imaging (CT or MRI). Staging is essential for treatment decisions.
4. Specialist consultation — referral to a veterinary oncologist for treatment planning, particularly for cancers where chemotherapy, radiation, or specialist surgery is being considered. The UK has several specialist oncology centres; most general practices will refer when appropriate.

Not every cancer needs full staging. For an older pet with limited treatment options or where extensive workup isn't appropriate, your vet will discuss the most useful tests rather than the full panel.

The treatment landscape in UK veterinary oncology

Modern veterinary oncology has substantial treatment options:

• Surgery — the most curative treatment for many local cancers (some skin tumours, mammary tumours, splenic masses, certain bone cancers when amputation is appropriate)
• Chemotherapy — widely available in UK practice, both at general practices for simpler protocols and at oncology referral centres for complex ones. Veterinary chemotherapy is dosed to maintain quality of life — most pets tolerate it well, with side effects much milder than in human medicine. Hair loss is uncommon (with the notable exception of Poodles).
• Radiation therapy — available at a small number of UK referral centres. Useful for tumours that can't be surgically removed completely.
• Targeted and immunotherapies — newer options for specific cancers (e.g. masitinib/toceranib for mast cell tumours and other tumours)
• Palliative care — pain relief, anti-nausea medication, appetite stimulants, and quality-of-life-focused care for cancers where active treatment isn't pursued

The right approach depends on the cancer type, stage, the pet's overall health and age, the owner's circumstances, and a thoughtful conversation with your vet (often informed by an oncology specialist consultation).

Questions to ask your vet at a cancer consultation

The shock of a cancer diagnosis can make it hard to think of questions in the room. Useful ones to take with you:

• What type of cancer is this and how confident are we in the diagnosis?
• What stage is it at? Has it spread that we know of?
• What are the treatment options? What are the realistic outcomes for each?
• What would happen if we did nothing? What would my pet's life look like?
• Is referral to a veterinary oncologist worthwhile in this case?
• What will treatment cost — a written estimate would be helpful
• How will I know whether my pet is responding well?
• What signs should make me come back urgently?
• How will we know when treatment isn't right anymore?

Our questions to ask your vet guide covers the general framework, and these specific questions are worth adding to that list for any serious diagnosis.