Signs and symptoms of cancer in pets

The thought alone of a beloved pet receiving a cancer diagnosis is upsetting. The sad reality is that cancer can affect around 50% of dogs and one in three cats over the age of 10 years.

The good news, however, is that early detection and prompt treatment can improve an affected pet’s outcome.

In this article, we’ll go through some of the warning signs and symptoms of common cancers in dogs and cats, and touch on the mainstays of diagnosis and treatment. Understanding the signs may support earlier diagnosis and treatment

Understanding cancer in pets

Cancer involves the abnormal and uncontrolled growth and multiplication of a particular cell type, due to a DNA mutation within those cells. It can develop in any tissue or organ within the body (including components of the blood), although some cancer types are more common than others.

When we diagnose a pet with cancer, it usually means that they have developed a malignant tumour, i.e. a group of abnormal cells that will tend to invade locally through tissues or spread around the body to cause disease. This is in contrast with benign tumours, which can grow but don’t tend to spread and damage other tissues in the same way.

Common types and symptoms of cancer in dogs

Some of the most common types of cancer in dogs include:

	What it is	Common symptoms
Lymphoma	A cancer of lymphocytes (a type of white blood cell). Is most commonly “multicentric” in dogs, affecting multiple lymph nodes, and often other organs such as the liver, spleen, or bone marrow.	Lethargy/fatigue Swollen lymph nodes, most often noticed under the jaw Unexplained weight loss Fever Difficulty breathing
Osteosarcoma	A cancer of bone cells, which frequently affects bones of the hindlimb or forelimb.	Lameness on the affected leg Painful, firm swelling at the tumour site Can lead to bone weakening and sudden fractures Difficulty breathing
Mast cell tumour	The most common skin cancer in dogs, developing in mast cells (a type of white blood cell involved in tissue swelling and allergic responses). Can range from relatively benign to aggressive in behaviour, depending on their grade.	May cause lumps on or underneath the skin, which can vary in appearance Lumps may fluctuate in size, and become swollen and itchy More aggressive types can trigger unexplained weight loss, gut ulcer unwellness symptoms, or general allergic reaction symptoms such as hives or even anaphylaxis
Soft tissue sarcoma	A cancer of connective tissue, muscle tissue or nerve cells, commonly occurring beneath the skin.	Soft or firm masses which can grow quite large, occurring under the skin anywhere in the body (including in the mouth or at nerve sites) Masses may cause pain, or may lead to ulceration and infection of the overlying skin
Hemangiosarcoma	A cancer of cells that line blood vessels, most commonly affecting the spleen, heart or skin of dogs. These tumours are filled with blood and can rupture, causing internal bleeding.	Internal tumours may cause: Weakness or collapse Pale gums Breathing difficulties Lethargy A distended tummy Sudden death Skin tumours may appear as bruise-like, blood blister-like, or raised dark masses on the skin.

Common types and symptoms of cancer in cats

Some of the most common types of cancer seen in cats include:

	What it is	Common symptoms
Lymphoma	A cancer of lymphocytes (a type of white blood cell). Most commonly affects the gastrointestinal tract in cats (the stomach or intestines), but can also affect other organs such as the liver, spleen or kidney.	Decreased appetite Vomiting Soft stools or diarrhoea Unexplained weight loss
Squamous cell carcinoma	A cancer of skin cells which commonly develops on pink-skinned, thinly-furred areas (such as the ear tips or around the eyes), or in the lining of the mouth.	On the skin: Non-healing scabs or red, crusty or ulcerated lesions, which gradually spread and deepen In the mouth: Bad breath Unusual drooling Difficulties eating Bleeding from the mouth Swelling of the jaw or face
Soft tissue sarcoma	A cancer of connective tissue, muscle tissue or nerve cells, commonly occurring beneath the skin.	Often slow-growing masses located beneath the skin, which may be firm or soft, and can be mobile or fixed to underlying tissues. May cause hair loss or ulceration of the overlying skin
Mast cell tumour	A cancer of mast cells (a type of white blood cell involved in tissue swelling and allergic responses). Can range from relatively benign to aggressive in behaviour. Commonly affects the skin or spleen of cats.	On the skin: May appear as hard, flat areas or bumps, particularly on the head and neck May be itchy In the spleen, they may cause: Weight loss Vomiting Loss of appetite

How is cancer in pets diagnosed?

For initial cancer diagnosis, the veterinarian will need to take a sample of cells from the pet’s tumour for analysis under the microscope. Depending on the tumour location and texture, this may involve:

• A fine needle aspiration (inserting a fine needle into the mass to collect some cells) – this may be done consciously or may require sedation.
• A surgical biopsy (cutting out a piece of the abnormal tissue or removing the entire mass to send to the laboratory) – this requires anaesthesia.

Some cancer types may then require additional diagnostic testing so the vet can better understand its likely behaviour and assess if it has already spread within the pet’s body or not. This can require tests such as:

• Additional specialised laboratory testing of tumour samples
• Blood and urine tests to check the pet’s blood cells and organ health parameters
• X-rays of the pet’s chest
• Abdominal ultrasound to look at organs such as the liver, spleen, intestines or internal lymph nodes
• Advanced imaging, such as a CT scan or MRI

How can pets with cancer be treated?

Once the veterinarian has confirmed a diagnosis, they will then be able to discuss appropriate treatment options for that pet’s particular cancer. The two overall treatment goals are:

1. Treatment with an aim for cure or remission (which may or may not be permanent): some pets may be able to have their cancer completely treated with surgery alone, while some may require alternative or combination therapies such as chemotherapy, radiation therapy, or cryosurgery
2. Palliative care: this involves keeping the pet comfortable by managing symptoms of pain, discomfort, fatigue or nausea, which can be done with medications and sometimes other therapies such as palliative surgery or radiation therapy to stabilise tumour growth, relieve pain, or remove tumour bulk

H2: Supporting a pet through cancer

It’s important to remember that there’s no “one size fits all” when it comes to treating pets for cancer. Some owners may want to go all in with an aim for a cure, while others may simply wish to keep their pet comfortable for as long as possible.

The best treatment for an individual pet will depend on their tumour type and stage of progression, the owner’s financial situation, the veterinary facilities that can be accessed, and the pet’s temperament and general health.

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21 Jul 2026