Mammary Gland Disease in Dogs: Recognising Lumps Early and Understanding Treatment

Mammary gland disease in dogs is one of the most common health concerns affecting unspayed female dogs, particularly as they age. While not every lump is cancerous, mammary tumours can range from benign growths to aggressive malignancies that spread throughout the body.

Because early detection significantly improves outcomes, understanding what mammary disease looks like and how it develops is essential for every dog owner.

What is Mammary Gland Disease?

Dogs have multiple mammary glands arranged in two chains along the chest and abdomen. Mammary gland disease refers to any abnormal condition affecting this tissue, including:

• Benign (non-cancerous) growths
• Malignant (cancerous) tumours
• Inflammatory or infectious conditions

Unlike in cats, where most mammary tumours are malignant, around 50% of mammary tumours in dogs are benign. However, distinguishing between benign and malignant masses requires veterinary assessment.

Types of Mammary Gland Disease in Dogs

Mammary conditions in dogs can present in several forms:

• Benign tumours – such as adenomas or fibroadenomas
• Malignant tumours – including carcinomas and sarcomas
• Mixed tumours – containing both benign and malignant components
• Mastitis – infection or inflammation of the mammary glands
• Mammary hyperplasia – hormone-related enlargement of tissue

Each type varies in severity, behaviour and treatment approach.

Causes and Risk Factors

Mammary gland disease is strongly influenced by hormones, particularly oestrogen and progesterone.

Hormonal Influence

The risk of mammary tumours is closely linked to reproductive status:

• Dogs spayed before their first heat have a very low risk
• Spaying after one or more heat cycles significantly increases risk
• Unspayed dogs have the highest risk

Hormones stimulate mammary tissue, and repeated exposure over time increases the likelihood of abnormal cell growth.

Age

Mammary tumours are most commonly diagnosed in middle-aged to older dogs, typically between 7 and 12 years of age.

Breed Predisposition

Certain breeds appear more prone to mammary tumours, including:

• Spaniels
• Dachshunds
• Poodles
• Terriers

Genetics likely play a role in susceptibility.

Obesity

Dogs that are overweight, particularly at a young age, may have an increased risk of developing mammary tumours later in life.

Hormonal Medications

Use of hormone-based drugs can contribute to mammary changes and increase disease risk.

Signs of Mammary Gland Disease

The most common sign is the presence of a lump along the mammary chain.

Owners may notice:

• Small, firm nodules under the skin
• Larger masses that may feel irregular
• Multiple lumps in different glands
• Rapid growth of a mass
• Ulceration or bleeding from a lump
• Swelling or redness of the surrounding tissue

Some masses are freely movable, while others may feel fixed to underlying structures.

Dogs with advanced disease may also show:

• Lethargy
• Reduced appetite
• Weight loss
• Coughing (if the cancer has spread to the lungs)

Benign vs Malignant Tumours

It is not possible to determine whether a tumour is benign or malignant by touch alone.

• Benign tumours tend to grow slowly and remain localised
• Malignant tumours may grow quickly, invade surrounding tissue and spread (metastasise)

Because of this uncertainty, all mammary lumps should be evaluated by a veterinarian.

Diagnosis

A veterinary assessment is essential for any suspected mammary disease.

Diagnostic steps may include:

• Physical examination – assessing size, location and number of masses
• Fine needle aspiration – collecting cells for preliminary analysis
• Biopsy or surgical removal – for definitive diagnosis
• Chest X-rays – checking for spread to the lungs
• Blood tests – assessing overall health before treatment

Histopathology (laboratory analysis of tissue) is required to confirm tumour type.

Treatment Options

Surgical Removal

Surgery is the primary treatment for mammary tumours in dogs.

Depending on the situation, this may involve:

• Removal of a single lump (lumpectomy)
• Removal of one gland (mammectomy)
• Removal of an entire mammary chain (mastectomy)

The surgical approach depends on tumour size, number and location.

Early removal, when tumours are small, offers the best prognosis.

Spaying

If the dog is not already spayed, your vet may recommend spaying at the time of tumour removal. This can:

• Reduce the risk of new tumours developing
• Eliminate ongoing hormonal influence

However, spaying later in life does not eliminate the risk entirely.

Chemotherapy

Chemotherapy may be recommended for malignant tumours, especially if:

• There is evidence of spread
• The tumour type is aggressive

Its goal is typically to slow progression and improve quality of life.

Treatment of Non-Cancerous Conditions

• Mastitis is treated with antibiotics and supportive care
• Hyperplasia may resolve with hormonal management or spaying

Prognosis

Prognosis depends on several factors, including:

• Tumour size – smaller tumours (<3 cm) have better outcomes
• Tumour type and grade
• Presence of metastasis
• Timing of treatment

Dogs with small, benign tumours often have an excellent prognosis following surgery.

Malignant tumours carry a more variable outlook, particularly if diagnosed late.

How to Check Your Dog at Home

Regularly examining your dog can help detect changes early.

Gently feel along the mammary chain from chest to groin, checking for:

• Lumps or nodules
• Areas of thickening
• Changes in skin or temperature

Incorporating this into grooming or petting routines makes it easier to notice changes over time.

Prevention

The most effective preventive measure is early spaying.

Spaying before the first heat cycle dramatically reduces the risk of mammary tumours.

Additional steps include:

• Maintaining a healthy weight
• Avoiding unnecessary hormonal treatments
• Scheduling regular veterinary check-ups

When to Seek Veterinary Advice

You should consult a vet if your dog:

• Has any new lump or swelling along the mammary chain
• Shows rapid growth of an existing lump
• Develops ulceration or discharge
• Appears unwell or lethargic

Even small lumps should not be ignored, as early treatment significantly improves outcomes.

Supporting Your Dog Through Treatment

Dogs generally recover well from mammary surgery, especially when the condition is caught early.

Supportive care includes:

• Providing a calm recovery environment
• Preventing licking of surgical wounds
• Administering medications as prescribed
• Attending follow-up appointments

Most dogs return to normal activity levels within a short period after surgery.

Conclusion

Mammary gland disease in dogs is common but often manageable, particularly when detected early. While not all tumours are cancerous, every lump should be taken seriously and assessed promptly.

By understanding risk factors, performing regular checks and seeking early veterinary care, owners can significantly improve outcomes and help their dogs live longer, healthier lives.

Preventive measures, especially early spaying, remain one of the most powerful tools in reducing the risk of mammary tumours and supporting long-term wellbeing.