Individual mammary gland tumours can start as benign but evolve to become malignant. The chances of malignancy increase with tumor size. The chances of metastases are smaller than 50% in malignant tumours. When metastases occur, they are usually found in the regional lymph nodes, liver, lungs and bones.
Mammary gland tumours are common in dogs (the most frequent tumour in non-spayed bitches). After all, in dogs the exposure to ovarian hormones is an important factor in the development of mammary gland tumours. Mostly middle-aged and older dogs are affected. It is very rare in dogs under 5 years of age.
Risk factors include:
- hormones: hormonal exposure plays an important role in the development of mammary gland tumours. Spayed dogs have a lower risk of developing a mammary gland tumour than non-spayed dogs. If the spaying takes place before the first heat, the dog only has a risk of 0.5% of developing a mammary gland tumour, 8% before the second heat, 26% before the third heat vs a dog who hasn’t been spayed. Afterwards, when it comes to tumour development, it makes little difference whether the bitch was spayed or not. Dogs who are administered “the pill” (progestagens) are twice as likely to develop mammary gland tumours and often develop them at a younger age.
- age: this tumour type mostly occurs in dogs at 7-8 years of age, after which the risk increases each year. The older the bitch gets, the higher the chances are the tumour is malignant.
- diet and body weight also play a role to a lesser extent. Being overweight during puberty could increase the risk of developing a mammary gland tumour.
A higher risk to develop this tumour type has been reported for the following breeds: Cocker Spaniel, Chihuahua, Maltese, Poodle, Dachshund, Yorkshire Terrier (more in smaller breeds). Some larger breeds with a greater risk include: Boxer, German Shepherd, Dobermann, English Springer Spaniel, English Setter, Epagneul Breton, Pointer.
This tumour type manifests as a palpable lump in the mammary gland. Most lumps appear at the glands closest to the tail. More than 70% of non-spayed bitches have more than 1 mammary gland tumour at the time of diagnosis. The tumour size, disease stage and presence of general disease symptoms vary a lot.