Individual mammary gland tumors can evolve from benign to malignant. The chances of malignancy increase with tumor size. For malignant tumors, the chances of metastases are smaller than 50%. This tumor type usually metastasizes towards local lymph nodes, the liver, lungs and the bone.
Mammary gland tumors appear regularly in dogs and are the most frequent tumor in non-sterilized bitches. After all, in dogs the exposure to ovarian hormones is an important factor in the development of mammary gland tumors. 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 tumors. Sterilized dogs have a lower risk of developing a mammary gland tumor than non-sterilized. If the sterilization takes place before the first heat, the dog only has a risk of 0.5% of developing a mammary gland tumor, 8% before the second heat, 26% before the third heat vs a dog who hasn’t been sterilized. Afterwards, it makes little difference whether the bitch was sterilized or not when it comes to the tumor development. Dogs taking the pill (progestagens) are twice as sensitive for the development of mammary gland tumors and often develop them at a younger age.
- age: they mostly occur in dogs at 7-8 years of age, after which the risk increases each year. The older the bitch, the higher the chance it concerns a malignant mammary gland tumor. In approximately 1 in 10 dogs as from the age of 10, a mammary gland tumor is present.
- to a lesser extent diet and body weight also play a role. Being overweight during puberty could increase the risk of developing a mammary gland tumor.
A higher risk to develop this tumor type has been reported for the following races: Cocker Spaniel, Chihuahua, Maltese, Poodle, Dachshund, Yorkshire Terrier (more in smaller races). Some larger races with a larger risk include: Boxer, German Shepherd, Dobermann, English Springer Spaniel, English Setter, Epagneul Breton, Pointer.
Sensible lumps at the mammary glands. Most lumps appear at the glands closest to the tail. More than
70% of non-sterilized bitches has more than 1 mammary gland tumor at the time of diagnosis. The tumor size, disease stage and presence of general disease symptoms vary a lot.