Mammary Tumours in Rats

Tumours, particularly mammary tumours, are commonly seen in rats. We look at why they occur and how to treat them

Posted: 14 September 2017

Mammary Tumours in Rats


Tumours, particularly mammary tumours, are commonly seen in rats. They’re usually diagnosed in females over 1 year of age, they also occur in males.

Most are benign fibroadenomas that can be removed by simple surgical excision. Some recur at the original site after removal.

Around 1/5 of mammary tumours are malignant adenocarcinomas and can recur after excision. They tend to metastasise slowly.

Why do rats develop mammary tumours?

Hormones – prolactin has been known to be a hormone that stimulates the proliferation of mammary tumours for some time. Since rats come into oestrus every 4 or 5 days, prolactin is almost always present. The release of prolactin also increases in both males and females as they age. Benign tumours of the pituitary gland will also secrete higher levels of prolactin.

Overfeeding - rats fed ad lib tend to have a higher incidence of mammary and pituitary tumours.

Genetics – there is a genetic predisposition in many rats. Breeding from rats with a known risk should be discouraged.

What are the signs?

  • Spherical and firm – many mammary tumours grow into single or multiple large, spherical masses. They feel firm and well differentiated and are not usually attached to deeper underlying structures.
  • Rapid growth - the tumours tend to grow rapidly, becoming very large in a short period of time (weeks).
  • Large in size - masses can grow to become very large, often larger than the animal’s head.
  • Ulceration – left untreated for a few weeks, the skin on the surface of the mammary tissue can become inflammed, broken, infected and ulcerated.
  • Fixed or floating – some tumours may be freely movable which usually suggests they are benign. However if they are fixed to skin or body wall they may be malignant.
  • Well tolerated – most rats seem to be unaffected by the growths, initially eating and acting normally despite the mass. As a result many owners fail to bring the animal to our attention until the growth is advanced and difficult to treat. Surgery should be recommended as soon as possible.
  • Difficulty walking and grooming – some tumours are allowed to grow so large that they interfere with movement of limbs, making it awkward or difficult to move, as well as interfering with normal cleaning and grooming.


Diagnosis first – before proceeding with surgery, we need to confirm the tumour. Whilst cytology from fine needle aspirates may be helpful, the results are often inconclusive if the tumour has a necrotic centre or contains inflammatory cells.

Differential diagnosis for mammary tumours includes abscesses, mastitis and lymphoma.

Surgical excision – this is the treatment of choice, especially in the early stages. The benign tumours are usually well demarcated, firm and relatively easy to remove by blunt dissection. Malignant adenocarcinomas are generally more attached to the underlying tissue and can be frustrating to resect cleanly. With malignant tumours, the skin should be resected to minimise local invasion.

Arteries before veins - with larger tumours, ligate the main arteries before the veins as blood can flow back into the circulation before excision.

Analgesia – it’s important to give preoperative and postoperative analgesia to all patients. The usual nsaid’s and opiates are used.

Ovariohysterectomy – ‘spaying’ at time of surgical removal of the mammary tissue (mastectomy) may improve survival. To minimise surgical time and trauma, it may be wise to remove large masses and then wait a few weeks before spaying.

Have you seen our article on chemical restraint in rodents? >

Preventing mammary tumours

Spay early - ovariohysterectomy may reduce the likelihood of mammary tumours by slowing or preventing the development of mammary tissue.

Hormone regulation - gonadotropin-releasing hormone agonists (eg, Deslorelin) may be useful in rats when ovariectomy is not possible.

Lifestyle changes – avoiding overfeeding, encouraging exercise and reducing stress triggers (cats, dogs, noise) are all lively to reduce the likelihood of tumour development.

10 top tips for surgery

  1. Early removal - remove mammary tumours as soon as they are diagnosed
  2. Fit and stable – ensure the animal is in good condition prior to surgery
  3. Discuss diet - provide a balanced plane of nutrition
  4. Avoid fasting – it is not necessary to withhold food before surgery as rats are unable to vomit
  5. Pain relief - provide good preoperative, perioperative, and postoperative analgesia
  6. Fluids – ensure your patient is sufficiently hydrated by using fluids, either IV or SC
  7. Monitor temperature – small mammals can lose heat easily and rapidly become unstable. Monitor rectal temperatures and provide heat during surgery
  8. Avoid blood loss - limit haemorrhage by careful surgical technique
  9. Electro-cautery - use radiosurgery when possible
  10. Keep an eye on the clock - minimise time in surgery; surgical staples to close skin, single- or double-layer closure, no drains


To help all our supporting practices working with pet rats, we have a special offer on our NEW range of Rat Nuggets. Take a look by clicking the link below