Tumours, particularly mammary tumours, are commonly seen in rats. We look at why they occur and how to treat them
Posted: 14 September 2017
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.
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.
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.
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.
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