Mammary Neoplasia - Rat

Introduction

Mammary Neoplasia is common in the rat. The most common tumour type is a benign fibroadenoma. Mammary tissue is extensive in the rat, extending from the neck to the flanks, so mammary neoplasia should be considered when a rat is presented with a mass within this region.

Signalment

Mammary neoplasia can occur in both female and male rats.

Clinical Signs

The rat may present with one or more subcutaneous masses, which are encapsulated in the cases of benign fibroadenomas. The masses can be of variable size and may become very large. They may ulcerate and begin to prevent normal activities such as eating, drinking and moving.

Treatment

Treatment is either complete surgical excision of the mass or euthanasia.

Surgical removal:
Remove the entire mass and collect regional lymph nodes to stage the disease. Unless the mass is ulcerated or suspected to be malignant, the skin overlying the tumour should not be removed. Ligate any blood vessels supplying the tumour and close the wound in a routine manner. Use a subcuticular suture pattern or skin staples because rodents frequently chew at sutures and may remove them. An Elizabethan collar may be needed. Although metastasis is rare, radiographs should be performed prior to surgery. Common sites of metastasis are regional lymph nodes, abdominal viscera and the lungs.

Euthanasia:
Euthanasia should be considered if the mass is not amenable to surgical resection or if the mass is preventing normal locomotion.

Prognosis

Prognosis is good with successful surgical excision, however it is common for the mass to recur either elsewhere or in the same location as the original mass. It is rare for the mass to become malignant. In comparison, mammary neoplasia in mice is frequently malignant, invasive and difficult to remove. It is associated with infection by the mouse mammary tumor virus. In guinea pigs, about 70% of the mammary gland tumors are benign fibroadenomas and 30% are mammary adenocarcinomas.

Control

The incidence of mammary neoplasia may be decreased by performing and ovariohysterectomy at a young age. This can be recommended to owners of female pet rats.

References

Merck & Co (2009) The Merck Veterinary Manual (Ninth Edition), Merial
Bond, Hendricks, Loeffler (2009) Veterinary Dermatology RVC Intergrated BVetMed Course, Royal Veterinary College