Because mammary tissue is so extensive, tumors can develop anywhere from the cervical region to the base of the tail.2,8 The tumors grow rapidly, becoming very large within weeks. Rats appear to tolerate the tumors, initially eating and acting normally despite the presence of a mass. Thus, owners often do not present their rat until the tumor has grown very large. If the patient is stable, owners should be advised to schedule surgery immediately (see Treatment).
Complications of large mammary tumors include difficulty ambulating, difficulty grooming, and ulceration of affected tissue, with potential risk for blood loss anemia secondary to ulceration of the mass. Weight loss despite good appetite may occur because the nutrients being consumed by the patient are supporting growth of the mass. Necrosis and secondary infection can also be factors, possibly leading to septicemia, organ failure, and death.3 As tumors grow larger, surgical removal becomes more risky because of the loss of blood and subsequent shock (from rapid changes in fluid balance) associated with tumor removal.
The differential diagnosis for mammary tumors in rats includes abscesses, mastitis, and other forms of neoplasia, including lymphoma. Although samples can be obtained for fine-needle aspiration and cytology, the results can be misleading if the tumor has necrotic areas that contain inflammatory cells.4
Related Article: Zoonoses of Small Mammals