Both mice and rats are very susceptible to formation of tumours. Rats over 2 years of age are reported to have an 87% chance of developing one or more types of tumours. Mice frequently develop tumours representing a wide variety of tissue types. The tumours may be external or internal.
Leukaemia (cancer involving the white blood cells) is quite common in mice and rats. Both males and females develop benign mammary (breast) tumours, and females develop benign tumours of the uterine and vaginal linings. These are the most common tumours of rats.
Because rats have mammary tissue in locations beneath the skin other than along the underside of the belly, it is not uncommon to find lumps and bumps representing mammary tumours over the shoulders, flanks and base of the tail.
These tumours are relatively easy to surgically remove under general anaesthesia. If you find a lump, bump or unusual mass on your pet mouse or rat, you should take your pet to see your vet straight away. Your vet will probably be able to surgically remove the mass and undertake a biopsy to determine if it is benign or malignant which, in turn, helps to determine the treatment and long-term outlook for the patient.
Tumours tend to grow continuously larger and may ulcerate and become infected if they reach a very large size. For this reason, it is always preferable to remove them while they are still small.