Tumours (neoplasia) and respiratory diseases are the most common conditions seen in rats by vets. There are many different types of tumour which may occur in rats; the most common are mammary tumours (in females), and adrenal, pituitary, and thyroid tumours which occur in both sexes. As in other species of animal it tends to be the older individuals which develop tumours. Neoplasia is most often seen in female rats over one year of age.
Tumours may be benign or malignant. Benign tumours are localized growths, usually encapsulated, which do not spread to other parts of the body. Malignant tumours are more serious and will invade the local tissue and may spread to other parts of the body such as the liver and lungs (this is known as metastasis) and eventually cause the demise of the animal if they are not effectively treated. Malignant tumours are what most people are thinking of when they discuss cancer. Malignant neoplasia can occur externally or internally. External malignancies are likely to grow quickly, often look angry and inflamed, may cause the rat pain or discomfort (often expressed by excessive grooming of the area) and may become ulcerated or bleed. Internal malignant tumours are harder to spot but there may be signs of a distended abdomen, weight loss, inappetance and lethargy. If tumours affect the brain then neurological signs may be noticed. Unfortunately if rats have malignant tumours then it is often kindest to consider euthanasia.
Benign tumours are fortunately the most common type of neoplasm in rats. The most common form of benign neoplasia seems to be mammary tumours occurring in females. I have come across many lipomas (fatty lumps) as well. They can generally be removed under general anaesthetic by a veterinary surgeon. The prognosis is usually good if the tumour can be completely removed while it is relatively small. Sometimes it is impossible to remove all neoplastic tissue during surgery and in this situation the growth will usually come back. If benign tumours are allowed to get too large they will become so big and heavy that the rat is unable to move around properly and it will become thin and unwell. If this stage is reached then euthanasia is often the kindest option since surgery may be impossible. Therefore if you notice signs of a tumour take your rat to your vet promptly; the smaller the tumour is, the better are the chances of successful removal. If you decide not to opt for surgical removal of the tumour this is OK, but you should watch your rat closely for signs of distress or discomfort associated with the growing lump. It may be some time before any such signs show, but once they do treatment or euthanasia should be carried out.
Rats tend to do well following surgery to remove tumours. You can generally treat your rat as normal when it returns home from the vet but do not use bedding which is likely to stick to the wound for the first few days. Watch out for signs of the rat chewing the sutures and also check for signs of infection such as fluid oozing from the wound or excessive swelling or redness. If any of these problems occur contact your vet.
Since it is unclear what causes neoplasia in the first place it is difficult to know how to prevent it. Some people think that feeding rats a low fat diet and preventing them from becoming obese may help. I am definitely in favour of this whether it prevents cancer or not, since rats will almost certainly have a better quality of life and be generally more healthy this way. Adding some green vegetables to the diet is also believed to help and once again I would consider this a good thing to do anyway, but do not over supplement the diet with them. Spaying female rats may also reduce the likelihood of them developing mammary tumours.