Debbie Ducummon
From the Sept/Oct 1995 Rat & Mouse Gazette

Along with respiratory infections, tumors are one of the most common health problems and causes of death in rats, especially in females. There are two main types of tumors, benign and malignant. Benign tumors are almost always encapsulated in a membrane and separate from nearby tissues, and therefore easily removed by surgery.

Although they can grow as fast as malignant tumors, they don't cause as much damage and they don't metastasize (spread to other parts of the body). Although benign tumors won't usually cause death directly, a tumor can grow so large that the rat has difficulty moving around and can't eat enough to support both the tumor and normal body functions. Most rats are euthanized before reaching this stage, if the tumor isn't removed.

In contrast, malignant tumors, also called cancer, usually invade and damage nearby tissues and also metastasize. Death will be caused by the failure of damaged organs. Because cancerous tumors intertwine with normal tissues, surgery is usually impossible in rats. In most cases, cancer involves the internal organs, so symptoms often aren't seen until the disease is well advanced and euthanasia is the only alternative. The symptoms of cancer may include a skin ulcer, ulcerated tumor, or bleeding from a lump, a distended abdomen, weight loss, and lethargy.

Fortunately, most tumors in rats are benign. In nine years I've done surgery on about 100 rats to remove tumors, and only a few have been malignant. Of all these rats, I've removed tumors from only five males; the rest were females, almost all of them with benign mammary tumors. I've had a very high success rate removing this type of tumor.


The most common tumor in rats is a benign mammary tumor, with most of these being a fibroadenoma (Fibro-from connective tissue, aden-from a gland, oma-a benign tumor.) Mammary tumors can be quite squishy and flat, or they can feel firm, sometimes even as hard as cartilage. Because tumors grow so quickly, a portion will sometimes outgrow its blood supply, die and become necrotic and filled with pus. Benign mammary tumors are usually distinct lumps just under the skin that can be moved around slightly with the fingers. If your rat gets one of these tumors, you can be 99% sure it's benign. If a tumor feels tightly attached to the underlying tissue, this can be a sign that it is malignant, but some benign tumors also feel attached. I have successfully removed two of these that I first suspected were malignant, so it's worth it to try to remove even tumors that appear to be attached.

The most common locations for mammary tumors are in the armpits and in the belly or groin area; although because female rats have widespread mammary tissue, they can occur anywhere on the body. It's a good idea to feel your rat all over frequently so you can identify tumors early. The smaller the tumor, the more easily (and cheaply) it can be removed. It's never too late to have a tumor removed as long as your rat is otherwise in good health. I've successfully removed mammary tumors the size of a small apple! I've also removed tumors from rats well over two years old, so age should not be the only factor when considering surgery. Because mammary tumors are just under the skin the surgery to remove them is minor, as opposed to major surgery which enters a body cavity, and rats usually recover quite quickly. As long as a rat is otherwise in good health, this minor surgery can be performed on a rat of any age. While there is a chance that any rat could develop another tumor later, the surgery will probably save her life and definitely improve the quality of her life for months to come.

The cost to remove rat tumors can vary greatly. In Chico (a town of 70,000) some of the vets will remove tumors from rats for a reduced rate ($35 - 50) compared to the same surgery on a dog or cat ($150 - 200). This doesn't include an exam fee which can be $25 - 30 extra. If you're sure it's a benign tumor, you might be able to avoid an exam fee by calling different hospitals and asking how much they would charge for just the surgery to remove an adenoma from a rat. Tell them the location and size of the tumor. Then see if you can take your rat in only for the surgery and not an exam.

If you decide not to have a benign tumor removed, you can expect it to keep growing until it's almost as big as the rat herself. Within a few months, or even weeks, you must be prepared to have your rat euthanized when she is no longer enjoying life. In the meantime, you can help your rat maintain her quality of life by making sure she eats a very nutritious diet, high in protein, carbohydrates, and vitamins, because the tumor will be drawing resources and energy from her body.


Although benign mammary tumors are most commonly seen in rats, they can also get a variety of other tumors. I've seen three female rats with benign adenomas of the pituitary gland, which lies beneath the brain. All three of these rats developed the same neurological signs. The first symptom is loss of coordination followed by a quite distinctive posture: both the from and hind legs are held out stiffly in the forward direc tion and cannot be flexed by the rat.

Unfortunately, I don't know anyone who can perform brain surgery on rats, so rats with these symptoms should be euthanized. I have recently also seen two cases of tumors growing in the vagina. The first one I saw was huge, and although I attempted to remove it, during the surgery I decided it was best to euthanize the rat. In the second case, I failed to remove the whole tumor during the first surgery and it immediately grew back. This tumor was diagnosed by a pathologist as a fibroma. Unfortunately, this rat was euthanized due to complications five days after a second surgery.Other tumors I've seen include one, probably cancer, of the ovaries, one involving the urinary tract, and a malignancy that arose in a rat after I had earlier done two surgeries in the same location to remove benign mammary tumors. One very fast growing tumor in the thigh of one of my female lab rats was diagnosed as a fibrosarcoma. (The terms sarcoma and carcinoma refer to types of cancer.) I've also seen two squamous cell carcinomas, one on the face and one under the jaw (and both also in females). This type of tumor is most common on the face and head and should be suspected in any case of swelling in this area on a rat.


There are only three ways to try to prevent tumors in your rats. First, by feeding your rat a low fat, low calorie diet which is also low in amines and nitrates (when these natural components of some plants combine in the stomach they are carcinogenic). Many studies link obesity and a high fat diet with tumors. Second, you can try to locate a breeder who has been selectively eliminating the tendency for tumors from their stock. This type of breeding can be difficult, because tumors usually occur in rats over a year old, after they can already have great-great grandchildren. Currently, the only tumor resistant rats I know of are in California. The third way, and probably the most reliable at this time, is to choose male rats over female rats. While males can also get tumors, the benign mammary tumors that are so common in females are relatively rare in males. Hopefully in the future, more rats will be tumor resistant, but until then, you need to be on the lookout for this life-threatening disease in your rats.

ED. NOTE-According to an article in TIME Magazine (4.24.94), scientists have shown that a substance found in crucifers (broccoli, cauliflower, Brussels sprouts and other crunchy vegetables) actively blocks formation of cancerous tumors in rats.