2020 updates by Cathy Bickel
While gerbils are not as prone to tumors as rats, they are not uncommon.
The most common type of external tumor is a scent gland tumor. It begins as a small lump or skin flap on the scent gland but over time and with the gerbil picking at it, will probably grow to a large mass. Growths on the scent gland should be checked by a veterinarian. Small tumors can often be removed, preventing metastasis. Some growths can start off as benign but turn malignant.
A rarer type of external tumor is a melanoma. It appears as a hairless bump, dark brown or black in color. The most common areas for a melanoma to grow are near the base of the tail, in the genital area, or near the ears, but it can occur anywhere. It will grow, and may bleed spontaneously.
Internal tumors are not uncommon in animals older than 12 months. Sometimes, a lump can be felt or seen in the gerbil's abdomen. More often, it is not noticed until the animal's general health has declined. Fatigue, weight loss, loss of fur condition, and partially closed eyes are some general symptoms the gerbil is not feeling well. A look of general emaciation combined with a bulging abdomen is a classic sign of an internal tumor. Animals with internal tumors will lose considerable weight and gradually weaken until they cannot move around or care for themselves. At this point, humane euthanasia should be considered.
Female gerbils, particularly those who have not been bred, can develop fast-growing ovarian cysts. These appear as a bloated, soft abdomen. It is difficult for an owner to differentiate a tumor from a cyst but cysts tend to grow far more rapidly, with a bloated look coming on over a few days. Cysts will sometimes resolve without intervention.