Lumps: Abscess or Tumor?
Information by Melissa Pandajis

Most lumps are either abscesses or tumors. If you discover a lump on your rat, you need to determine whether it is an abscess or a tumor before you can treat appropriately. If you are unable to determine the cause of a lump, you should get your rat to a veterinarian ASAP.  Here are some things to consider:
Where is the lump located:
Most mammary tumors are located in the leg, genital area (inguinal mammary tissue) or in the chest, "armpit" area( pectoral mammary tissue).  However, any tumor on the underside of a female rat should be suspect as a mammary tumor as they have very diffuse mammary tissue.

A lump in the groin area of a male can be testicular, prostate, or an abscess in a recently neutered male. Males CAN get mammary tumors, as they have pectoral mammary tissue.
A rat with a lump in the jaw, eye, or ear area, regardless of cause, should be taken to a vet immediately for diagnosis and treatment.

If the lump is not in an area that contains mammary tissue, an abscess should be considered.
Age of rat:
Mammary tumors can occur in young rats, but most commonly occur after 18 months of age. Most cancerous tumors are found in rats over one year. In young rats, rule out abscess first.
Physical Properties:
Size: Pea-sized, walnut-sized? The earlier a tumor is identified, the more options are available for treatment. Large tumors begin to cut off the blood supply to surrounding tissue. Lumps of any size can be considered an abscess.

Is there an entry wound: Is there a scab or a "head" to the lump? If so, it is most likely an abscess.
Soft or hard: If the lump is soft, it can indicate an abscess or a benign tumor. If it is hard, it can indicate a more serious tumor or an abscess filled with a waxy pus, indicating that it has been there for some time.
Solid or fluid: If it feels like it may contain fluid, it is more likely an abscess.
Consistency: If it feels uniform, it is probably an abscess or a benign tumor. If it feels like there are small beads in it, it can indicate a malignant tumor. These nodular tumors tend to be more invasive.  They are also harder to remove surgically, as they are not well encapsulated.
Movement: Tumors that move about freely, almost as though you could pop them out, are usually well-encapsulated and easily removed by an experienced rat surgeon. Deeply connected tumors are usually invasive and indicative of a more serious type of tumor. Abscesses can vary depending on the properties of the pus—from liquid to waxy. Your vet can do a fine needle aspiration to determine if there is pus in the lump, which would indicate an abscess. 
Smell: Abscesses can have a foul odor associated with the bacterial infection. Tumors will have no odor. A black line indicates necrosis, or dying, of tissue. This can occur with either an abscess or a tumor, and requires that a vet debride the tissue, actually trimming off the dead tissue down to healthy tissue so the necrosis will not spread.

Treatment of abscesses
Treatment of tumors

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