A sac having a definite wall, distinct from surrounding structures, which contains fluid, solid material, or mixture of both.

Clinical Signs

May observe the following:

  • Firm movable, round, globular mass that may be less than 3 cm.
  • Enlarged abdomen in a female rat if there are ovarian or uterine cyst(s).


Cysts can occur in any body organ, and are formed in different ways. They can occur as a result of genetic predisposition, infection and inflammation, or trauma. The contents of a cyst may contain fluid, or a solid material which can sometimes have a yellowish-white color, or have a cheesy type appearance that may also include a rancid odor. Cysts can vary in size from that of a small pea to several centimeters, and depending on type and location may be benign posing little or no discomfort to the rat.

Cysts can be classified into types based on histopathology.

  • Cutaneous cysts which include epidermal inclusion c., epidermoid c., and the epidermal cyst, are usually found on the face, neck, or back. Those that are traumatic inclusion cysts are more likely to expand in area.
    These type of cysts contain keratinized material and are lined by keratinizing squamous epithelium.
  • Extravasation cysts develop from blood found in tissues that are surrounded by a definite wall.
  • Retention cysts are tumor-like, and are formed when the outlet to a gland becomes blocked, and the gland continues to secrete. They may be found in mammary tissue, pancreas, kidney, salivary, mucous membranes and sebaceous glands.
  • Exudation cysts are formed through slow seepage of exudate material into a closed cavity.
  • Parasitic cysts are those that form around larval stage parasites that enter the body. Usually seen in the liver, these cysts are referred to as hydatid cysts.

Cysts can frequently reoccur even with aspiration of contents or lancing unless the entire sac is removed, and may on occasion change to a malignant growth.

Ovarian/uterine/endometrial cysts, can vary in size and types. They may be retention cysts containing clear fluid, or dermoid cysts that can grow slowly and are filled with thick yellow sebaceous fluid along with partially developed hair , teeth and bone tissue. They may also be a mucinous cystadenoma (which in actuality is a tumor) that can grow to a large size putting pressure in the abdominal-pelvic cavity and distending the abdomen, and at times becoming malignant.

The most common type of cyst, found in male rats, are retention cysts often occurring on the back or side where oily sebaceous glands are most active.


Case Histories of Cysts (*note: graphic photos)

  • Fig. 1: Uterine Horn Cysts. *Warning: some of these photos are taken postmortem, and are quite graphic.*
  • Fig. 2: Sebaceous cyst in 13-month-old male rat (Schnapps)
  • Fig. 3: Massive follicular/retention ovarian cyst and mammary tumors in 2.6-year-old female rat (Charlotte)


Palpation of a characteristic lesion.

Excision/removal with histological examination.

Ultrasound (if available) for suspected ovarian/uterine horn or endometrial cysts.

Culture and Sensitivity, if infection is present, will help in choosing appropriate antibiotic.


The treatment of cysts can depend upon location, type, size and recurrence. Benign epidermal or sebaceous (retention) cysts may remain small, being asymptomatic and posing no problem for the rat; thus requiring no treatment other than observing for any unusual changes such as increase in size, appearance, or pain.

Where surgical intervention is required for cyst removal effort should be made to completely remove the sac along with its contents to prevent recurrence.

In the event infection is present the site may be left open to drain, and antibiotics initiated such as:

The use of a corticosteroid may be required to reduce swelling of tissue if not able to remove cyst, such as:

  • Prednisone or Dexamethasone are examples of corticosteroids that may be prescribed.
  • Meloxicam is a non steroidal antiinflammatory that may be used as an alternative to corticosteroids.

In female rats, where there are ovarian and / or uterine cysts, a spay is the most likely course of treatment.

Rats do experience pain with surgical procedures. The type of pain medication used post-op should be determined based on extent of procedure and the anticipated severity of pain.

  • For severe pain or first 24 hours post-op: Buprenex (buprenorphine), or Torbugesic (butorphanol).

  • For mild to moderate pain : Banamine (flunixin meglumine), Metacam (meloxicam), or carprofen. Do not use if a corticosteroid has already been prescribed.
  • For mild pain: Tylenol (acetaminophen) if no contraindications.

For information regarding medications refer to the Rat Medication Guide.

Nursing Care

  • Continue treatment or follow-up care until absorption of cyst or wound incision heals.
  • Provide hospital cage during recovery if there are concerns that their cage mates may groom sutures or wound site.
  • Provide clean bedding daily such as felt, soft t-shirt type material or ink-free paper towels. Avoid using material such as terry cloth type towels that can ravel. Also avoid litter-type bedding, post-op, until healed to prevent the chance of wound contamination or infection.
  • Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
    • You can use an isothermic product that is heated in the microwave such as SnuggleSafe®. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe® will provide heat for 12 hours before needing to be reheated. Other similar types of product may vary in re-heat time. Check directions for individual product.
    • If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
    • If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
  • Provide pain medication as needed post-op. Check with veterinarian for most appropriate pain medication to give.
  • Provide high calorie foods during recuperation.
  • Encourage fluid intake while recuperating, such as water, Jell-O water , or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
    • Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
    • *Note: a juicy type of fruit also provides an additional fluid source in the diet.
  • Observe and contact veterinarian if there are: any further signs of infection, if the wound does not appear to be healing, or if the wound heals but elevation or lump returns.
  • Clip nails if scratching to prevent infection.


  • An excised cyst, where sac removed, does not return.
  • Pain is relieved.
  • Those cysts that are small and are not excised should show no redness, increased swelling, or pus.
  • The infection is resolved.


  • Practice good sanitation of the cage environment.
  • Cover wire floors, or rough surfaces, with a sturdy covering to prevent abrasions. Examples of coverings can include vinyl floor covering, plastic needlepoint canvas, carpeting, towels, self stick tiles, plastic place-mats, fiberboard, and Plexiglas.
  • Treat all wounds promptly and seek attention at the first sign of infection.



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