(Mastocytoma)

Definition

A local aggregation of mast cells in the tissues.

Clinical Signs

May observe any of the following:

• One or more raised nodules
• Be well circumscribed
• Nodule(s) may be soft or firm (may resemble a lipoma, or a cyst)
• Nodule may be ulcerated
• Center of nodule may appear red, or nodule may or may not be alopecic (without hair covering it)
• If more than one tumor present may see: loss of appetite, lethargy, enlarged lymph nodes, delayed wound healing, bleed easily if injured, have unusual bleeding

*Note: for additional information on recognizing various signs of pain or discomfort refer to: Signs of Pain In Rats.

Etiology

Mast cell tumors, also known as a mastocytoma, are soft tissue sarcomas that can arise from an aggregation of mast cells found in the mesenchyme, including the dermis and subcutaneous connective tissue.

Mast cells play an important role in the body’s immune surveillance system. When confronted with allergens they release granules containing histamine, heparin, and in the rat and mouse, serotonin. These chemicals respond by setting up an allergic response attracting additional immune surveillance cells to the site. This leads to the inflammation and swelling that is observed with allergies. Sometimes for unknown reasons mast cells begin to grow out of control. The result is a mast cell tumor, or if mast cell proliferation is systemic is known as mastocytosis. Mast cell tumor growth may arise as a single nodule in the skin, or there may be more growths in other areas or organs of the body.

Mast cell tumors are sometimes called the great pretender since its appearance can mimic that of a lipoma, cyst, or a histiocytoma. Mast cell tumors may progress from benign tumors to tumors that are metastatic. Since it is often impossible to determine just by appearance if the tumor is benign, the recommended treatment is to have the tumor surgically removed. The prognosis is good in early detection and tumor removal.

Some factors thought to be involved in mast cell tumor formation include heredity, viral infections, chronic inflammation, or environmental factors. As with many cancers, mast cell tumors tend to be associated with older animals.

Figure

Case History of Mast Cell Tumor

• Fig. 1: Mast cell tumor in 1-year-old male rat (Griffen).

Diagnostics

Since it is difficult to diagnose a mast cell tumor by visual inspection it is advised to have all suspicious growths examined by a veterinarian.

X-ray or ultrasonography (if available) to determine hepatomegaly (liver enlargement), splenomegaly (enlargement of spleen), or lymph node involvement

Fine needle aspirate (FNA) for cytology.

Lab test: CBC (complete blood count) to check for low platelet count, low red blood cell count, low white blood cell count, and elevated mast cell count to rule out the spread of the tumor. WBC (white blood cell count) may be elevated if there are circulating inflammatory compounds, or if gastrointestinal ulceration is present.

Histopathologic examination of excised tumor.

Treatment

Excision of the growth with margin of clean tissue is recommended along with the sending of the tissue for histology.

An antihistamine such as Benadryl may be used prior to surgery, or in the event surgery is not an option, to reduce the effects of histamine release and give comfort.

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.

• *Note: for pain not controlled by the use of an NSAID (e.g., Banamine, meloxicam, or carprofen), alone, consider alternating or co-administering with a narcotic (e.g., buprenorphine or butorphanol) or narcotic-like (e.g., tramadol) medication.

A corticosteroid (e.g., prednisone/prednisolone) may be used to reduce tumor size, or associated inflammation, if surgery is not an option. However, it is important to keep in mind that the use of a corticosteroid will not eliminate the tumor itself.

If tumor(s) are metastatic, and stomach ulceration is thought to be a concern, H2 antihistamines such as cimetidine or ranitidine may be recommended.

For information regarding medications refer to the Rat Medication Guide.

Nursing Care

• Provide hospital cage during recovery, or 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 products 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.
• Medicate for post-op pain as needed.
• Body wrap may be required if thread sutures are used.
• In the event of dried or excess drainage, the incision site may be cleaned with a moistened Q-tip (swab), using warm water or normal saline.
• Assess nutritional status:
  • For 24 to 48 hrs post-op, feed iron-rich foods to prevent anemia (cooked liver, scrambled or hard-boiled eggs).
  • Provide additional nutritional supplement, such as soy baby formula, Ensure, Boost, NutriCal paste (for dogs and cats, found in pet stores), mashed avocado, and baby foods during recuperation. Include additional multi-vitamin supplement (can be found in pet stores) if food intake is poor.
  • 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.
• Contact veterinarian if any of the following are observed: swelling, redness, or pain at the incision site, or if there are signs of increased weight loss, lethargy, or changes in habits.

Outcome

• Incision site free from infection
• Increased comfort and mobility
• Increased quality of life

Prevention

• Although mast cell tumors may not be able to be prevented, doing a regular Health Check on your rat and treating illnesses as they arise can help. For a reference on how to perform a health check, refer to Basic Health Check or Advanced Health Check in the Rat Guide.

References

1. Fraser, C. (1991). The Merck Veterinary Manual: A Handbook of Diagnosis, Therapy, and Disease Prevention and Control for the Veterinarian (Merck Veterinary Manual), 7th Edition. Rahway, NJ: Merck Publishing Group.