Melanoma

Definition

Melanoma is a cancer that originates from melanocytes (pigment producing cells).

Clinical Signs (Skin Melanoma)

May observe any of the following:

Light mass (amelanotic melanoma)
Dark mass (may resemble a mole or hematoma)
Nodular mass
Flat discolored area (superficial spreading melanoma)
Bleeding
Ulceration
Itching (pruritus)

Clinical Signs (Intraocular Melanoma)

Bulging of eye from the orbit (exophthalmia)
Opacity
Swelling

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

Etiology

Melanocytes are the cells that produce melanin (the pigment found in the hair, skin, and eyes) which are responsible for color. Melanoma occurs when these cells become neoplastic. In rats melanomas have been found on the pinna (ear), scrotum, tail, eyelids, anal area, and in the eye (intraocular).

Melanomas can be either melanotic (containing melanin pigment) or amelanotic (without melanin pigment). Melanomas can differ in shape. Some are nodular masses that progress downward (nodular melanoma) while others tend to spread out (superficial spreading melanoma).

Nodular melanomas can be misdiagnosed initially as small hematomas (blood blister). However, a dark nodule that does not resolve and increases in size most likely is not a hematoma and should be looked at by a veterinarian.

Natural melanoma is not a common occurrence in the rat and most of the information found comes from spontaneous melanomas that occur during carcinogenicity studies.

During one extensive study the issue of metastases was addressed:
Only the tumors arising in the pinna metastasized to the lung and/or mandibular lymph nodes. The metastatic rate was 19% (16/84) of the tumors and was clearly increased with an increase in tumor size. (Yoshitomi et al 1995)

Causes of melanoma are:

Exposure to UV light
Genetic predisposition
Exposure to carcinogens

Prognosis for rats having melanoma is dependent upon the location of the growth and severity.

Figures

Fig. 1: Melanoma of the ear (Apollo)

Diagnostics

FNA (Fine needle aspiration): A small sample of cells can be removed from the tumor and examined microscopically for melanocytes and melanin.

Histopathology: Tissue (either from the removed mass or from a biopsy) can be put in formalin and sent to a diagnostic laboratory. A cytologic examination of prepared tissue can sometimes be difficult for a pathologist to interpret, particularly if the melanoma is amelanotic. In such cases a special immunohistochemical test can be performed using the S-100 protein marker to support the diagnosis of a melanoma.

Treatment

Surgical removal of the tumor and surrounding marginal tissue is the best option for a melanoma. Laser surgery has been used successfully to remove a hematoma and marginal tissue from an ear.

Detection and removal of affected lymph nodes is most likely not feasible in rats.
Intraocular melanoma in a rat would most likely be asymptomatic or misdiagnosed. Those with severe clinical signs (unresponsive to treatment) involving the eye may benefit from an enucleation (eye removal).

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.
If surgery is required, the following is recommended for pain control post-op:

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 been prescribed.
For mild pain: Tylenol (acetaminophen).

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 surgical 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.
Provide proper nutrition
Keep the rat well hydrated
Contact the 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

Post-op pain relieved
Incision site free from infection
Recurrences of melanoma detected early and treated

Prevention

There is no known prevention for melanoma in rats. Routine health checks will aid in early detection of reoccurring melanomas. Detection and treatment when the growth is small decreases surgical time, enhances recovery period, lessens the chance of metastasis, and improves outcome.

References

Pathology and Incidence of Amelanotic Melanomas of the Skin in F-344/N Rats
Katsuhiko Yoshitomi , Michael R. Elwell , Gary A. Boorman