Skinny Pig Melanoma

[bats]

Many of you have seen cute pictures of Angel, the rescued skinny pig Jim and I adopted from Chary. I'm sorry to report he has melanoma -- a skin cancer that, after brief search of the VIN and internet, does not seem to have been documented in guinea pigs before.

This reference thread will be used to document his condition and ongoing progress.

This first picture was taken in late November, 2003, when Angel was perhaps 5 months over. Note the single freckle on his left flank, and a cyst on his front shoulder. The rest of his skin is clear -- although he had several spots of ringworm at the time (not shown).



Over the course of the last year, Angel developed 15-20 "freckles" -- spots that could have been anything from simple grizzling (hair faults and associated skin discoloration) to skin cancer.

In the last 2 months, several of those spots became raised and showed signs of darkened centers. We decided to have two of the largest and most threatening spots biopsied.

This second picture was taken 3 weeks ago, just before his surgery:



A larger photo, showing the darkened center of one of the spots more clearly, can be found here.

The lab results came back two days ago -- melanoma in both places that were biopsied. The mitotic activity appears to be low, meaning this may be slow growing. One of the skin samples showed clear at the edges, but the other had signs of cancer even at the margins.

[bats]

Here is the lab report:


Histopathology Report:

SOURCE/HISTORY: 1.5 year old hairless, castrated male guinea pig. Several months’ history of increased size of darkened pigmented areas on hairless guinea pig. Biopsies from A) Right lateral thorax B) Left lateral thorax submitted. All tissue processed.

MICROSCOPIC FINDINGS: SLIDES (A&B) BOTH SPECIMENS: Acanthosis with fairly prominent junctional activity by mildly atypical round oval cells, some with distinct brown granular pigment; suspicious of multicentric melanoma formation.

PROGNOSIS: See comments.

COMMENTS: A similar process was observed in the two specimens. There were round oval cells observed exhibiting fairly prominent junctional activity. In some areas these cells contained no appreciable cytoplasmic pigment; however in other areas they contained a small amount of brown granular cytoplasmic pigment. Similar cells were observed in low numbers in the underlying dermis. The cells in general exhibited mild atypia only and mitotic figures were rarely seen. Overall morphologic characteristics with routine stains are most suggestive of multicentric melanoma formation. In a brief literature search I found no documentation of cutaneous melanomas in pet guinea pigs or in lab animal guinea pigs. Thus, predicting the behavior of this melanocytic process is quite difficult. The fact that it is multicentric is worrisome. Additional cutaneous tumor formation is possible and metastasis must also be considered a possibility. I would appreciate any further feedback/follow-up on this case. The lesion from the right lateral thorax appeared completely excised. Some mildly atypical, poorly pigmented malanocytes were observed extending very close to lateral margins in the specimen from the left side.

[bats]

Here are 3 more shots, 10 days after surgery. The stitches were removed a few hours after these shots were taken.







Larger images of these can be found
here
here
and here.

Size of those photos may vary depending on the size of your browser screen. But if you save the photos to your computer, you can see the photos in full resolution.

Note that there are 4 or 5 other spots still remaining which appear troublesome.

[bats]

Angel is a small pig. Throughout the year, he weighed between 800 and 900 grams... typically 850. Just prior to surgery, his weight was down to 790 grams. Ten days later, he weighed in at 750. But he has a voracious appetite and his energy remains high.

And in case you were wondering, he is an indoor pig -- with no exposure to direct sunlight.

Our approach will be "watchful waiting". We will monitor the growths and look for signs of enlarged lymph nodes. We may excise the worst of the other spots if this doesn't appear to be aggressive.

[Lynx]

The rules for distinguishing a normal mole from an abnormal one for humans are:

Asymmetry. Normal = symmetrical. Abnormal = half the mole does not match the other.

Border. Normal = regular edges. Abnormal = irregular, ragged, notched, or blurred.

Color. Normal = uniform. Abnormal = not uniform, may be differing shades of tan, brown, or black, so metimes with patches of red, white, or blue (remember this is in a human, and it sounds like an American human).

Diameter. Normal = about size of pencil eraser. Abnormal = larger in size, perhaps increasing in size.

Do any of these seem to apply to Angel?

[Josephine]

I'm also wondering if animal-safe sunscreen would be of benefit to all Skinnies or Baldwins, even though direct sunlight is limited.

[bats]

Here are some closeups of the moles.



The one on the lower right was one of those removed. I don't have a good picture of the other one that was biopsied -- however, it was shaped like the Greek capital letter Phi.

Both of those that were removed were nearly the diameter of a pencil eraser. The others are half that size.

[bats]

Pigpal, the pentultimate detective, was successful in finding the following on induced melanoma in guinea pigs.

Needless to say, I find the following line troubling: "Metastases to the skin and internal organs were multiple and eventually fatal for the animals."

By the way -- I forgot to mention the excellent support given by Dr. Lynn of the All Pets Hospital in San Francisco... and the work of pathologist Chris Schiller of Antech Diagnostics. Both are interested in Angel's welfare and in following up on whatever develops from here.

[Charybdis]

Bats, you did a wonderful job documenting -- I am just sad that you had to do it.

Angel may be the oldest of our Skinnies. He was certainly the biggest, but still I don't think he was an adult when he arrived here. He is probably between 1 1/2 and 2 years old. All the Skinnies from our group are over 1 year old and most, including the black ones, have similar spots -- although not nearly as many as Angel.

The person who was breeding these Skinnies once told another breeder that his sows lived only 2 years, but that might have been because he was backbreeding them. My sources say that their Skinnies live 3 to 4 years on average. I sent some queries around to Skinny fanciers and none had pigs with melanoma.

Have you asked your vet whether or not the severe ringworm could be a factor in the melanoma, other than the fact that it indicates immunodeficiency?

[bats]

Pinta said Bliss, one of her skinnies from the same group of rescued pigs, also has spots. Bliss is a himi-skinny who strongly resembles Angel.

I didn't ask about the correlation between ringworm -- or other skin trauma such as nips and scrapes -- and melanoma.

[Nurgle]

My guess would be that the pigs that should have had white hair would be the most susceptible, in the same way that humans with the lightest skin would be the most susceptible to skin cancer.

[pinta]

Bliss has got spots cropping up all over. I took her to the vet probably a month before Angel went in and she was given the all clear. Nothing looked suspicious. The vet thought they were age related freckles and said they were very common on other animals. I have taken detailed photos of all 3 skinnies for monitoring.

Heart has had no changes at all. Gretchen also no changes. Bliss is very Himi in looks.