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Hi Debbie.

My rat Possum went into surgery today to remove two suspected mammary tumors. I only discovered them a few weeks ago and they were only pea sized when they were operated on. I have had rats with mammary tumors before and I did think these were quite unusual as they were quite prominent and both right under her left and right nipples under her arms. I did suggest abscesses to the vet when she first checked her out but we both couldn't find any lesions or cuts on the bumps so the vet suggested a tumor removal.

Anyway she came around from the op just fine (thank god) but the vet was stumped by what she found. Possum had two abscesses under the nipples but not attached or connected to the nipple, and buried within the mammary tissue. The tissue around the abscess was also infected. She removed the abscesses and the surrounding infected tissue.

What is odd is we haven't a clue how they came about! The vet said the first idea that came to her head was mastitis, as the liquid inside the abscess looked quite milky but Possum has never mothered a litter and in the 2 years I have had her, she has never been in contact with a male rat.

Do you have any idea what could have been the problem. Obviously we are hoping the problem will have been resolved with the surgery but I am always curious as to the reasons behind my rat's illnesses.
The only two ideas I have are either an internal injury but I can't think of any time in which she could have had such an injury. or a phantom pregnancy, but she has always been a skinny rat and I have never noticed her gaining a lot of weight or her belly swelling which I thought would also occur during a phantom pregnancy.

I'd really appreciate any ideas you could throw my way which I could discuss with my vet. I think it is important to know the reasons behind my ratties illnesses so I will be more able to perhaps prevent such a problem in the future.

Thank you so much!


Hi Sam,
When I remove mammary tumors, it is very common to find lumps filled with a whitish liquid that I have always assumed is milk, although I've never had it tested. It just seemed reasonable to me that a mammary tumor, made of mammary tissue, could make milk. The liquid is sometimes quite thick and ranges from white to yellowish. Often these pockets of milk are within a benign mammary tumor, but they can also be just an isolated pocket off by itself. However, I have never seen one that is infected around it. You might ask the vet for more details about why she thought it was infection. Did it smell bad like a bacterial infection? Was there pus? Could it have just been milk that leaked out of the pocket rather than pus?

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Debbie Ducommun


I can answer any questions about pet rats, but you will probably be able find answers to simple questions more quickly on my website at If you have a life-threatening emergency you can try calling me at 530-899-0605. I am not usually on the computer on the weekend.


I have been "The Rat Lady" since 1985 and am recognized as one of the world's experts on pet rats. I have 3 published books and already answer lots of questions about rats daily.

President of Rat Assistance & Teaching Society

I am a monthly columnist for Pet Business magazine, and my writing has appeared in other magazines. I have 3 published books.

BA in Animal Behavior

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