Growth on side

Question
My sister's female fancy mouse Honey has a growth on her left side. We just noticed it today. When I touched it she didn't react so I assume it wasn't hurting her. She is a year old. Maybe slightly older. My assumption was that it could be due to not being fixed as I've read with rats. I wondered if you had any other ideas of what it could be.

Answer
Hi Kayleigh,

Growths like this, while odd-looking, don't generally bother mice, as you noticed.  It's almost impossible to spay a mouse and I have never found a vet who would do it (their body temperature drops too quickly for anesthesia), so I wouldn't blame that at all.

That being said, some mice are indeed prone to mammary tumors, but these growths are isolated and don't spread like a cancer would.  The only reason a tumor would worry me was if it was growing very quickly and dragging, which might become uncomfortable for her, but this is certainly not always the case.  Some lumps grow, some lumps just hang out for a bit.  Either way, a tumor, while scary sounding, isn't actually a big deal at all to little Honey.

The other possibility is something called an abscess, which looks much like a tumor, but is an isolated infection resulting from a scratch or other kind of irritation that has become infected and grown beneath the skin.  An abscess will eventually go away, but is a risk since it can burst inward just as easily as outward and cause the mouse to become septic.  I have seen these in the same location as mammary tumors (which typically occur in the undearm area or toward the back legs), so location isn't a good way to tell which it is.  A vet who works with pocket pets may be able to tell just by feeling it if it is a tumor or an abscess, and if it is the latter, can lance it for you safely.  Lancing at home is NOT safe or recommended, very especially if you are unsure what the lump consists of.

Not being able to see it in person, I'd like to also review the other possibilities, in case I'm misunderstanding what you mean by a growth.  Other things a bump, swelling, or irritation could be include:  a bug bite (it happens!), a pimple (weirdly enough, it also happens!), a swelling from a recent scratch or ouchie, or even an allergy.  Again, a vet can tell you very quickly and usually without lab costs, so if you're not sure what it could be, that's your best bet for diagnosing it.

Let me know if there's anything I can clarify, or anything else you have questions about!
-Tam