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Mice/my mouse has a lump on her side... What should we do?


QUESTION: About a year age, we got two mice for my brother after his fish died. The next day, we got another mouse from the same litter. All the mice seemed to like each other. But then, two days later, we got two more. That totaled at five. One of the mice from the first little hated the fact that we mixed them. We hoped things would get better, and that they would fighting, but they didn't. About a week after we got the last two, my mom had taken one out, and she jumped out of my mom's hands. The mouse was missing for two weeks, and occasionally we saw her run by. We finally purchase a have-a-heart trap. A week after we got it, we caught her. After we put her back with the other mice, she started to cause some issues, the other mice started to get bloody eyes. At this point, she was a lot smaller then the others And this mouse, right near her tail, she stated getting scars, and lost fur.Then, about two weeks ago, we noticed a huge lump on her side. It had just appeared overnight. We are all really worried, and my brother doesn't know yet.

ANSWER: Hi Susan,

Do you have a second cage, or can you get one?  This mouse needs to be separated from the others immediately.  Everything you have described sounds like injuries from fighting, and mice can and will kill each other once real aggression starts.  Normal scuffling for a day or two after introducing (or reintroducing) is normal, but any time there is blood drawn, biting, scratching, or any kind of injury, they must be separated.

Sometimes a scratch or bite can become infected and form a large pocket of bacteria and pus beneath the skin called an abscess.  If I had to guess from what you've told me, that is what I would think her lump might be.  The good news is that abscesses can and should be treated.  Please find a vet in your area that works with pocket pets like mice and bring her in by herself - while you can provide some antibiotics at home, the only real way to treat an abscess is by having a professional drain and clean it.  That is not something one can do at home, and the danger of not treating it is real - abscesses can burst inward and spread that infection throughout her body, which would obviously be very bad and life threatening.  The other good news is that after one visit to the vet and a round of antibiotics, as well as keeping her in her own cage from now on, she should be good as new and start feeling much happier!  I'm betting you see a whole new mouse in her when she is removed from that stress and pain.

Please let me know how she does, and best of luck!

---------- FOLLOW-UP ----------

QUESTION: OK. Thank you. Should we put her with another mouse? We don't want her to get lonely. Also, we discovered another mouse has passed away ON Sunday, after I messaged you about this. A mouse in the cage named Sandy is seemingly very depressed. What should I do?


If she has not been getting along with the others, there is no reason to try again, especially not while she is in less than healthy shape.  While most female mice are social, every mouse is different, and some do truly want to be alone.  If she seems like she is getting bored you can try to give her some more one-on-one time with you, cycle out her toys to keep her busy, or even have supervised playtime with the other mice a few times a week in an escape-proof, safe area.  I would not recommend getting any new mice for her, and I would also not recommend trying to put her back in the group.  Once violence like you've described has occurred it would not be worth the risk to their health.

I'm sorry to hear about your loss, and Sandy is probably grieving, which is healthy and normal.  It's important to check her over (and bring her with you if you already have a vet appointment for the separated mouse) to be sure she is not actually suffering an injury from the previous fighting, just in case.  If it is her coping with the loss of her buddy, you can help her by giving her extra love and attention as well as giving her a bit of time.  The others will help her deal with her grief, too.  Introducing a new toy can give her something new to focus on, and can sometimes help as well.

Best of luck,


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I can answer questions regarding mice as pets, mouse behavior, color and coat genetics, breeding techniques, and general health questions. I can help with caging and setup, nutrition, social issues, and what to do in most mouse emergencies (such as unplanned litters, injuries, fighting, etc.). I can also assist with questions pertaining to orphaned mouse pups, weaning litters, and questions of mating and birthing. I cannot answer questions about exotic or wild varieties of mice such as spiny or pygmy mice. *****FOR EMERGENCIES, anything requiring immediate medical intervention, PLEASE take your mouse to a professional veterinarian or wildlife rehabilitator who works with mice as soon as possible! IMPORTANT RESOURCES: Raising Orphaned Mice: Orphaned Mice Videos: Natasha's Your First Mouse: General Mouse Help: Mouse Info and Exotic Breeds:


I have enjoyed the companionship of mice nonstop since 2004, and spent a year caring for them in a lab where I learned a great deal about their breeding, social needs, and health. I spent a few years breeding them, specifically working with albinos, marked mice, angora mice, and satins. My education never stopped - I am learning something new every day from current and well-established research thanks to the wonderful folks at the Jackson Laboratory, as well as from my wonderful mousey friends online. I also love learning from my terrific questioners here on AllExperts - you folks keep my passion for these amazing animals alive and well!

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