pet fancy mouse sick?

Question
Hello,
I have x2 female fancy mice for 3 years and they have always lived together and sleep together and groom each other. Recently, however, Spring Water (the albino mouse) looked ill-her fur was dull, looked like small patched were missing from her face/neck. She became so thin and wasn't moving much. I was going to bring her to the vet for pain management as I also observed a large bulge(tumor?) From her left side. I saw that Penelope (her sister) was still caring for her + sleeping with her as usual. Suddenly, Spring Water has become more active,looks like she's gaining her weight back as well as eating, cleaning herself and even running on the wheel! I still see the lump, but I also noticed today that the pointed end of her tail is gone! I don't know if she did it or Penelope? Penelope doen't appear to be aggressive twds her... I want just the best for both of them and I know that I have been lucky that they both have lived over 3 yrs. Also: Penelope is just fine; her coat is shiny, she has all of her fur. Spring Water just seems a bit weak still, but is coming around. I also noticed a patch of fur missing from her left rear area. She doesn't make any noises; she  doesn't appear to be in pain, but only getting better. What do you think this is? What does it sound like? Also, why is the small pointy part of her tail gone? Should I separate them opr did she do it to herself? I just don't want her to hurt and wonder if she needs treatment  if she's getting better?

Answer
Hi Erin,

First of all, congratulations on having a pair of such old mice!  You must be doing a lot of things right!  :)

Unfortunately, there are so many possibilities about what could be causing her loss of weight and recovery, the bump, the fur loss, and even the tail biting that figuring out which it is from here is pretty tough for me to do.  I would still recommend a vet visit if you can, because a professional will be able to tell what's going on and what you can do to aid her recovery and any continuing pain or ailments.

In the meantime, here's what I'm thinking:

Don't split them up - Penelope could be responsible for both the missing tail tip and the missing fur, but it's unlikely that these behaviors would pop up suddenly for her after three years, and female mice need companionship.  Splitting them might solve a couple of your problems but there would be no way to be sure, and you could do serious damage to their health and mentality by splitting them up at this point.  I'd only recommend pulling one of them out of the cage if you witness serious aggression or see proof of any - namely, scabs, bite marks, small places of missing fur that show a bite mark when blown aside, or blood anywhere in the cage.  Also if you see either of them boxing or tail rattling (boxing is standing on the back legs and punching at a chasing mouse, and tail rattling is a quick vibrating of the tail against toys or the wall of the cage) - this means one of them feels very seriously threatened and could become aggressive.  I don't expect this at all, but should you ever see it, you'll know it's cause for separation.

Is the skin with the missing fur irritated and/or itchy?  If it is, it could be a result of mites, an allergy, or neurological self-mutilation (we'll get to that in a second - I promise it's not as scary as it sounds).  An allergy can be acquired, so look to her bedding and diet for new changes or causes.  Mites would need a treatment, but is unlikely if the other mice is showing no problems and is still grooming her normally (write me back for mite treatments if you suspect this is the cause).  If the patch on the left side of the body is near or over the suspected tumor, it could simply be rubbing the fur away as she moves around normally, and will need softer bedding and to be watched for irritation.  Finally, if there is no irritation at all, Penelope could be overgrooming her or barbering, which is just trimming away the hair in a pattern.  Since this is new and barbering is usually a lifelong habit, I don't think this is your culprit, either.

The missing tail tip could be either mouse's doing, but if there isn't continuing damage that would need cleaned and prevented, it isn't something you can fix now.  If she did it to herself it could be what I mentioned before - self-mutilation.  Self-mutilation is pretty much like a nervous habit people get - chewing on nails too far or nervously pulling out your hair - unless you notice something seriously or dangerously wrong (open wounds MUST be cleaned and treated), there isn't much you can do except keep them comfy and happy.  If you want to be positive it isn't Penelope but not stress either of them out, you can try moving them to separate cages but keeping the cages side-by-side.  They will still be able to hang out together, but you will be able to quickly determine the cause of a couple of the symptoms.

Hope I helped, and let me know if you have any  more questions!
-Tam