Hello and thanks for researching this very important question...I am so sorry that Basil is showing these signs of illness!
Unfortunately, mice sold for "feeders" are often sickly and poor-doing since they are not meant to live a long time, genetically speaking. That is, their breeding lines are not screened against developmental problems and weak immunity.
Mice live about 20-24 months, on average...this means that many animals will fall to the "longer" side of the bell curve, and many to the "shorter" side. Feeders may be any age...even old "breeders" that have been culled from colonies.
Your tank type and bedding are Right On! I like to give mice strips of plain white paper towel or tissue paper so that they can weave warm nests for themselves, as they would do in nature.
Mice prefer warm "closer" environments...this is what attracts them to the walls of human structures. I'd rather see her kept around 75F degrees day and night. You may need to increase your heat tolerance, or carefully use a heating pad under or around her tank during cold months. Cover the tank with a thin towel overnight to prevent drafts.
If not kept warm enough, the immune system can weaken and make a mouse susceptible to many bacterial and fungal agents that they ordinarily would quickly "brush off".
Mice can fall victim to Scurvy (vitamin C deficiency) or Rickets (calcium deficiency). Both of these problems can cause the body to shrink up and look kind of "cramped". Offer her a tiny nip of orange and a bit of crushed egg shell (save them from boiled eggs and dry them for storage). Since it is hard to know how long a particular packaged mix has been in storage (and what nutrients may have decayed)...I like to make my own rodent diet.
My mix contains:
4 cups of a barley-wheat based granola cereal (Grape-Nuts or generic equivalent).
4 cups of an oat-based granola cereal that is at least 15% VITAMIN C on the label (it will need to contain whole fruit like raspberries in order to gain this level of vitamin C).
4 cups of song-bird seed mix (more small seeds than sunflower)
2 cups dry lentils
shells from 6 standard chicken eggs (save shells from boiled eggs...dry and crush to a powder)
3 times weekly give a leaf of Curly Endive (or similar vegetable with high VITAMIN K content)More dietary tips here
Although I cannot see her and cannot promise that things can turn around at this point, it is certainly worth a try to get some of these changes in place and see how Basil responds! She may need some veterinary support if she is sneezing or has any red discharge around her eyes or nostrils (indicating active infection).
If you need additional support at this time, please "REPLY