Mouse Losing Weight

Hello! I am not a mouse breeder, but I own 3 mice as pets.
Recently my fuzzy haired dumbo mouse has been losing weight rapidly and now I can see her ribs. I am very scared for her. She and her sister are a year old. Her sister is very healthy and active compared to her, and they live in the same cage and eat the same food. I don't know why she stopped eating. Her feces are also very small and dry compared to what she usually drops about. I feed them Essentials Regal Rat Adult Rat food because other foods have unhealthy seeds like corn. I don't know what I'm supposed to be suspicious of--is my mouse sick? Is the food bad? Is she allergic?
Should I do a trial run and change the food? If so, are there any recommendations?
I will be grateful for any help!
Thanks,
luca

You can supplement her diet with FerretVite, a high calorie vitamin and mineral paste for small pets. Just a dab for a mouse. To them it's really sweet, though it's not full of sugar. If she won't lick it off your finger, smear some on her "fur" and she'll lick it off. You can also make "Ferret O's" by filling the hole of a plain Cheerio with FerretVite and giving it to your mouse.

Make sure she's getting enough water. Maybe a dab of water on her "fur", too. If she licks it right off, she's thirsty.

But mainly, this may be a dental issue. She may be having trouble eating solid foods because of her teeth. Give her some scrambled eggs and see if she eats them. Scrambled eggs would be good for her, anyway. You may want to take her sister out of the cage for a while so she doesn't eat them first. It is also OK to give her some high fat items at this point. She needs to gain weight! High fat and high protein (like eggs [hardboiled or scrambled], cooked chicken, yogurt).

Others may have other suggestions. Good luck! I hope your little girl gains some weight.

I never thought of that! I don't have any eggs on me right now so I boiled some lima beans and mashed it and placed it in her cage. It's during the day so they're all asleep but I'll be vigilant when night falls. I will go buy eggs soon!
Thank you!

Hard poops could also indicate dehydration. Have you checked your water bottle's function?

Yes, the water bottle works. I quarantined my fuzzy haired mouse just a hour ago but she refuses to eat the mashed lima beans or the scrambled eggs. She's so weak now. I tried the smearing technique but she doesn't really care and leaves it. I tried smearing it on her mouth but she won't even care about that either. She won't even try to nip at me so maybe it's a dental problem...
I don't know anymore, this is really depressing me. Nothing seems to be helping and she just keeps on getting weaker everyday. Is. . . putting her down the humane option if this continues? I feel horrible if she is suffering and I can't afford to take her to the vet.

A vet likely couldn't help you at this point. If she's not cleaning herself when stuff gets on her coat, I'd put her down. They don't usually get better after that point. There are a million things that could have gone wrong. Your food is not likely the issue, so the other mouse/mice are probably fine.

If you want to check her teeth, especially since she's so listless, hold her in your hand with her back against your palm. Put your thumb and forefinger on either side of her cheeks and squeeze lightly until you can see her teeth. Missing, one longer/shorter than the other, or both very long could indicate problems.

I just checked her teeth and they seemed perfect. Both are of average length.
I see. . . when I have time in the next few days, I will take her to the vet to get put down. I can't trust myself with any home euthanasias. Thank you all for your help.