Why does my pet mouse have a mouth lump and a limping leg? What to do?

Hello there! I'm sorry to hear that Marvin is having some health troubles. The lip and the leg issue are probably unrelated, but depending on the age of your mouse, several things may be going on. If he's a young dude, I strongly suggest checking the length of his teeth, as overgrown teeth can poke into the lip and cause swelling and abscesses. The lump could still be an abscess even if he's older, and especially if he lives with other mice that may have bitten him. It is also possible that the lump is a cancerous tumor. While the location is uncommon, older mice can develop cancers anywhere. The best thing for him is to visit a vet to determine the cause. They will likely palpate the mass, and may take a small sample of cells from within the mass using a very small needle. These cells may then be examined under the microscope, which may help determine what is causing the mass (bacteria, cancer, etc). Treatment will depend on the cause - he may need antibiotics, or the vet may recommend removal of the mass.

Limping in mice is usually caused by injury. Take a look at his foot pad to check for redness or lesions, especially if he's housed on a grate or wire mesh as these can cause pododermatitis (a skin infection of the foot). This should also be evaluated by the vet to determine the cause and to get him some pain medication, if needed.

Good luck, and I sure do hope that Marvin the mouse recovers!