Tumour

Unless you are 100% certain that the mouse has a tumour, ask your vet to check. Sometimes abscesses are mistaken for tumours, and prompt treatment could be important. Mice are prone to mammary tumours, which occur most commonly in the armpit, along the sides and on the abdomen, and sometimes on the back of the neck.

Mammary tissue can be found in places you might not expect! Tumours start as a small lump and grow into larger lumps, some lumpy and some smooth. Generally the tumours that feel like smooth balls of fat under the surface, and which are not tightly attached to the underlying tissue, are the least serious and are easiest to remove.

True tumours will not go away on their own, and there is at present no real 'cure' except surgery, although you can do other things to improve the quality of life - see steroid treatment below. Your vet may be able to operate to remove a tumour, but bear in mind that a secondary tumour often grows within a few months of surgery.

If you decide not to operate, mice can live very happily even with large tumours for some months. If the tumour starts to ulcerate it will become sore and raw on its surface; at this stage, either removal or euthanization is necessary. Steroid treatment can make the mouse's last few months more comfortable.