This veterinary journal (not a research animal study) is the record of the occurance of a specific form of Lymphoma in an elderly pet Eastern Gray Squirrel. Since most with squirrels in non legal States may not take a pet squirrel to their vet, this journal is of special importance as it provides information on the occurance of a specific form of lymphoma in a tree squirrel.

After the removal of the lesion, that the squirrel didn't have a reoccurance within the time recorded, is notable.

https://www.jstor.org/stable/20460390
In rats lymphomas are mostly fatal.
Other forms of lymphoma occur in rodents, both in rats and tree squirrels; in rats all forms are commonly fatal. In this tree squirrel after the surgical removal of the lesion it apparently didn't return. This isn't to sa that it couldn't, for as the journal specifically notes it was expected to return.

This is just one form. for photos (Fig. 1) see Ratguide.com for this form of lymphoma. It doesn't start out looking crusty, but ends up this way when the lesion breaks open. Other forms of lymphoma in rodents manifest in lumps just under the skin. In most all cases lymphoma is a disease common to rodents of advanced age.

https://ratguide.com/health/neoplasia/lymphomas.php

What is noted in research is advanced age in which the immune system lowers. Exposure to bacteria, viruses, herbicides and pesticides are amongst the other promoters of this disease in aged rodents.

Should you have had a squirrel in the past or present that developed one of these three forms of lymphoma;I I would welcome you to post your experience on this thread, along with any photos of your or another's squirrel while it was alive. Pleases also include a warning to other members that viewing the photos may be upsetting.

Thank you, DF