Red, hardened nipple on a 8 year old neutered male cat.

Hello, I noticed that my neutered male cat, who is 8-9 years old, has a red, hardened nipple on the upper right side of his chest, as seen in the attached photo. He didn't seem bothered when I checked it. Can anyone please tell me if this could be a serious issue?

Hello - in this case, it is difficult to appreciate over the net that this nipple is red or hardened, but as you have seen a change, it is absolutely worth checking out.   Your vet should examine the nipple, compare it with its friend on the other side of the chest, feel the local lymph nodes and decide from there whether biopsy, treatment or observation is merited.   In general, causes of nipple changes include pregnancy (females) or hormonal changes, inflammation, trauma, cancer, infection and skin disease.  Comodones, ticks, foreign bodies and other keratinised skin changes can sometimes throw in a curveball too.  Please will you let us know how you get on?