Now as I am sure you can appreciate when we see any lump on our older cats, we do have to consider a range of issues. Still if this is a mass is on the skin level and he hasn't had any recent injections, then we can at least put vaccine reactions and nasty injection site sarcomas to the bottom of our list. That said, if it has been present for a week already and isn't reducing, then we'd also have to drop allergic reactions from stings (even though that could explain the itch) lower on our list as well. Therefore, our main concerns for what you have reported would be soft tissue swelling, abscesses, cysts, and skin based benign or cancerous growths. And I do have to note that if this is growing and stretching the skin that can prompt them to scratch.
Now with these in mind, we can start some supportive care to try and rule out some of these. To start, if there is any chance or a bee/spider/wasp sting or bite, then we can reduce allergic type swelling using antihistamines. Commonly we will use Benadryl (Diphenhydramine; More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/diphenhydramine-benadryl). A low dose (ie. 0.25mg per pound of body weight twice daily) is often enough to reduce these signs over a few days. We do usually like to keep the dose low, as they can have drowsiness with this medication (just like people). As well, of course, this medication shouldn't be used if your wee one has any pre-existing conditions or is on any other medication without speaking to your vet first.
Furthermore, to reduce swelling with any of these sudden appearing concerns, you can also start warm compressing this lump. This can reduce inflammation as well as encourage soft tissue inflammation and allergic reactions to settle. Just to note, you can make a safe warmer for use as a warm compress by filling a clean sock 2/3rds full with uncooked white rice. Tie it closed and microwave (approx 1-1.5 min). Before use, do make sure to shake to allow the heat to distribute before using as a compress. (If it cools, you can re-warm as required).
Now if you use the above, but the mass doesn't settle with antihistamine treatment then it does rule allergic reactions. And in that case leaves us with the others to consider for Gizmo. In that case, the best way to approach an abnormal mass like this is to have your vet evaluate the mass via fine needle aspiration (FNA). This is where the vet uses a needle to harvest cells from the mass. If the remove pus, then this tells us that there is infection present and antibiotics can be dispensed. If clear or blood stained fluid is removed, then a cyst will be suspect and can be drained or removed if need be. Or if there is creamy discharge removed, then sebaceous cysts would be suspect and is also cosmetic for them. Otherwise, if the above are not found, then the cells they harvest can be stained and the identity of the nature of the mass can be determined and whether it is something that is concerning or needs more serious treatment.
Overall, to see a lump here at Gizmo's age means we do need to tread with care. Still, we can use the above to rule out some of those aforementioned concerns. Otherwise, if this doesn’t settle; then we’d want to have your vet sample this mass to identify its cause so that appropriate treatment can be initiated to address it for your wee one.
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