Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Now as I am sure you can appreciate when we see any lump on our older dogs, we do have to consider a range of issues. Still, if you have only just found this, its soft, and non-painful; then we'd be most suspicious of a possible trauma induced hematomas (blood blister like lesions), soft tissue swelling, a cyst, an abscess, an insect sting induced allergic reaction, or a fatty lump called a lipoma.
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 of 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). A low dose (ie. 0.5mg 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 in dogs, 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 allergic reactions or those traumatic differentials, you can also start warm compressing this lump. This can reduce inflammation and encourage hematomas 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, then we'd have those other concerns here. At that stage, it'd be ideal to have his vet evaluate the mass via fine needle aspiration (FNA). This is where the vet uses a needle to harvest cells from the mass. If they remove pus, then this tells us that there is infection present and antibiotics can be dispensed. If blood or blood stained fluid is removed, then trauma was most likely and pain relief/dog-safe anti-inflammatories can be used to settle the swelling. If they remove clear fluid, then a cyst would be likely. And if the material is greasy and clear, then a fatty lipoma would be likely. And I would note that those last 2 tend to only be removed if causing the dog bother. 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, we do have some concerns here for this mass. Therefore, as long as it’s not painful, growing, or obviously draining pus, then you can try the above to rule out those aforementioned concerns. Otherwise, if this doesn’t settle, then it'd be ideal to have a check of this (if he is due a booster
soon, you can move that appointment up so that this could be checked at the same time). That way we can get his vet to sample this mass to identify its cause so that appropriate treatment can be initiated to address it for him.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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