Spot on underarm?

Hi Vets, thank you for your time! Just noticed this on my boy’s underarm. He’s a very temperamental cat (doesn’t like being handled) and actually didn’t mind at all as I looked at it and touched it to see if it was a lump or anything, so seems it doesn’t hurt or bother him. Has a sore hole and another spot that looks like maybe it was previously a hole but healed over. Doesn’t have a bump but looks raised or like it’s healing from being raised and obviously some balding around it. He has been prone to some over grooming in the past but usually his belly and tail, and we hadn’t noticed him over grooming lately. Does it look familiar? I know skin issues can be a wide spread of possibilities but just curious for another set of eyes/opinion. Possibly useful context: I did start my cats on a new food about 3 weeks ago, but overall it has made their fur and skin really great, silky and no dandruff. They’ve been fed this food before. I was away for 8 days isolating with Covid, he was with his human dad who we already live with so he wasn’t alone at all really, but maybe it did stress him more. He has gotten sick while I’ve been gone before. Could it be a spider bite? I don’t believe it could be fleas, we’ve never had them and he stays fully indoors. Haven’t noticed him itching more than usual or anything of the sort. He’s been calm and content. He’s 10, male, 16 lbs of big bones and a bit of chonk, and has refused to eat anything but kibble his entire life despite dozens of attempts with wet food. Bathroom goings are normal. We have a second cat and they do like to wrestle, not sure if maybe his brother somehow managed to get ahold of his armpit? I have some topical treatments I’ve used when he gets kitty acne from nuzzling a blanket too much (before we can dissuade him). I’m not sure if these could help, or if it looks on its way to healing already anyway? I haven’t applied anything yet.

Reposting the picture because it isn’t loading on my end

Hello - there is a long list of possibilities that something like this might be, some of them barely significant and others much more so.  Fleas remain on the list I'm afraid, because fleas can - and do - enter a house even when a cat is unable to leave.  This wouldn't be the typical presentation.  Bites by numerous other small insecty things e.g. midges may be likely.   I can see that there may be hair-loss around this lesion, so it is possible that overgrooming (a sign of pain or itchiness) has been involved and sometimes, this causes a viscious cycle.  Ringworm can sometimes present as non-itchy hair-loss to the extremities; although this is not of classical appearance, it may still be possible (be vaguely aware of the zoonotic risk - i.e. Ringworm can pass on to human ).  A tiny foreign body / splinter / bee-sting may be a possibility, as could e.g. a nettle sting.  A bite from another cat can look insignificant from the outside but go on to swell, with fever and tiredness, a few days later as bacterial infection takes hold.  You don't mention whether he's an only cat, so it may be possible.  So may follicular cysts / swellings / demodex and self trauma.  This latter could be the result of underling joint pain, for example.  Owners tend to expect that if a cat were uncomfortable, then he would make it obvious but sadly, the reverse is true; cats tend to live independently and 'co-operate' in colonies, rather than being evolved for group living as dogs are.  One disadvantage of this from a veterinary perspective, is that they often hide their pain / irritation and owners frequently underestimate it, so we cannot rule joint discomfort out.   Obviously, all of these different possibilities (I'm sure there are more) require different treatments, so we are in no position to recommend a treatment online.  The cream and the licensed feline chlorhexidine wash would be prescription only in the UK and steroid can make things worse (e.g. in the event of ringworm), so our answer would always be to seek veterinary advice, particularly if this looks worse rather than better, in order that the area may be assessed in full context.

Below are some blogs that may help for a couple of the more serious possibilities; however, we hope that this is resolved soon.  Wishing you both all the best. Atopy (allergy, long-term) -  https://vethelpdirect.com/vetblog/2023/07/27/do-cats-get-atopy/; hair loss:   https://vethelpdirect.com/vetblog/2019/09/23/why-is-my-cats-fur-falling-out/    

I did mention we have another cat.. ”We have a second cat and they do like to wrestle, not sure if maybe his brother somehow managed to get ahold of his armpit? ” They do bite at each other sometimes when they wrestle, at which point we have to shoo them off each other. Their wrestling has lessened since we changed their eating routine (got rid of our automatic feeder as it was creating some food competition and switched to their own individual bowls again). As for fleas, I haven’t noticed any itching, and we live on a second floor unit with our own door, no hallway entrance. I don’t see how we could have fleas at all.   If it continues to heal and hair grows back is it likely not a concern? He had balded some of his tail out grooming earlier this year and it’s all grown back.

Hello - if the fur grows back on its own and there is no underlying abscess, it may be of no concern.  However if both of these incidents are related to overgrooming, it may be worth wondering what is causing the overgrooming, as this is indeed a sign of stress or pain (cats dom't show pain in the way that most people would expect). The reason that I keep harping back to infection is, that nearly all cat bites get infected, so that would be a big concern. It is almost never appropriate for an online vet to say 'yes your cat is okay' because there is so much unchecked potential for it not being (our job is very much to spot what can go wrong).  Therefore for me, this one is worth a check.

With apologies - I do indeed seem to have missed that you told me about the other cat already.