Mystery Lumps- Vet baffled

I'm looking for some advice as my vet was just as baffled as I am about this. My 4 year old GSD/Cattle/Amstaff mix developed these pea sized bumps (I've counted 15) on his legs and shoulders. Most are individual some happen to be next to another. It's been 6 days and they have not gone away despite benadryl. He has been hiking recently but I've found no engorged ticks on him or my other dog (cattle/husky). My other dog does not have any bumps. He doesn't seem to be bothered by them except he's now licking the one I shaved to get a better look at. They appear skin tone colored and I can easily pinch them to move but they feel solid or fibrous in nature. We started anti biotics but I really don't know if that's what this is.   Update: Day 3 of anti biotics and there are more of them for sure now and others are more pronounced. They almost look like they might become pustule but aren't.

Hello - has your vet seen these lesions?   What antibiotics did they prescribe?  Are you in the UK? In light of the development, I wonder if this is more inflammatory than infected?   It's rare for a perfectly healthy dog with perfectly healthy skin just to 'get' a skin infection - the skin is usually very good at protecting itself.  It might have to be penetrated eg by fleas or flies or so on (that would have been quite a cloud of them) or the defensive layer weakened by scratching on your dogs' part (eg as happens sometimes in patients with allergies), or by a weakened immune system by other diseases. Alternatively, there are sterile, auto-immune diseases that might explain such lesions as you are describing.   I don't know where in the world you are, but there may also be 'infectious diseases' - not as in skin infection, but as manifestation of eg virus. I have listed things here that do not have antibiotics as the mainstay of treatment, so if they are getting worse despite antibiotics it might be that a further consultation is in order.

I am interested that you called your vet baffled -   Most vets don't know what is causing clinical signs on first presentation - this is normal.   It sounds as though you need to ask them what, in their opinion knowing the patient, it could still be, and how to start ruling possibilities in or out.

Thank you but I’ve done that and so has my vet hence why we are stumped by this. Anything environmental would have affected my other dog as well and it has not. They are on regular flea and tick treatment as well as heart worm prevention. As I stated I’ve never pulled an engorged tick off of him and there have been no signs of fleas. I’m in New England (America) and the ground has frozen so pests are not a concern right now. I’ve also asked her opinion and she didn’t have one which is why she’s trying anti biotics and so I asked here.

He is on 10 days Cefpodoxime

Thankyou.  That's important background.  However, it isn't true that all environmental 'things' affect all dogs the same.  Some dogs have sensitivities to environmental allergens that other dogs simply don't have, even amongst littermates, just the same as with people.  This is especially true with things like fleas in a clean household environment, where they could only possibly be present in small numbers if they're there.   In the UK the weather is milder but fleas survive indoors all year long (central heating) and it only takes a few cells from one to set an allergy off.  Autoimmune conditions, as mentioned earlier, are generally intrinsic to the dog so are also worth looking into. Regarding the plan from here, it is surprising for a vet to have no opinion as to what something is and no plan as to how to find out, but if that's where you're up to I would suggest asking them to refer you to a dermatologist who can help you.   Thanks to Covid, a lot of dermatologists consult online now, which could prove handy.  Another way to find more information would be to ask them to take appropriate skin biopsies, which might be helpful if this was eg autoimmune disease, but when it comes to identifying allergens may be less so. I hope that some of this helps to move the situation on a little; please do let us know if you get to the bottom of it.