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This happened to our cat - it turned out to be fleas as we were lax in making sure treatments were exactly monthly. We reverted to being more meticulous and no more cuts / lesions. Vets advice was to continue using back of neck drops but to avoid open wound.
About 6 weeks ago, I noticed that our cat had what I thought was a cut on the back of her neck. She is a bit of a fighter and so I assumed it was from that. It didn't seem to bother her and healed up fine, so we didn't take her to the vet or anything.
Last night I gave her her flea treatment (advantage) and this morning she has the exact same thing again. I've now realised that the last time she had it was probably just after her last flea treatment.
Is it possible that she's having an allergic reaction to the flea treatment? Or is it more likely that the old cut wasn't quite healed and the flea treatment has aggravated it?
In either case, do you think she needs to go to the vet? She seems fine in herself. I've cleaned the spot with salt water and it looks red but not infected.
Oh and we've been using advantage for a while so if it is an allergy she must have just developed it.