Should I worry about this mysterious scab on my cat's neck?

• My cat, Mint, has a round scab on the back of her neck, right between her shoulderblades. It seems to have spontaneously appeared a couple of nights ago. http://picasaweb.google.com/beccamorte/RecentlyUpdated?authkey=Gv1sRgCPSiy5Cu7dWuEg&feat=directlink Google results all seem to suggest it's an allergic reaction to a food (possible) or a vaccine (she hasn't been to the vet lately). I checked for more wounds, but couldn't find any. Her sister, Clover, is unscathed, as far as I can tell. Of possible relevance:We moved 6 weeks ago, but I don't think that's the problem. Both cats seem happier in the new place. Mint, in particular, mellowed out a lot.I took Clover to the vet a couple weeks ago, and when she returned, Mint hissed at her for a few days. Soon after that, my husband woke up in the middle of the night to the sound of fighting cats, saying it sounded like it was coming from inside the house. It is possible that our cats fought, but that was after they'd gone back to being their http://www.flickr.com/photos/beccamorte/3956481070/. We do have some ferals hanging around the house, and I've heard them fighting in the yard in the middle of the night.I'm pretty sure it wasn't there long before I found it--Mint loves being scratched between her shoulderblades.They're indoor cats, but Clover has ventured out onto the porch. Could she have brought something in with her (fleas?) in the couple minutes she was outside?There's been no change in either cat's behavior. Am I worrying about nothing> Should I have this checked out by the vet, or just let it heal on its own? She's not scratching at it, and it only seems to bother her when I inadvertently touch it.

• Answer:

  I'm not a vet. IMHO, it doesn't look like an allergic reaction to diet or a vaccine. A reaction to diet is generally not localized in one spot; it tends to present as a flaky coat, or a thin coat, or symmetrical patches in areas where the cat can reach itself for repeated chewing or scratching. Fleas will also result in a more generalized response, with lots of scratching and perhaps some hair loss. You should also be able to observe fleas on the cats, or at least the presence of 'flea dirt,' which looks like fine black dust or sand found close to the skin in the animal's coat. From the photos, the lesion doesn't look inflamed. There does not appear to be much, if any redness, or swelling, or oozing. It looks scabby. I would guess it was from Clover. My cats also like to bite each other on the neck while playing, though they are usually quite cuddly. The biggest danger with bite wounds in cats is the tendency for abcesses to form. Because cats have very loose skin and a great deal of subcutaneous space, and oftentimes fight other cats who make puncture wounds that, in effect, inject bacteria into that space, a simple bite can turn into a huge pocket of pus and a very sick cat. As long as Mint's lesion stays small and dry, without swelling, you will probably be fine. If she gets lethargic and a big hump of fluid starts to collect around the wound, you will need to have the vet lance and drain the abcess and treat her with antibiotics. Good luck! Keep us updated!