Just missing fur does not mean it's a cat scratch. You should notice some sort of wound or scab? If it's not openly blooding or looking infected.
Than the vet won't do much. You yourself could have done something but once the wound is scabbed and no longer fresh there is less you can do. You might still be able to do something but I'm not used to dealing with wounds on hamsters.
I know there are things you can do to treat wounds on a small degree. So is the wound bleeding? Or still looks really fresh?
In reality the hamster should be fine. An infection is about the only problem you could have. Is there anyway to put the hamster up to a place the cat cannot easily reach?
Or better yet while more expensive you could go and buy a cage that the cat could not possibly claw into! Like a plastic tube cage, glass tank, even a properly made bin cage could work. If the new cage is even larger or better then your current cage it won't just be a buy to prevent your cat from getting at your hamster.
You could look at it as an upgrade to your hamsters life. Hamster cages should have over 360 square inches of floor space anyhow. About equal to the size of a 20 gallon long tank.
Not that is has to be a tank... I'm just giving you a size comparison. So if it's smaller than that you kind of have a reason for getting an other cage anyhow. That's about the only real advice I can give about keeping the cat away.
Besides trying to keep the cat out of the room, or teaching the cat not to do it. Which might be as difficult as traveling though time since cats do what they want! Other things besides cats can leave bald spots on your hamster.
I am sure you noticed a scratch as well but if it's just a bald spot look into other things like mites and such. Unless it's a really old wound there should at least be a scab.
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