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Hi, i`m Richard, a Veterinarian from the UK. Hopefully I can help today.
It could well be a reaction to the fly spray, have you used this particular product on him before?
If it is a mild allergic reaction it should be self limiting. If you are seeing any blistering or crusting bathing the area with a chlorhexidine solution such as hibiscrub can help soothe the lesions and prevent any secondary infections. If you don`t have access to chlorhexidine then a salt water solution is a reasonable substitute.
they would help with an infection, but for a genuine uncomplicatd allergy they would have no effect.