This is probably not the answer but I have to ask was the injured area, touching the wire/tube connecting area.
I once got a naste frequency burn that lasted aprox 5 or 6 months before the scab fell off. I fell asleep with the tubes held to my sides with underpants. The skin where the wire connected to one tube was touching. May have been 3 or 4 hours. Very painfull for very long. Mine blistered as I recall.
One thing was that this zapper had battery, and a wall power source, and I was using the wall source.
I just thought I would mention this in case it might be burn like. I hope it isn't, would take a very long time to heal.
Good luck, Naturalman