Fly struck sheep die from the ammonia produced by the maggots as they feed on the flesh which then circulate through their system and kill them. Death typically occurs from poisoning 3-6 days after they have been struck.
They are also prone to secondary infections hence you should have given your sheep a shot of LA Penicillian as well as the chemical treatment to kill the maggots.
A bit premature to blame the chemical when you don't really understand the problem.
