Blowflies are the brightly coloured flies that lay their eggs on dirty wool or wounds and then the maggots hatch out and start to devour our stock alive

Prevention wise - we dag the sheeps bums, shear and/or use (in our case) Clik which is supposed to prevent the process. I know others use other preventative treatments. If we actually have strike we've used crovect for severe or battles maggot oil for mild cases.
Regular checks over your sheep during warm/damp days and if you see any acting 'odd' or hanging away from the flock do a good thorough check over.
I've known cats with flystrike. Lost 2 jacobs that we'd lent our land to and last year caught it early on 2 lambs and it was successfully treated.