Here are some very good threads to read to see how others have dealt with fly strike:
https://www.backyardchickens.com/t/...has-huge-hole-below-vent-fly-strike-confirmed
https://www.backyardchickens.com/t/...autionary-tale-with-graphic-photo-progression
http://www.the-chicken-chick.com/2014/07/flystrike-in-backyard-chickens-causes.html
https://www.backyardchickens.com/t/822173/fly-strike-picture-of-wound-progression-of-healing
https://www.backyardchickens.com/t/...has-huge-hole-below-vent-fly-strike-confirmed
https://www.backyardchickens.com/t/...autionary-tale-with-graphic-photo-progression
http://www.the-chicken-chick.com/2014/07/flystrike-in-backyard-chickens-causes.html
https://www.backyardchickens.com/t/822173/fly-strike-picture-of-wound-progression-of-healing