Fly Strike is
maggots living in the vent and into the body living off dead and then live tissue of the chicken.
flys are attracted to the chickens vent as usually some veces is there on the feathers.
called blow fly strike and fly blown.
It is deadly if not treated immediately.
Many fly species favor the stinky wet moist area of the chickens vent to lay their eggs.
Maggots hatch in24 hours and begin eating dead and live tissue, causing sores and infection.
Keeping the chickens clean is a necessity.
Provide a dusting area for the birds by using lots of clean sand for them to dust in.
Thus they will keep the wet feces off the vent feathers.
Also keep the fly population down especially in warm weather.
Use necessary fly protection.
If hens have wet dirty feces covered feathers.
Clean with a bath and or cut feathers off the chicken in vent area
If the fecal droppings are runny etc, abnormal loose.
then you need to have a vet lab test the fecal droppings.
Thus you will avoid the FL STRIKE.
http://www.the-chicken-chick.com/2014/07/flystrike-in-backyard-chickens-causes.html