Might be lice- some products you can buy are
Dri-Kill
Sevin
try hardware stores (ag division), tack shops, co-ops, ag supply stores like 
TSC.
Have some on hand at all times.
Any sign of waxy egg clusters near the vent?
Important you begin treatment immediately.  
Either of these products can me mixed with food-grade DE to dust the coop, and as Miss Prissy recommends, the mixture can be placed in the foot of a stocking to make a powder puff to dust under wings and along their bottoms.
You can also add a sand box with the same mixture in the sand for follow-up and future preventive treatment.
Bedding will have to be removed,floor and all cracks and crevices, roosts and platforms treated too and new bedding added, also treated with that mixture.
Go to the Search feature above for comprehensive conversations, and check Pests and Predators Section.
I think there is also a link from the home page to this topis.