Chooks can also get mites and lice (from native pigeons etc) and this is easy to treat for by dusting the birds and coop with a powder called Pestine, or a similar product. We sell Pestene, it works a treat.
If you suspect this is a problem (ie. you see them or you are inexplicably not getting eggs) flip the chook over and have a look at its underside. Chooks will stop laying if they are being hassled by pests.
We sell many chook medicines and they are also available from petshops, vets usually have them too (if you have very deep pockets).