@aart, Yes, Depluming Feather mites feed on the fluid inside the feather shafts and would be undetectable to the eye. Those must be treated with systemic treatments. However, feather mites feed on the feather dander and can be treated with dust.
@Silkie2 A good visual of the bird will help you determine which pest you are dealing with. The type of infestation will determine the type of treatment.
Permethrin spray rids most pests, however, aart is right that systemic treatment should be used for those mites inside the shafts that dust can't reach as well as for heavy infestations of blood sucking parasites which have gotten beyond the control of dusts and sprays.
Systemic insecticides, such as Ivermectin, only work for those parasites that eat blood or body fluids, such as the Northern Fowl Mite, the Scaly Leg Mite, and the Depluming Mite. For those blood/fluid eating types, Ivermectin is usually very effective.
Ivermectin is generally not effective for parasites that eat feather dander or skin scales, such as poultry lice and feather mites.
Unfortunately, the Red Roost mite, which is a blood eater, has high tolerance to Ivermectin, so other insecticides are necessary. Since it lives predominately in the coop, coop treatment is necessary if carried out completely and diligently. Permethrin spray can be effective, but if resistance has been built up, other insecticides must be considered. (And some even resort to burning down the old coop and rebuilding with fresh...but that obviously is only extreme cases).
As all of the lice and mites can be hard to glimpse, it is best to look for body signs such as nits at the base of the feathers (the eggs of poultry lice) or the blackened blood trails of mites.
Observing the birds at night with a flashlight shined upon roosts and at vents can be very helpful is viewing the quick moving beasts.
Feather damage can happen with most of the external parasites but in particular with the depluming mite. In that, look for crusty skin (from shaft fluids oozing out) and feather pulling by the birds.
I'll link some articles below that give photos of what to look for and suggested treatments.
LofMc
Types of parasites, Merck Manual
http://www.merckvetmanual.com/poultry/ectoparasites/mites-of-poultry
Photos of infestations
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
http://anrcatalog.ucanr.edu/pdf/8162.pdf (pictures of infestations)
