The advice hub Northern Fowl Mite
The Northern Fowl Mite (Ornithonyssus bursae), is an oval shaped mite about 1mm in size. Like the red mite, it starts off life a pale grey colour and feeds on the chicken by sucking its blood turning the engorged mite a black/brown colour. This feeding irritates the bird leading to much stress and a likely drop in egg numbers. Birds may lose weight and become depressed. If the number of these mites is large enough then the chicken can suffer from anaemia characterised by a pale comb and wattles. The Northern Fowl mite lives on the bird, laying its eggs at the base of the feathers around the vent, these eggs hatch after a few days and mature into adult mites as quickly as 12 days, allowing their numbers to build up rapidly under ideal conditions.
The most significant difference between Northern Fowl Mites and red mites is that Northern Fowl Mites will spend their entire life on the chicken (they can only survive 10 days off a chicken). You will not find them hiding away in cracks and crevices like the red mite. Control is slightly different to red mite because you need to concentrate on treating the chicken as this is where the mite will be found.
For preventative treatment, use Diatomaceous Earth on a weekly basis. Apply to the bird under the wings and around the vent too.
During a known infestation, use Dergall. Apply diluted Dergall directly onto the vent area using a hand sprayer. This will need repeating once a week for 3 weeks. Do this treatment at night when the birds are roosting to reduce stress trying to catch them.