Mite infestation can kill a healthy chicken in three days or less. Examine the bottom of the hen, where the egg comes out. If you find the area to be dark in color and it seems to be moving, then your chicken probably has a mite infection. The most commonly found mites are the Red Mite and Northern Fowl mite.
The chicken can also show physical signs of a mite infection. Check the comb and wattles and you will be able to see if there is a mite infection. Also, if the chicken has turned pale or looks to be sulking in a corner, they’ve probably been infested. Check the vent area to see if that’s where the infection is coming from. Also, check the ears, thighs and neck.