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While not a disease per se, lice and mites can cause problems for your chickens, such as anemia. If you think your chickens might have lice or mites, inspect the base of the feathers near the back end of the bird to look for lice or mites.  You may notice clusters of tiny white eggs. Alternatively, you might see parasites moving around on the bird, either small black specks or white parasites the size of rice. Lice and mites can be treated by dusting the birds a parasite powder or spraying them with a parasite solution for chickens. This intestinal parasite can make your chickens lose weight, and they may not feel like taking care of their feathers. Also, the skin around their mouths may be paler than normal.  This parasite may cause diarrhea, and you may see matted feathers near their rear ends. If one bird gets this, you'll need to treat the whole flock. This condition is called being egg-bound. You may notice the bird hanging out in the coop more. Its behavior may be erratic. You may see her straining to lay the egg. Think of someone with constipation and how that would look in a chicken. It may make noises while trying to lay the egg. There's not a lot to be done for egg-bound chickens, though a warm water bath around her back end or applying lubricant may help. This disease is caused by a herpes virus, and it is fairly widespread. In some cases, it can cause lymphoid tumors that press on or enlarge nerves. In turn, that can cause paralysis, which may only be temporary or could be permanent. This condition can't be treated.

Summary:
Look for itchy birds and ruffled feathers to detect lice or mites. Notice birds who are thin with bad feathers, which could indicate roundworms. Pay attention to a bird having trouble laying eggs. Watch for temporary paralysis for Marek's disease.