Some possibilities:
(Quoted from:
http://www.welphatchery.com/poultry_health.asp )
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Botulism
Symptoms: Extreme weakness. Paralysis of legs, wings, neck. Bird cannot swallow. Feathers easily removed. Head hangs, swollen wattles, difficult breathing, dark head & wattles.
Causes: Toxic birds eat decaying animal or vegetable material. Toxin is soluble so can be in water or maggots.
Prevention: Clean yards. Don't use spoiled food.
Treatment: Place bird in shade. Fill crop with water twice daily. Give Epsom salts (1 lb. to 5 gal. water) into crop. Remove dead animals.
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Marek's Disease (Range Paralysis)
Symptoms: Gray eye, enlarged feather follicles, paralysis of wings, legs, neck.
Causes: Herpes virus. Airborne or other contaminated skin & feather dust (dander). Contaminated litter. Infected birds.
Prevention: Vaccinate day-old chicks. Buy vaccinated chicks. (I think isolating sick birds & cleaning areas where sick birds have been will also help.)
Treatment: None (supposedly, but there is a thread on treatment at
https://www.backyardchickens.com/forum/viewtopic.php?id=290501 )
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Best wishes!