Chicken Disease Encyclopedia
Every condition our AI Chicken Doctor knows about, in one searchable list. Click any disease for symptoms, treatment, and prevention written for backyard keepers.
comb wattles
Anemia (Pale Comb)
sev 3/5Pale comb is a symptom, not a disease. Common drivers are heavy mites or lice (anemia from blood loss), internal worms, broody hens losing condition, or chronic disease.
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Fowl Pox
sev 3/5A slow spreading virus carried by mosquitoes. Dry form (scabs on comb and wattles) usually heals on its own in 3 to 4 weeks. Wet form (mouth and throat lesions) is dangerous.
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Frostbite
sev 2/5Cold and humidity damage exposed tissue. Tips of large combs and wattles go black or gray. Wet bedding plus cold makes it much worse than dry cold alone.
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crop
Impacted Crop
sev 3/5A blockage of fibrous material (long grass, straw, bedding) jams the crop so food cannot pass. The crop stays full and firm by morning instead of empty.
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Sour Crop (Crop Mycosis)
sev 3/5A yeast (Candida) overgrowth in the crop, often following antibiotic use, spoiled feed, or an impacted crop. Crop feels like a water balloon and the bird's breath smells like sour milk.
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droppings
Coccidiosis
sev 4/5A protozoal gut infection that destroys intestinal lining. Bloody or mucousy droppings in a young chick are nearly always coccidiosis. Fatal if untreated.
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Internal Worms (Roundworms, Capillary, Cecal)
sev 2/5Common internal parasites that steal nutrients and cause weight loss, pale combs, and droppy lay. Most free range flocks have a low level burden, problems start when it gets out of hand.
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feathers skin
Annual Molt
sev 1/5A normal annual feather replacement, usually in fall as daylight shortens. Birds look ragged, stop laying, and act subdued. Not an illness.
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External Mites and Lice
sev 3/5Lice live on the bird, mites often live in the coop and feed at night. Heavy loads cause anemia (pale comb), feather damage, and a stressed flock that stops laying.
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feet legs
Bumblefoot
sev 3/5A staph infection that enters through a cut or pressure sore on the foot pad. A black scab is the calling card. Easy to treat early, ugly if left.
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Scaly Leg Mites
sev 2/5Tiny mites burrow under leg scales and raise them up, leaving crusty white or yellow buildup. The legs look thick and lumpy. Treatable but uncomfortable.
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neurological
Botulism (Limberneck)
sev 5/5Paralysis caused by a toxin from Clostridium botulinum, usually picked up by eating decaying animal matter or wet rotting feed. Limp paralysis that creeps up the body is classic.
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Marek's Disease
sev 5/5A herpesvirus that causes paralysis (classic split leg posture), tumors, and a gray eye. No cure. Survivors stay lifetime carriers. Vaccinate chicks at hatch to prevent it.
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Wry Neck (Torticollis)
sev 3/5Neurological signs caused by vitamin E and selenium deficiency, head trauma, or sometimes Marek's. Pure vitamin cases often reverse fully with treatment.
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reproductive
Egg Binding
sev 4/5An egg gets stuck inside the hen. She strains, walks upright like a penguin, and stops eating. Untreated egg binding past 24 hours is often fatal.
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Egg Yolk Peritonitis
sev 5/5Yolk material misses the oviduct and drops into the abdomen, triggering infection. The hen develops a swollen belly and waddles. Often fatal even with treatment.
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Soft Shell or Shell Less Eggs
sev 2/5An occasional soft egg is normal for new or stressed layers. Persistent soft shells point to a calcium deficiency, illness (often IB), heat stress, or aging reproductive tract.
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Vent Gleet (Cloacitis)
sev 2/5A yeast and bacterial inflammation of the vent. Yellow paste, fishy smell, and a generally unhappy hen are the classic combo.
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Vent Prolapse
sev 5/5Part of the reproductive tract pushes outside the vent. The exposed tissue attracts pecking, which can kill the hen quickly. Isolate first, treat second.
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respiratory
Aspergillosis (Brooder Pneumonia)
sev 4/5A fungal lung infection from inhaled Aspergillus mold spores in damp bedding or moldy feed. Often hits chicks in brooders. No discharge, unlike bacterial respiratory disease.
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Avian Influenza (HPAI)
sev 5/5Highly pathogenic avian influenza is a reportable disease that can wipe out a flock in 48 hours. Dark combs, swollen heads, and sudden death are red flags. Report immediately.
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Heat Stress
sev 4/5Chickens cannot sweat. Above 90 F they pant and dump heat through their comb. Heavy birds, dark feathered birds, and those with large fluffy bodies are most at risk.
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Infectious Bronchitis (IB)
sev 3/5A very contagious coronavirus of chickens that causes mild respiratory signs in adults and lasting damage to the egg making system, soft or misshapen eggs are a giveaway.
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Infectious Coryza
sev 4/5A fast-spreading bacterial respiratory infection that causes swollen faces, smelly discharge, and one-sided sinus swelling. Recovered birds often stay lifetime carriers.
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Infectious Laryngotracheitis (ILT)
sev 5/5A severe herpesvirus that attacks the windpipe. Classic signs are gasping, neck stretching, and coughing up bloody mucus. Often fatal. Survivors stay lifetime carriers.
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Mycoplasma Gallisepticum (MG)
sev 3/5A slow-burning bacterial respiratory disease that produces classic foamy eyes and a rattling chest. Birds carry it for life and shed it under stress.
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Newcastle Disease
sev 5/5A reportable viral disease with respiratory, digestive, and neurological signs. Virulent strains kill flocks fast. Contact your state vet or USDA APHIS at the first suspicion.
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wounds
Cannibalism and Feather Pecking
sev 3/5Once chickens see blood they will not stop. Triggers include overcrowding, boredom, protein deficiency, or a sick bird singled out by the flock. Isolate any bleeding bird immediately.
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Predator Wound
sev 4/5Bites, scratches, or punctures from raccoons, dogs, hawks, foxes, or other birds. Chickens are stoic, the wound is often worse than the bird looks at first.
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How to use this encyclopedia when a bird looks sick
Most keepers land here with a hen that looks wrong and no clear idea why. Start with what you can see, not with a disease name. Write down the main sign, how long it has been going on, how many birds have it, and whether anything changed recently, like new birds, new feed or a heat wave. It is exactly what a vet will ask for.
If you are not sure where to begin, the Chicken Doctor lets you describe what you see and works through likely causes with you.
First moves for any sick bird
- Move her to a quiet, warm, draft-free spot away from the flock, with her own food and water.
- Check her crop, vent, feet, eyes and breathing. Look under the feathers for mites and wounds.
- Look at her droppings once she is alone, so you know which ones are hers.
- Wash your hands and change shoes before you go back to the healthy birds.
Problems you must report
Some diseases are a legal matter, not just a flock matter. If several birds die suddenly, or you see swollen purple combs, twisted necks, heavy breathing and a crash in laying all at once, call your state veterinarian or extension office before you do anything else. That covers avian influenza and Newcastle disease,.
Prevention beats treatment
Keep new birds apart from your flock for about a month before mixing, keep wild birds away from feed and water, and do not share equipment with other flocks. The small flock biosecurity guide walks through the basics.
Common questions about chicken diseases
How do I find out what killed my chicken?
Ask your state animal diagnostic lab about a necropsy. Keep the body cool in a fridge, not the freezer, and call them quickly. It is the most reliable way to protect the rest of the flock.
Can I eat eggs from a sick hen?
Play it safe and do not, until you know what she has. If she is on any medication, check the product label or ask a vet about the withdrawal period.
Do I need a vet who sees chickens?
It helps a lot. Look for an avian or poultry vet, or ask your extension office who in your area will see backyard birds. Find one before you need one.
