Egg Drop Syndrome (EDS '76):

Symptoms: Sudden drop in egg production, eggs become pale, soft-shelled, or shell-less. Birds remain otherwise healthy.
Post-mortem: Flaccid oviduct, edema of uterine wall, atrophy of ovarian follicles.
DDx: Infectious Bronchitis (nephropathogenic strains), Mycotoxicosis, heat stress.

Infectious Bronchitis (IB - reproductive form):

Symptoms: Reduced egg production, poor egg quality (misshapen, rough, thin or soft shells, watery albumen).
Post-mortem: Persistent oviduct lesions, cystic oviducts, undeveloped oviducts in pullets exposed early.
DDx: EDS '76, Mycoplasmosis, stress factors.

Fowl Cholera:

Symptoms: Acute septicemia causing sudden death, facial swelling, lameness, diarrhea, reduced egg production. Chronic forms lead to localized infections (joints, wattles, oviduct).
Post-mortem: Petechial hemorrhages, necrotic foci on liver, peritonitis, salpingitis.
DDx: E. coli infection, Newcastle Disease (acute).

Salpingitis/Peritonitis:

Symptoms: Reduced egg production, distended abdomen ('penguin stance'), depression, sudden death.
Post-mortem: Inflamed, distended oviduct filled with caseous material or inspissated yolk, fibrinous exudate in abdominal cavity.
DDx: E. coli, Fowl Cholera, Mycoplasmosis, trauma, foreign body.

Marek's Disease (MD):
- Symptoms: Paralysis of legs/wings (classic form), ocular lesions ('grey eye'), tumors in internal organs.
- Post-mortem: Enlarged nerves (sciatic, brachial, vagus), tumors in liver, spleen, kidney, gonads, skin.
- DDx: Newcastle Disease (nervous form), Avian Encephalomyelitis, Lymphoid Leukosis.
Newcastle Disease (ND - nervous form):
- Symptoms: Tremors, torticollis (twisted neck), opisthotonos, paralysis. Often accompanied by respiratory and digestive signs.
- Post-mortem: Similar to respiratory/digestive forms; no specific gross nervous lesions for diagnosis without lab.
- DDx: Marek's Disease, Avian Encephalomyelitis, Vitamin E deficiency.
Avian Encephalomyelitis (AE):
- Symptoms: Ataxia, tremors (especially head/neck), paralysis, dullness, prostration. In layers, can cause a transient drop in egg production and hatchability.
- Post-mortem: No gross lesions visible. Microscopic brain lesions.
- DDx: Newcastle Disease, Marek's Disease, Vitamin E deficiency.

Marek's Disease (MD - skin form):
- Symptoms: Feather follicle tumors (skin leukosis).
- Post-mortem: Enlarged feather follicles with tumors.
- DDx: Fowl Pox (wet form, but lesions are different), external parasites.
Fowl Pox:
- Symptoms:
  - Dry Pox: Warty lesions on unfeathered skin (comb, wattles, eyelids).
  - Wet Pox: Diphtheritic membranes in mouth, larynx, trachea.
- Post-mortem: Characteristic pox lesions.
- DDx: Marek's Disease (skin form - different lesion type), Vitamin A deficiency (wet form).
Staphylococcosis:
- Symptoms: Bumblefoot (swollen footpads), arthritis, septicemia.