Introduction and History
Infectious bronchitis (IB) is a highly contagious viral disease in chickens. It affects the respiratory, renal, and reproductive systems. Farmers first noticed infectious bronchitis in chickens in the 1930s. Experts in North Dakota, USA, described it in 1931. Schalk and Hawn saw young chicks gasping and acting weak. By 1933, scientists named it a new virus. In 1936, they showed it differed from other illnesses like laryngotracheitis. Researchers grew the virus in eggs by 1937. This helped study it more. The disease spread fast around the world. South America saw it in Brazil during the 1950s. Early cases linked to Massachusetts-type strains. Poultry farms grew bigger from the 1970s. This led to more outbreaks. Now, it hits farms everywhere. It causes huge losses in money. Birds die or lay bad eggs. Farms spend on vaccines and care. New types keep appearing. They come from changes in the virus genes.
The virus changes often. It mixes genes or mutates. This makes control hard. Global trade moves birds and spreads it. In places like Asia and Europe, types like QX harm kidneys. South America has unique types like GI-11. These stay local but hurt farms bad. History shows we need better watches. Surveillance tracks new strains. It helps make right vaccines.
Etiology
The infectious bronchitis virus causes this disease. People call it IBV. It belongs to the coronavirus group. Specifically, it is a gammacoronavirus. The virus has a round shape. It measures about 120 nanometers wide. Spikes on its surface look like a crown. These spikes help it stick to cells. The genome is RNA. It has one strand and positive sense. Size is around 27.6 kilobases. It codes for proteins like spike, envelope, membrane, and nucleocapsid. The spike protein splits into S1 and S2 parts. S1 binds to host cells. It has hypervariable regions. These change a lot. Changes make new types. No cross-protection happens between types.
Many strains exist. Serotypes include Massachusetts, Connecticut, Arkansas, and more. Variants like QX hit kidneys. Others like 793/B or D274 spread wide. In South America, lineages GI-1, GI-11, GI-13 show up often. Mutations happen at 10^-4 to 10^-5 per site per year. Recombination mixes genes from different strains. This creates new variants. Vaccines use old strains. New ones escape them. The virus lives in air or on surfaces. Heat kills it quick. But it spreads easy in farms.
Host and Transmission
Chickens suffer most from IBV. All ages get hit. Young chicks under five weeks face worse signs. Layers lose egg output. Broilers grow slow. Pheasants and peafowl catch it too. They often show no signs. Wild birds might carry it. But main hosts are domestic chickens.
The virus spreads through air. Sick birds cough out drops. Healthy ones breathe them in. It also goes via feces. Birds shed it in poop for weeks. Contaminated feed, water, or tools carry it. People bring it on clothes or shoes. Farm visits spread it. Equipment like egg crates helps too. No eggs pass it to chicks. But shells can hold virus. In farms, it moves fast. Whole flocks get sick quick. Carriers shed it for up to 20 weeks. Even after they look well.
Incubation Period
Signs appear in 24 to 48 hours. The virus acts fast. It enters via nose or mouth. Then it copies quick. Peak shed happens in 3 to 5 days. All birds in a group catch it soon. Short time helps spread.
Morbidity and Mortality
Morbidity reaches 100% often. Every bird gets ill. Signs last 10 to 14 days. Mortality stays low at 5% in simple cases. But bad strains or extra bugs raise it to 60%. Young birds die more. Kidney types kill chicks fast. Layers survive but lay less. Coinfections with E. coli or Mycoplasma make it worse. Farms lose weight gain in broilers. Eggs drop by 70%. Recovery takes 8 weeks.
Pathogenesis (Step-by-Step)
Virus Entry – IBV enters through the respiratory tract or conjunctiva.
Replication in Respiratory Tract – The virus multiplies in tracheal epithelial cells.
Viremia – The virus spreads via blood to kidneys, oviduct, and other organs.
Tissue Damage –
Respiratory system: Inflammation, mucus buildup, cilia damage.
Kidneys: Nephropathogenic strains cause kidney damage (tubular necrosis).
Reproductive tract: Oviduct damage in layers leads to misshapen eggs.
Secondary Infections – Bacterial infections (E. coli, Mycoplasma) worsen disease.
The spike changes affect harm. QX type kills more cells. It boosts apoptosis. NSP proteins block immune signals. Like NSP14 cuts JAK1. This stops IFN response. Virus adapts to new spots. Brain damage happens in some.
Clinical Signs
Chickens cough and sneeze a lot. They make wet rale sounds. Noses run clear fluid. Eyes tear up. Faces swell. Young chicks huddle near heat. They eat less and grow slow. Weight drops. In bad cases, they gasp for air. Kidney strains add wet poop. Birds drink more. Feathers ruffle. They act weak and die.
Hens lay fewer eggs. Drop hits 70%. Eggs look odd. Shells turn thin, soft, wrinkled, rough, or pale. Insides get watery. Pigment fades. Young hens get oviduct harm. They become false layers. Ova fall inside but no eggs come out. Signs last two weeks. But egg issues linger.
Necropsy Findings
Throats fill with mucus. It starts clear then turns thick or cheesy. Nasal spots swell. Sinuses hold fluid. Air sacs cloud up. They get foam or caseous stuff. With E. coli, sacs get yellow pus. Hearts and livers swell. Kidneys look pale and big. Tubes pack with urates. Like white stones. Ureters block. In chronic, kidneys shrink.
Oviducts in young hens form cysts. Fluid fills them. Layers have short oviducts. Ovaries shrink back. Eggs inside look bad. Lungs congest. Trachea bleeds sometimes. Gut tonsils hold virus.
Diagnosis
Signs look like other diseases. Like Newcastle or laryngotracheitis. Labs confirm it. Test blood for antibodies. Use ELISA or HI tests. Rising levels show infection. Find virus with RT-PCR. It spots RNA fast. Or grow in eggs. Embryos curl and die. Sequence S1 gene. Compare to known types. This types the strain. Quick tests help pick vaccines.
Prevention and Vaccination
Keep farms clean. Use all-in all-out system. Stop wild birds entry. Clean tools with soap. Heat kills virus at 56°C quick. Limit visits. Quarantine new birds. Feed good diet. Cut stress. Good air flow helps. Low ammonia stops weak spots.
Vaccines fight IBV. Use live ones on day-old chicks. Spray or drop in eyes or water. Types like H120 or Ma5 from Mass. Boost at 2 weeks. Use different types for wide cover. Killed vaccines for layers. They boost long immunity. Match local strains. New variants need custom vaccines. Like autogenous ones. Watch for recombinants. They come from vaccines mixing with field bugs.
In South America, Mass vaccines led to new types. Surveillance picks right shots. No cross-protect between types. Revaccinate breeders. This cuts shed. Global work shares data on strains.
Economic Impact and Recent Developments
Farms lose big from IB. Birds die or grow slow. Eggs drop and sell low. Vaccines cost money. Treatments for extras add up. Worldwide, it ranks high in costs. In 2025, new studies show QX changes. It hits immune paths more. NSP16 blocks antigen show. This helps escape.
Research finds S1 maps. Key amino acids bind cells. This aids new vaccines. In Brazil, surveillance spots escapes. Whole genome tests help. Machine learning predicts types. Herbs like Shuanghuanglian fight it. They cut key genes.
Control needs global teams. Share strains. Make broad vaccines.