Infectious bronchitis (IB) is an acute or subacute highly contagious disease of chickens characterized by respiratory signs and high mortality in young chicks and it is also described by diminished production and deteriorated egg quality in laying birds. Infectious bronchitis was first discovered in 1931 in a flock of young chickens in the USA. Since the time the disease has been identified in broilers, layers, and breeder chickens throughout the world.
Infectious bronchitis virus belongs to the genus Coronavirus. It is an enveloped single standard RNA virus it has a lipid membrane that is driven from the endoplasmic reticulum of the host cell.
IB Virus is antigenically highly variable, new serotypes continue to emerge with time. The principal site of replication of the IB Virus is the ciliated epithelial cells of the respiratory tract mucosa of chickens. Some strains show an affinity for kidney tissues while others have an affinity for the tissues of the respiratory system.
Infectious Bronchitis (IB) Virus persists for about 4 weeks in contaminated chicken houses. This virus can be easily destroyed by common disinfectants, heat, and sunlight.
- The disease occurs more frequently in younger chicks.
- The resistance of chickens to the fatal infection increases with time.
- Occurrence is high in winter as compared to summer.
- Heavy breeds and male birds are more prone to a severe uraemic form of the disease.
Clinical signs of infectious bronchitis
- Sneezing, gasping, dyspnoea, coughing, rattling, nasal discharge, frothy exudate in the eyes. Abnormal respiratory sounds.
- In young chicks, this virus damages the oviducts of birds and ultimately produces false layers.
- Drastic drop in egg production occurs ( from 60-90% to 5-20% within 1-2 weeks)
- It causes the production of inferior quality eggs. Eggs are small, thin shelled, soft shelled, misshapen, rough-shelled or have calcareous deposits. The albumin becomes watery and yolk running. Chalazae are often broken, small haemorrhages may be seen in the yolk or albumen.
- Morbidity rate is very high ( can upto 100%)
- Mortality may be upto 25-30 % when cases are not complicated.
Postmortem lesions of infectious bronchitis
- Congestion of whole carcass
- Deep pectoral myopathy (white colour degeneration)
- Sinuses are usually swollen (rhinitis).
- Air sacs may be cloudy, and abdominal airsacs may contain yellow caseous exudate.
- Muco-catarrhal or caseous exudate in bronchi
- Aberrant development of oviduct.
- Abdominal ovulation and egg peritonitis may occur due to abnormality in oviduct.
- Swollen or inflamed kidneys (nephrosis, nephritis)
- Distention of the ureters with buildup of urate deposits.
ND (Newcastle Disease)
- Nervous signs are never seen in IB
- Mortality may be much higher in ND
- Both ND and IB cause a sudden drop in egg production.
- IB birds rarely go completely out of production while in ND birds go completely out of production.
ILT (Infectious laryngotracheitis)
- Not seen in young chickens
- Spreads in flock more slowly than IB.
- Respiratory signs are more severe in ILT than in IB.
- Spreads rapidly from bird to bird but there is facial swelling.
- The nephrosis in IB resembles the terminal kidney changes seen in several disease conditions such as IBD, drug toxicosis, severe dehydration and nutritional deficiencies.
- Mycoplasmosis: similar signs to IB initially but spread through the flock is much slow than IB.
Treatment of infectious bronchitis
No specific treatment is recommended. Antibiotics are used against secondary bacterial or mixed infections of E.coli and mycoplasma. Electrolytes should be offered to birds in the kidney form of Infectious bronchitis in chicken.
Prevention and Control
Vaccinate only healthy birds. Live vaccines often induce a mild respiratory infection, so vaccination may be inciting factor in airsacculitis when pathogenic mycoplasma is present in the flock.
Modified live vaccines provide better stimulation of cell mediation and elicit a superior local antibody response. Massachusetts strain of virus is most commonly used.