Infectious bronchitis (IB)
Infectious bronchitis is a highly contagious respiratory disease of poultry, caused by a virus belonging to the family Coronaviridae. Its existence in India is known since
1969. The disease spreads rapidly through aerosol and indirect transmission through contaminated feed, water troughs and other fomites can also occur. Transovarian transmission does not occur. Birds at all stages are susceptible but chicks below 4 weeks of age are severely affected.
Symptoms and lesions: Clinically the birds show depression, coughing, gasping, tracheal rales, sneezing and discharges from the eyes and nostrils, and occasionally swollen sinuses. Sometimes the disease may go unnoticed. Early infection of the chicks may lead to oviduct damage that result in laying failure. Infection of laying flock may be responsible for lowered egg production and eggs laid may be misshapen, rough and soft shelled. These characters of eggs alongwith poor internal quality and very low hatchability are the strong indications for suspicion of IB infection. In young chicks mortality may go upto 25%. In the renal form of the disease mortality may occur in broilers. The course of the disease is up to 3 weeks. Recovered birds may act as carrier for a variable period up to 1 month. On PM examination, congestion of lungs and the cloudy air sacs may be seen. Excess mucus and catarrhal exudates in the trachea and lungs, caseous plugs may be seen in the lower trachea of chick. When oviduct is involved, it may be regressed, absent or non-patented or cystic vestiges are seen. When nephropathic strains are involved, the kidney may be swollen and pale with tubules and ureters distended with uretic deposits.
Diagnosis: This is based on clinical symptoms and gross lesions. Virus isolation can be made from trachea, air sacs, caecal tonsils, oviduct and kidney. The virus isolation attempts in chicken embryos invariably show characteristic dwarfing and stunting of the embryos. Demonstration of viral antigens by fluorescent antibody test and intra- tracheal inoculation of susceptible chicks with original samples or first passage culture fluids are also helpful. Demonstration of antibodies in recovered bird by ELISA, hemagglutination inhibition and agar gel diffusion is used for confirmatory diagnosis.
Prevention and control: Proper hygiene and good management are necessary. Live vaccines are used in broilers and for initial vaccination of breeders and layers. Inactivated oil adjuvanted vaccines are primarily used at the point of lay in breeders and layers. Best protection with inactivated vaccine can be obtained only if it is given after priming the birds with live vaccines. It is necessary that the vaccine should contain the serotype prevalent in the field. Indiscriminate use of the vaccine has to be discouraged as this may lead to dissemination of variant strains. Though aerosol vaccination is said to be more effective, this method should be adopted only in flocks having a base immunity to avoid penetration of the vaccine virus deep into the lungs.