Pathogenesis and molecular diagnosis of infectious bronchitis in poultry

Infectious bronchitis, probably the commonest respiratory disease of chickens caused by a coronavirus. Its effects vary with the virulence of the virus; the age of the bird; maternal immunity and complicating infections (Mycoplasma, £. coli, Newcastle disease). The virus which is antigenically highly variable and several sero-types and eight sero-groups have been recognised by sero-neutralization. These variations are due to structural differences in the spike proteins (SI fraction). The infectious bronchitis virus (IBV) targets not only the respiratory tract but also the uro-genital tract and can spread to different organs also. Infection initially causes respiratory disease in the infected birds and also drop in egg production in layers and breeders. Infection occurs via the conjunctiva or upper respiratory tract with an incubation period of 18–36 hr. The virus is highly contagious and spreads rapidly by contact, fomites or aerosol. Some birds may act as carriers. Affected birds show signs of depression, huddling, loss of appetite, coughing, gasping, dyspnoea, wet litter, diarrhea and diuresis. Morbidity may vary from 50–100% and mortality 0–25%, depending on secondary infections. Post-mortem lesions include mild to moderate respiratory tract inflammation, tracheal eodema, tracheitis, air-sacculitis, caseous plugs in bronchi, kidneys and bronchi may be swollen and ureters may have urates. Various serological tests like haemagglutination inhibition (HI) test, agar gel precipitation test (AGPT) and enzyme-linked immunosorbent assay (ELISA) are used for detection of antibodies specific to IBV. Other tests include immunoflourescence test (IFT), immunoperoxidase test (IPT), electron microscopic analysis and dot immunoblotting assay. The virus is confirmed by isolation in embryonated chicken eggs (ECE) and identified by virus neutralization (VN) test. Tracheal organ culture (TOC), prepared from 20-day-old SPF embryos, have proved very successful for the isolation, titration and serotyping of IBV. Various molecular techniques like reverse transcription-polymerase chain reaction(RT-PCR), real time-RT-PCR, hybridization assay, restriction endonuclease (RE) analysis, gene cloning and sequencing are being used for the differentiation of various pathotypes of IBV. Control of IB is best achieved by improved biosecurity and vaccination.