Abstract
In the United States, more than 90% of chicken meat is produced in the southeastern states, and most egg production resides in the eastern half of the country and Texas. Several molecular epidemiological studies have indicated that most infectious bronchitis (IB) virus (IBV) isolates obtained from outbreaks of respiratory disease in these regions correspond to Ark-type IBV in spite of extensive vaccination programs which include IBV ArkDPI-derived vaccines. Accumulating evidence suggests that Ark-type strains may have a distinct capacity to circumvent preventive measures. Two strategies by which Ark-type IBV strains may maintain a high prevalence in commercial chickens are: (1) Unusually high genetic and phenotypic variability, and (2) synergism with concurrent viral immunodeficiency. Support for the first strategy includes epidemiological findings showing continued isolations of Ark-like viruses from respiratory disease affecting flocks vaccinated with serotype-specific homologous (ArkDPI-derived) vaccines, experimental data demonstrating selection of new predominant phenotypes occurring rapidly after a single passage in the host, and recent findings indicating changes of the predominant IBV population occurring within the host during the invasion process. The second strategy is supported by epidemiological data indicating increased isolations of Ark-type IBV showing minor geno-/phenotypic variation occurring in chickens simultaneously affected by immunosuppressive viruses. In addition, experimental results have shown that viral immunodeficiency leads to more severe and prolonged IB signs and lesions, delayed and reduced specific antibody responses, and increased and persistent IBV shedding. Finally, accumulating evidence confirms high genetic and phenotypic heterogeneity in commercial ArkDPI-derived vaccines. The rapid selection of new predominant phenotypes occurring in these vaccines may be facilitating the emergence of Ark-like strains. Thus, improvement of Ark-type vaccines and prevention of viral immunodeficiency's seem to be essential for an effective control of the disease.
Highlights
Infectious bronchitis (IB) virus (IBV), a member of the Coronaviridae family, is likely one of the most prevalent avian pathogens in the poultry industry worldwide
We found significantly higher (P
At 7 DPV, chickens vaccinated with vaccine C had more severe epithelial necrosis and deciliation in the trachea than chickens vaccinated with vaccine D, and more lymphocytic infiltration of the trachea than chickens vaccinated with the other three vaccines. These results indicated that the distinct predominant IBV populations selected during the vaccine production process from the same parent virus (ArkDPI-derived vaccines) exhibit different virulence in chickens
Summary
Infectious bronchitis (IB) virus (IBV), a member of the Coronaviridae family, is likely one of the most prevalent avian pathogens in the poultry industry worldwide. A mechanism to explain the prevalence was first reported in 1956 by Jungherr et al (Jungherr et al, 1956) They demonstrated antigenic differences (sufficient to prevent crossprotection) between IBV isolates obtained in the states of Massachusetts and Connecticut. Different combinations of 2 to 3 IBV live vaccines were used including serotypes Mass, Conn, ArkDPI, GA98, and DE072 These authors evaluated the relative amount of IBV in tracheal swabs from birds from 10 different flocks and followed the clearance of the vaccine virus. Only Arktype viruses were recovered, indicating a unique persistence of Ark-type vaccine viruses in the chicken respiratory tract These findings indicate that Ark-type IBV is currently the most relevant IBV type in the United States poultry industry. There is strong evidence supporting at least two important mechanisms by which Ark-type IBV strains succeed in the environment and maintain a high prevalence in commercial chickens: (1) High genetic and phenotypic variability, and (2) synergistic effects of concurrent viral immunodeficiency
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