Infectious bronchitis virus: dominance of ArkDPI-type strains in the United States broiler industry during the last decade

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.