An in vitro comparison of the virulence of seven strains of infectious bronchitis virus using tracheal and oviduct organ cultures.

The virulence of seven infectious bronchitis virus (IBV) strains for tracheal and oviduct ciliated epithelium was assessed using an in vitro tracheal organ culture (TOC) and oviduct organ culture (OOC) system. The OOC were prepared using oviducts obtained from oestrogen-treated chicks. All strains tested stopped ciliary beating in the oviduct by day 5 post-inoculation (p.i.) and in the trachea by day 3 p.i. This corresponded with the absence of immunofluorescent-stained cells in the epithelium at that time. The time taken for a 50% reduction of relative ciliary activity (RCA) of oviduct cilia was shortest for strain 6 (serotype D207) and longest for strain G (enterotropic variant). Strains 7 (serotype D3896) and M41 were the most pathogenic for tracheal cilia, while strains 6 and G were less pathogenic. A calmod-ulin (CAM) assay was standardized to quantify the epithelial cell damage to oviducts caused by IBV. It was found that strains 6 and M41 were the most pathogenic. The use of time taken to achieve a 50% reduction in RCA and the CAM assay for in vitro pathotyping of IBV isolates is discussed. The susceptibility of OOC or TOC for five IBV strains was compared. It was found that strains 25 and 793B had equal predilection for both, while for 25, G and M41, TOC was more susceptible.