Chicken-embryo-origin modified-live ILT vaccine virus showed increased virulence and mortality after serial bird-to-bird passage, whereas tissue-culture-origin virus did not.
Serial bird-to-bird passage of chicken-embryo-origin modified-live infectious laryngotracheitis vaccine viruses increases their virulence.
Modified-live (ML) infectious laryngotracheitis (ILT) vaccine viruses, both tissue-culture-origin (TCO) and chicken-embryo-origin (CEO), were passaged 20 times in specific-pathogen-free chickens. After serial bird-to-bird passage, increased virulence was observed for CEO virus but not TCO virus. Increased mortality and increased severity and duration of respiratory disease were observed in chickens inoculated with chicken-passaged CEO viruses; only mild respiratory disease (no mortality) occurred in chickens inoculated with chicken-passaged TCO viruses. These findings suggest that ML ILT vaccine viruses may increase in virulence after bird-to-bird passage.
Guy et al. (1991) studied this question. Chicken-embryo-origin modified-live ILT vaccine virus showed increased virulence and mortality after serial bird-to-bird passage, whereas tissue-culture-origin virus did not.