Studies on a live virus vaccine against infectious laryngotracheitis of chickens. I. Biological properties of attenuated strain C7.

The biological properties of tissue-culture-modified strain C7 of infectious laryngotracheitis (ILT) virus were studied. Strain C7 formed well-defined small white pocks that were distinguishable from those formed by the original strain when it was inoculated onto the chorioallantoic membrane of embryonating chicken eggs (pock marker). Strain C7 grew in chicken kidney cell cultures and embryonating chicken eggs better than the original strain. When inoculated intratracheally into chickens, strain C7 propagated primarily in the larynx, trachea, and lung, but the virus was unrecoverable after 6 days postinoculation. No contact infection was demonstrated among chickens. These biological characteristics of strain C7 were stable through five consecutive back-passages in susceptible chickens. In chickens vaccinated ocularly at 14 days of age or older, strain C7 gave good protection against challenge with the virulent strain of ILT virus. These results indicate that strain C7 may be an adequate candidate for a live virus vaccine against ILT.