Studies on a Tissue-Culture-Modified Infectious Laryngotracheitis Virus

Abstract

The ASL L-6 virulent laryngotracheitis virus strain was adapted to primary monolayer avian-origin tissue-culture systems. Systematic passage of the strain in cell culture resulted in 2 changes in virus characteristics, which were manifested by the ability of the virus to form atypical lesions on the chorio-allantoic membrane of chicken embryos, and by significantly decreased virulence of the virus for susceptible chickens. Serological tests showed that the modified virus is closely related to the parent strain from which it was derived. Initial studies on strain stability demonstrated that the modified virus strains its distinguishing characteristics through 2 back passages in embryos and chickens. Desiccated vaccine prepared from the modified virus provided good protection against virulent challenge of chickens vaccinated ocularly, intranasally, or through the vent. Similarly, good immunity against a variety of challenge strains was demonstrated when the vaccine was applied ocularly. On the other hand, spray and water administration of the modified virus resulted in poor immunity. Studies also showed that ocular application of the modified virus to birds 2 weeks old or older resulted in good immunity, whereas those vaccinated younger than 2 weeks did not develop dependable resistance. Laboratory experiments indicated that immunity lasting for at least 22 weeks develops in birds vaccinated at 2 to 6 weeks of age. The modified virus spreads to unvaccinated contact controls without deleterious effects.