Studies on the live-virus vaccine against infectious laryngotracheitis of chickens. II. Evaluation of the tissue-culture-modified strain C7 in laboratory and field trials.

Laboratory and field tests were conducted to evaluate the immunogenicity of a live-virus vaccine against infectious laryngotracheitis (ILT) of chickens that was prepared using tissue-culture-modified strain C7. Eighty-three percent or more of chickens vaccinated by the ocular (OC) or intranasal (IN) route with 10(4.75) TCID50 of the attenuated strain C7 at 50 days of age were protected against challenge with a virulent strain of ILT virus without showing any clinical signs for 4 weeks post-vaccination (PV). When vaccine was administered by aerosol, however, only 65% of vaccinated chickens were protected against challenge. Fifty-seven percent of chickens vaccinated at 70 days of age maintained immunity for 6 months PV. Immune response in younger chickens was inferior to that in older ones. In the field trials, clinical observation revealed no adverse reactions caused by the vaccination, and 60% or more of broilers and 80% or more of layers vaccinated by the OC route were protected against challenge at 4 weeks PV. These results confirmed the safety and efficacy of the vaccine.