Evaluation of the protection elicited by direct and indirect exposure to live attenuated infectious laryngotracheitis virus vaccines against a recent challenge strain from the United States.

In a recent study (Oldoni & García, 2007), some field strains of infectious laryngotracheitis viruses (ILTV) were characterized as genotypically different (group VI) from ILT vaccine strains. The objective of this study was to evaluate the protection elicited by one chicken embryo origin (CEO) and one tissue culture origin (TCO) vaccine against a field isolate from group VI after direct and indirect exposure to ILTV live attenuated vaccines. In phase 1 of the experiment, non-vaccinated chickens were placed into contact with the eye drop vaccinates for a period of four weeks after vaccination. Transmission of the vaccine virus to these in-contact birds was demonstrated by real time PCR and antibody production, although the in-contact birds did not become protected against disease when subsequently challenged in phase 2 of the experiment. This emphasized the importance of uniform vaccination to obtain adequate protection, both to avoid the occurrence of susceptible chickens, and to minimize the potential for reversion to virulence of live-attenuated vaccines. In phase 2, protection against challenge with a group VI field virus was assessed four weeks after vaccination by scoring clinical signs and mortality, and quantifying weight gain. Sentinel birds were added to the groups one day after challenge to assess shedding of challenge virus, using real time PCR and virus isolation, during the period 2 to 12 days post challenge. The results showed that the CEO and TCO eye drop-vaccinated chickens were protected against challenge with the group VI virus, even though it was genetically different from the vaccine strains, and that challenge virus was not transmitted from these protected birds to the sentinels.