Subclinical circulation of recombinant infectious laryngotracheitis virus in vaccinated commercial layers.

Vaccinated layers shed ILTV subclinically; sterilizing immunity wanes within weeks of vaccination.

This study investigated infectious laryngotracheitis virus (ILTV) circulation in vaccinated commercial layer hens in Australia, where outbreaks occur 15-20 weeks post-vaccination. Eighty-four ISA Brown hens of varying ages (22, 49, and 62 weeks old) with different vaccination histories were evaluated. Upon arrival, 82% tested positive for ILTV DNA via choanal cleft swabs, with detection rates and viral loads increasing with age. Notably, a vaccine-like recombinant strain (Class 7b) associated with recent Australian outbreaks was identified in 41.7% of mid-aged and 25% of older hens, while younger birds carried only Class 1 vaccine strain. Following challenge with virulent Class 9 ILTV, vaccinated birds remained clinically healthy but showed decreasing susceptibility to superinfection with age. The study demonstrates that current vaccination protocols prevent clinical disease but fail to establish sterilizing immunity, permitting silent viral circulation within vaccinated flocks. This subclinical shedding poses significant epidemiological risks for disease transmission and strain evolution, emphasizing the necessity for improved biosecurity measures and more effective vaccine formulations.

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