Subclinical circulation of recombinant infectious laryngotracheitis virus in vaccinated commercial layers

In Australia, outbreaks of infectious laryngotracheitis (ILT) have been observed 15–20 weeks after vaccination with attenuated ILT vaccines. This study evaluated the susceptibility of vaccinated commercial layer hens to ILT virus (ILTV) infection after virulent challenge. ISA Brown hens (n = 84) from a commercial farm were acquired at 22, 49, and 62 weeks old (wo), corresponding to 4, 6, and 7 doses of Class 1 attenuated ILT vaccine and 12, 14, and 2 weeks post-last vaccination (wpv), respectively, and transported to an isolator facility. Upon arrival at the facility, 82% (69/84) of hens were positive for ILTV DNA in choanal cleft swabs. Detection rate (%) and load (least squares mean ± standard error) were lowest in 22 wo (71%, 4.94 ± 0.61) and highest in 62 wo (93%, 7.09 ± 0.42) hens. ILTV typing of 12 swabs per age group revealed that 100% of 22 wo samples were Class 1. Surprisingly, a vaccine-like recombinant strain (Class 7b), associated with recent Australian outbreaks, was detected in 41.7% of 49 wo and 25% of 62 wo hens. Following challenge with Class 9 ILTV (n = 42), birds showed no clinical signs, with 53.8%, 15.4% and 0% of samples positive for Class 9 ILTV in the 22, 49 and 62 wo groups, respectively. This study highlights that current Australian ILT vaccination protocols appear to prevent clinical disease, but sterilising immunity wanes with time, permitting infection as early as 13 wpv. Silent circulation of field strains heightens epidemiological risks, underscoring the importance of enhanced biosecurity and more effective vaccines.