Insights into infectious laryngotracheitis virus vaccine-mediated protection: a comparative assessment of eye drop and vent brush vaccination methods

Live attenuated chicken embryo origin (CEO) vaccines against infectious laryngotracheitis virus (ILTV) are commonly used in long-lived chickens in Australia and generally provide good protection when administered by eye drop (ED). Although vent brush (VB) vaccination provides excellent protection, the associated immune responses are not well characterized. This study compared vaccine-mediated protection conferred by SA2 ILTV vaccine strain administered via ED or VB to 4-week-old layer chicks against a virulent (class 9) challenge 2 weeks after vaccination. No significant differences in ILTV DNA in choanal swabs were observed between vaccinated-challenged and non-challenged groups from 4-11 days post challenge (DPC), indicating that both VB and ED vaccination prevented increases in viral load after challenge, although neither prevented infection. Both vaccination routes resulted in high protection indices against clinical signs after challenge (VB, 99%; ED, 90%) and pathological lesion scores in conjunctiva (VB, 83%; ED, 74%) and trachea (VB, 78%; ED, 69%). The non-vaccinated challenged group exhibited significant over-expression of complement component 5a receptor 1, interleukin 1 beta, interleukin 6, myxovirus resistance 1 (MX1), and signal transducer and activator of transcription 1 (STAT1) in both conjunctiva and trachea at DPC 4, whereas ED- and VB-vaccinated challenged groups displayed similar gene expression profiles, resembling those of vaccinated non-challenged group except for elevated MX1 expression at DPC 4 and 7, and STAT1 expression at DPC 4 in the trachea. In conclusion, VB and ED vaccination conferred equivalent clinical protection with gene expression profiles similar to vaccinated, non-challenged chickens, indicating comparable immunological efficacy.