SUMMARY. Infectious laryngotracheitis virus is thought to exit the host in respiratory aerosols and enter by inhalation of these. High levels of ILTV DNA have been detected in excreta raising the possibility of alternative routes of shedding from the host. However, it is not known whether the ILTV DNA in excreta represents infective virus or not. This study investigated transmission of wild type and vaccinal ILTV from infected to susceptible commercial meat chickens. Airborne and excreta mediated transmission of two field isolates of ILTV (Classes 9 and 10) and three vaccine strains (SA2, A20 and Serva) was tested. To test airborne transmission, air from isolators containing infected birds was ducted through a paired isolator containing uninfected chickens. To test excreta transmission, aliquots were prepared from excreta containing a high level of ILTV DNA within the first week after infection. Chicks were infected bilaterally by eye drop. Clinical signs were monitored daily and choanal cleft swab samples for ILTV detection by qPCR were collected at 4, 8, 15, 22 and 28 days post-infection (DPI) in the airborne transmission study and at 7 and 14 DPI from the excreta transmission studies. There was no transmission of ILTV from excreta suggesting that ILTV is inactivated during passage through the gut. All strains of ILTV were transmitted by the airborne route but only to a limited extent for the vaccine viruses. The field viruses induced clinical signs, pathology and greatly elevated ILTV genome copies in swabs. In summary, these findings confirm the suspected airborne transmission of ILTV, demonstrate differential transmission potential between wild type and vaccine strains by this route and indicate that excreta is unlikely to be important in the transmission of ILTV and the epidemiology of ILT.

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