Infectious laryngotracheitis (ILT), caused by Gallid herpesvirus type 1, is a highly contagious upper respiratory and conjunctival disease of poultry. The virus (ILTV) 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, dust, blood or plasma and in various organs outside the respiratory tract; raising the possibility of alternative routes of shedding from the host. However, it is not known whether the ILTV DNA in excreta, dust and blood or plasma represents infective virus. Wind borne transmission is also implicated in the epidemiology of ILT. However, despite the widespread acceptance of airborne transmission of ILTV there appear to have been no controlled experiments to investigate the efficiency of airborne transmission.
We investigated transmission of wild type and vaccinal ILTV from infected to susceptible commercial meat chickens in multiple experiments in a PC2 isolator facility. Airborne transmission (Class 9, Class 10, SA2, A20 and Serva) and transmission by direct contact (Class 9, A20 and Serva), infection by inoculation of blood/plasma (Class 9, A20 and Serva) or dust and extracts of dust (Class 9) or excreta (Class 9, Class 10, SA2, A20 and Serva) were tested. There was no transmission of ILTV in extracts of excreta or dust from infected birds administered via eye drop, by fresh dust (~10mg) collected from infected isolators and insufflated into the nares, choanal cleft and trachea or blood (1ml) from infected birds inoculated intra coelomically with plasma (60 µl) administered by eye drop. We also carried out a preliminary investigation of coarse spray vaccination with Serva and A20 vaccines.
All strains of ILTV transmitted by the airborne route, whether through a 2 m air hose between isolators, or by sharing a common airspace without physical contact. Wild type virus transmitted very effectively by this route, but transmission of the vaccine viruses was significantly less efficient, particularly when via the 2 m air hose. The field viruses induced clinical signs, pathology, and greatly elevated ILTV genome copies in airborne exposed birds. However, clinical signs were less severe and delayed compared to birds infected by eye drop, or in direct contact with infected birds. This was also reflected in reduced and delayed ILTV GC in choanal cleft swabs. When birds shared a common airspace, transmission of Serva vaccine virus was significantly greater than that of the A20 vaccine virus as assessed by ILTV GC. Birds in direct contact with each other had significantly higher ILTV GC than birds sharing a common airspace (P < 0.0001). Coarse spray vaccinated chickens (one dose in 1ml/bird) showed no adverse reaction and greater vaccine take with A20 than Serva.
These findings confirm the suspected airborne transmission of ILTV, demonstrate differential transmission potential between wild type and vaccine strains by this route, and the importance of degree of contact between chickens on the transmission of ILTV. ILTV GC detected in excreta appears to reflect non-infective virus inactivated by passage through the gut with no role in ILTV transmission. As excreta is the main component in poultry dust (Ahaduzzaman et al., 2021, these proceedings), this inactivated ILTV may explain the failure to transmit ILTV in dust or extracts of dust from infected birds. Similarly, the detection of ILTV GC in blood seems to be due to circulation of a non-infective form of ILTV and does not appear to be infectious.
ACKNOWLEDGEMENTS: We acknowledge AgriFutures Chicken Meat Project PRJ-010639 for funding and thank Danielle Smith for technical assistance.
Presented at the 32th Annual Australian Poultry Science Symposium 2021. For information on the next edition, click here.