Abstract
Understanding the mechanisms of transmission of infectious laryngotracheitis virus (ILTV) is critical to proper control as both vaccine and wild-type strains circulate within chicken flocks with potential adverse consequences. The relative efficiency of transmission by direct contact between chickens and airborne transmission has not been investigated. Furthermore, relatively high levels of ILTV DNA have been detected in poultry dust and blood but the infectivity of these is unknown. In this study, comparison of in-contact and airborne transmission of two vaccine and one field strain of ILTV revealed that all transmitted to 100% of in-contact birds by 6 days post-exposure (dpe). Airborne transmission without contact resulted in 100% transmission by 14 and 17 dpe for the wild-type and Serva vaccine virus but only 27% transmission by 21 dpe for the A20 vaccine virus. The infectivity of dust or extracts of dust and blood or plasma from infected chickens at various stages of infection was assessed by inoculation into susceptible chickens. There was no transmission by any of these materials. In conclusion, direct contact facilitated efficient ILTV transmission but the virus was unable to be transmitted by dust from infected chickens suggestive of a limited role in the epidemiology of ILTV.
Highlights
Avian infectious laryngotracheitis (ILT) is an important and widespread respiratory and ocular disease of chickens [1]
We have recently reported poor airborne transmission of vaccine strains of infectious laryngotracheitis virus (ILTV) (A20, SA2 and Serva) relative to virulent field strains [24] but the importance of direct physical contact between chickens in transmission within flocks has not been investigated to date
In the present experiment, direct contact between chickens was shown to enable rapid transmission of both virulent and vaccine strains of ILT, but sharing a common airspace without direct contact between birds greatly slowed the rate of transmission, for the A20 vaccine virus
Summary
Avian infectious laryngotracheitis (ILT) is an important and widespread respiratory and ocular disease of chickens [1]. The disease affects the conjunctiva and tracheal mucosa resulting in inflammation, serous or mucous discharge, haemorrhagic in severe cases, coughing and dyspnoea associated with tracheal necrotic plugs, In Australia, ILT remains a disease of significant concern to the broiler chicken industry due to a prolonged outbreak in high producing areas in Victoria and New South Wales (NSW) that has not been brought under adequate control, despite the availability and use of live vaccines [6, 7]. Live attenuated vaccines may show reversion to virulence following passage between birds after vaccination [13] and in Australia have been responsible for the emergence of new virulent strains due to natural recombination between them [14] with associated increased replication rate, infectivity and enhanced transmissibility to in-contact birds of some recombinants [15]
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