Growth Kinetics and Transmission Potential of Existing and Emerging Field Strains of Infectious Laryngotracheitis Virus

Abstract

Attenuated live infectious laryngotracheitis virus (ILTV) vaccines are widely used in the poultry industry to control outbreaks of disease. Natural recombination between commercial ILTV vaccines has resulted in virulent recombinant viruses that cause severe disease, and that have now emerged as the dominant field strains in important poultry producing regions in Australia. Genotype analysis using PCR—restriction fragment length polymorphism has shown one recombinant virus (class 9) has largely replaced the previously dominant class 2 field strain. To examine potential reasons for this displacement we compared the growth kinetics and transmission potential of class 2 and class 9 viruses. The class 9 ILTV grew to higher titres in cell culture and embryonated eggs, but no differences were observed in entry kinetics or egress into the allantoic fluid from the chorioallantoic membrane. In vivo studies showed that birds inoculated with class 9 ILTV had more severe tracheal pathology and greater weight loss than those inoculated with the class 2 virus. Consistent with the predominance of class 9 field strains, birds inoculated with 102 or 103 plaque forming units of class 9 ILTV consistently transmitted virus to in-contact birds, whereas this could only be seen in birds inoculated with 104 PFU of the class 2 virus. Taken together, the improved growth kinetics and transmission potential of the class 9 virus is consistent with improved fitness of the recombinant virus over the previously dominant field strain.

Development and application of a TaqMan single nucleotide polymorphism genotyping assay to study infectious laryngotracheitis virus recombination in the natural host

“…Virus titrations were performed to calculate plaque forming units per mL (PFU/mL) as described in Devlin et al [23]. In vitro multi-step virus growth kinetics and entry kinetics studies were performed as described in Devlin et al [23] and Lee et al [24], respectively.…”

Impacts of poultry vaccination on viruses of wild bird

“…The importance of recombination as a mechanism of genome evolution in many DNA viruses is increasingly being recognised, including in virus families that are favoured as vaccine vectors, such as herpesviruses and adenoviruses [37][38][39]. Potential outcomes of recombination include a shift in host range, increased virulence, enhanced viral replication and enhanced virus transmission [40][41][42].…”

Genomic recombination between infectious laryngotracheitis vaccine strains occurs under a broad range of infection conditions in vitro and in ovo

“…Growth kinetics (multi-step growth curve, entry kinetics and kinetics of plaque development) of Serva and A20 strains of ILTV in CEK cells were measured as described previously [22]. Briefly, for multi-step growth curve, CEK cells in 24-well plates were infected with a MOI of 0.001 and cells and supernatant harvested at the indicated time points.…”

“…The capacity of Serva and A20 ILTV to spread from cell to cell was also compared by examining the kinetics of plaque development in CEK cells. The plaque area of 13 to 22 individual plaques photographed at different time points were measured using ImageJ (version 1.50i) [25], and compared using Student's t-test as described previously [22].…”

“…Progeny viruses were isolated using ten-fold dilutions of AFs, CAMs, and homogenates of cells and supernatant fluids (SNFs) inoculated onto CEK monolayers in 6-well plates as described previously [18,22,26].…”