Marked differences in virulence of three Australian field isolates of infectious laryngotracheitis virus in meat and layer chickens.

The objectives of this study were to compare the virulence of contemporary infectious laryngotracheitis virus (ILTV) field isolates of classes 9, 10, and 14 in meat and layer chickens, and to evaluate cloacal and oropharyngeal swabs and dust as sample types for ILTV detection. A total of 211 chickens were divided into groups and inoculated with ILTV class 9, 10, or 14, or sham-inoculated via eye drop at 15 or 22 days of age. Chickens were euthanized at 5 and 9 days post-infection. Virulence was assessed by scoring of clinical signs (conjunctivitis, dyspnoea, and demeanour), ILTV genomic copies (GC) in oropharyngeal and cloacal swabs, mortality and microscopic lesions in conjunctiva and trachea. Class 14 caused subclinical infection, while inoculation with class 9 or class 10 resulted in severe clinical signs and microscopic lesions. Compared to class 14 (2.25 ± 0.36 log10 GC), higher viral load was observed in oropharyngeal swabs of classes 9 (7.86 ± 0.48) and 10 (7.53 ± 0.36), with a higher proportion of positive oropharyngeal and cloacal swabs in the latter groups (P < 0.0001). Viral detection in cloacal swabs was delayed at early stages of infection compared to oropharyngeal swabs. Dust samples from class 9- and class 10-inoculated groups showed a trend towards higher GC than that of class 14. Overall, clinical scores, mortality, viral load, and microscopic lesions were similar for classes 9 and 10, but class 9 caused more severe disease in layer chickens than meat chickens. In summary, ILTV classes 9 and 10 exhibited severe virulence, while class 14 exhibited very mild virulence. RESEARCH HIGHLIGHTS Wide variation in the virulence of three field Australian field ILTV strains. Class 9 and class 10 strains were highly virulent, while class 14 was mildly virulent. The highly virulent strains were associated with significantly higher viral genome copies in various sample types than the mildly virulent strain.