SUMMARY. Infectious bronchitis virus (IBV) is a gammacoronavirus that primarily induces an upper respiratory disease in chickens, also affecting the urogenital tract and occasionally leading to a condition called false layer syndrome (FLS), where sexually mature hens ovulate normally, but are unable to lay eggs. Here, we describe an outbreak of FLS in Arizona from which an IBV variant that is nearly 90% homologous to DMV/1639 using the S1 gene, named AZ/FLS/17, was isolated and used in challenge experiments. Three-day-old specific-pathogen-free (SPF) chicks were challenged with AZ/FLS/17 or M41 in high and low doses and the disease outcomes were compared. Overall, no differences in microscopic lesions or viral loads in the reproductive tract were detected between AZ/FLS/17- and M41-infected birds. To minimize the losses linked to FLS in the problematic flocks, an updated live-attenuated IBV vaccine protocol including the use of the Ma5 strain at the hatchery was implemented, resulting in a drastic reduction of false layers in the subsequent flocks. To monitor the circulation of wild-type and vaccine strains in this population, a molecular surveillance study was performed. Samples were collected at one, 7, 14 and 21 days of age, and from laying hens at 30 and 36 weeks of age. As the birds were older, the IBV strains detected were more diverse than at one and 7 days of age. Nevertheless, live vaccine combinations are still widely used to decrease the losses caused by FLS in commercial egg laying flocks worldwide.

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