Stable Population, Shifting Clades: A 17-Year Phylodynamic Study of IBV GI-19-like Strains in Spain Reveals the Relevance of Frequent Introduction Events, Local Dispersal and Recombination Events

Infectious bronchitis virus (IBV) remains a common pathogen in poultry production. Although its clinical and economic impact in Europe has markedly declined in recent decades due to extensive vaccination, ongoing viral circulation continues to pose risks to animal health and provides opportunities for viral evolution. In this study, we investigated the molecular epidemiology of GI-19 and related strains in Spain using samples collected between 2008 and 2025. Partial <i>S1</i> sequencing revealed a complex scenario involving three major clades and several minor ones, the latter likely resulting from independent introduction events from north-western Europe, particularly Denmark. Six distinct recombination events involving GI-13 and GI-19 parental strains-some apparently vaccine derived-were also identified, several of which showed wide geographical spread and long-term persistence. Both recombinant and non-recombinant variants were detected across multiple regions and production systems, indicating strong epidemiological connectivity among broilers, layers, and breeders. Although overall viral population size appeared stable over time, shifts in the predominance of specific clades and recombinant groups were observed, possibly reflecting fitness advantages of newly introduced or evolved variants and reduced cross-protection from existing immunity. These findings highlight the susceptibility of the poultry sector to repeated introductions, mixing, and the dissemination of IBV variants. Strengthened molecular surveillance and tailored control strategies, together with the periodic evaluation of vaccination practices and population immunity, are needed to limit viral circulation, reduce recombination opportunities, and mitigate the impact of IBV.