Two-year surveillance of infectious laryngotracheitis virus in layer farms from Southern Brazil: a seroepidemiological, molecular, and pathological approach

Infectious Laryngotracheitis (ILT), caused by Gallid Herpesvirus 1 (GaHV-1), is a contagious and notifiable disease. In Santa Catarina, southern Brazil, it was exotic until 2020, when it was diagnosed in a commercial layer farm in São Ludgero/SC. This study consists of two phases: the first assesses ILT prevalence in municipalities near São Ludgero in 2020, while the second examines the entire state of Santa Catarina in 2021. A total of 44 farms were sampled in 2020 and 49 in 2021, including commercial layer and rearing farms. Twenty chickens per farm were sampled for serology, and 10 were euthanized for necropsy, respiratory organ collection and histopathological evaluation. Conjunctiva, larynx, trachea, and trigeminal ganglion samples were subjected to real-time polymerase chain reaction (qPCR) for ILT virus. Fisher's exact test and multivariate logistic regression (α=0.05) were applied to identify risk factors. In 2020, 95.4 % of farms were seropositive, and 88.1 % of seropositive farms tested qPCR-positive. In 2021, these rates were 65.3 % and 21.8 %, respectively. Syncytial cells and intranuclear inclusion bodies were observed in three farms (7.1 %) in 2020 but were absent in 2021. Multivariate analysis confirmed flock replacement with older chickens as a significant risk factor for ILT in 2021 (p < 0.05). These findings underscore the widespread ILT prevalence in São Ludgero and its dissemination across Santa Catarina, emphasizing the importance of biosecurity measures in preventing outbreaks in commercial layer farms.