Turkey haemorrhagic enteritis live vaccination controls disease and field virus circulation: molecular, clinical and pathological evidence in field longitudinal studies

Haemorrhagic enteritis (HE) is a viral disease affecting turkeys. Live haemorrhagic enteritis virus (HEV)attenuated vaccines have recently been introduced in Italy. In this study, longitudinal field trials were conducted to assess the efficacy and safety of the live HE vaccine. The study was carried out on two turkey farms. Birds housed in sheds 1 and 4 were assigned to the control and vaccinated groups, respectively. Birds in shed 4 were vaccinated at 28 days with a live attenuated vaccine. From 3 weeks of age until the end of the production cycle, five birds per group were selected weekly for clinical evaluation, euthanasia, necropsy, and sampling. Clinical and post-mortem scores assessed health status and duodenal and splenic lesions, respectively. Cloacal swabs and spleen were collected for HEV detection by PCR, allowing differentiation between vaccine and field strains. Selected samples were analysed by shotgun next-generation sequencing (NGS). Spleens were histologically examined. Clinical and post-mortem scores were significantly higher in control groups at 49, 56, and 63 days of age. Molecular analyses showed persistent detection of vaccine-like strains in vaccinated groups from 35-118 days, whereas field strains were sporadically detected. NGS analysis did not reveal co-infections. In unvaccinated groups, field strains were consistently detected. Histological examination revealed a marked reduction in splenic follicles. Follicle numbers subsequently recovered in vaccinated groups. These results demonstrate that the live HE vaccine effectively protects turkeys from infection until slaughter age. Vaccination limits circulation of field strains, reduces environmental infectious pressure, and minimizes the risk of clinical disease.RESEARCH HIGHLIGHTS Live haemorrhagic enteritis virus (HEV) vaccination reduced clinical signs and lesions under field conditions.Vaccination limited field strain circulation and environmental infection pressure.Vaccinated turkey flocks remained protected from HEV until slaughter age.