Italian field survey reveals a high diffusion of avian metapneumovirus subtype B in layers and weaknesses in the vaccination strategy applied

The current information on prevalence of avian metapneumovirus (AMPV) infection in layers is fragmentary and its true impact on egg production often remains unknown or unclear. In order to draw an epidemiological picture of AMPV presence in layer flocks in Italy, a survey was performed on 19 flocks of pullets and layers based on longitudinal studies or sporadic samplings. AMPV was detected by RT-PCR, and blood samples were collected for serology by AMPV ELISA. Occurrence of respiratory signs and drop in egg production were recorded. Possible involvement of infectious bronchitis (IB) and egg drop syndrome (EDS) viruses which could have caused loss of egg production we ruled out by RT-PCR for IB virus and haemagglutination-inhibition (HI) for EDS virus. AMPV of subtype B only was found in both pullet and layer farms. Surveys of pullets showed that most groups become infected prior to the onset of lay without showing clear respiratory signs. At point of lay these groups were serologically positive to AMPV. In two layer flocks, egg drops were observed and could be strongly linked with the presence of AMPV infection. Results were correlated with AMPV vaccination programmes applied to the birds in three flocks on the same farm. Only a vaccination program which included two live and one killed vaccines gave complete protection from AMPV infection to the birds, while a single live vaccine application was not efficacious. In conclusion the current study gives an inside view of field AMPV diffusion in Italy and control in layers.