Field survey of Avian Metapneumovirus in Northern Italy

Avian Metapneumovirus (AMPV) is the casual agent of Turkey Rhinotracheitis (TRT), and also causes a respiratory infection in chickens, which can result in Swollen Head Syndrome (SHS). A survey of AMPV infection in 122 turkey and 48 broiler farms, in a highly densely populated area of Northern Italy, performed from January 2011 to February 2013, is reported. AMPV positive samples were detected using or RT-nested PCR or qRT-PCR both able to detect and differentiate AMPV subtype A and B. All samples but one resulted positive for AMPV subtype B confirming the high prevalence of this subtype in Italy. Only one AMPV subtype A was detected in a turkey farm located in Verona province.The majority of AMPV detections were of field origin, circulating mainly from 9 to 12 weeks of age in turkeys and from 5 to 7 weeks of age in broilers, and associated with respiratory symptoms. Turkeys were all vaccinated at 1 day of age in the hatchery. The reasons for vaccine failure could be field virus changes in key antigenic regions that allow replication and leading to disease in well vaccinated birds. Our study revealed that VCO3 vaccine strain can be detected in turkeys up to many weeks after vaccination, with a high prevalence in the first weeks after hatch. This confirmed the pattern seen previously for subtype A and B live vaccine which has been shown to persist on farm for 4-5 weeks. Uniquely in this study, apparently vaccine derived strains were observed in older turkeys of up to 84 days of age. It is unlikely that a vaccine persists on the same farm for such a long period, so it is likely that as observed in a previous study for AMPV subtype A, a vaccine-derived virus was present in the environment being able to circulate and affect turkey farms.