Avian Metapneumoviruses in Italy: Evidence of attachment protein evolution coincident with mass live vaccine introduction

Avian metapneumoviruses (AMPV) of subtype B dominate over other subtypes on Italian poultry farms in northern Italy. AMPVs from the Veneto region of Italy between 1987 and 2007 were sequenced in their attachment (G) and fusion (F) protein genes, together with other subtype B AMPVs from other parts of Western Europe, collected prior to 1994. All viruses in the survey had very similar predicted F sequences whereas the predicted G protein sequences found in Italy from 2001 were distinctly different from those found in pre 1994 viruses. Nonetheless pre 1994 Italian AMPVs were more similar to post 2000 AMPVs than other pre 1994 viruses from other parts of Europe, thereby showing that the later viruses had probably evolved from early Italian viruses. The occurrence of the later variants followed introduction of the mass administration with a single subtype B vaccine and its G protein sequence placed it clearly in the pre 1994 cluster. The possibility of the vaccine having driven field virus evolution in order to evade immune pressure cannot be excluded.