Molecular Characterization of Vaccine-Derived Mutants of Avian Metapneumoviruses Isolated from Turkeys in Egypt

Avian metapneumovirus (aMPV) causes acute respiratory infection in turkeys with sudden onset and rapid spread in the infected flocks. Vaccinated flocks frequently showed clinical signs typical like the non-vaccinated flocks in Egypt, this situation suggests a virulent vaccine virus was circulating in the field. Oropharyngeal and nasal sinus swabs were collected from 100 turkeys from 10 commercial farms with mild to moderate respiratory signs. Virus isolation and propagation was done in SPF embryos of chicken eggs via yolk sac inoculation and further passed in Vero cells. RT-PCR for detection of aMPV was done on field samples as well as on virus isolates. Genotyping of viruses was done through RT-PCR targeting Fusion (F) and the surface Glycoprotein (G) genes and partial sequencing of PCR products was used for genetic and phylogenetic analyses. The results were compared with two live attenuated commercial vaccines commonly used in the field. Analysis of nucleotide and amino acid sequences for F and G genes indicated that they were very closely related to each other and belonging to B subtype of aMPV isolated from Europe. The Egyptian viruses were very close to VCO3 vaccine strain, indicating their possible origin. Phylogenetic analysis indicates clustering of the Egyptian field viruses with European strains of B subtype. These results support that they may have a common progenitor from vaccine strains and suggesting evidence of reversion of attenuated vaccine virus to virulence. This is the first report describing the emergence of vaccine-derived mutants from the B subtype aMPV in broiler turkeys in Egypt.