A single polymerase (L) mutation in avian metapneumovirus increased virulence and partially maintained virus viability at an elevated temperature

Previously a virulent avian metapneumovirus (AMPV) farm isolate Italy 309/04 was shown to have derived from a live vaccine. Virulence due to the five nucleotide mutations associated with the reversion to virulence, was investigated by their addition to the vaccine using reverse genetics. Virulence of these recombinant viruses was determined by infection of one-day-old turkeys. Disease levels resulting from the combined two matrix (M) mutations was indistinguishable from that produced by the recombinant vaccine, whereas the combined three L gene mutations increased disease to a level (p<0.0001) which was indistinguishable from that caused by the revertant Italy 309/04 virus. Testing of the L mutations individually showed that two did not increase virulence while the third, corresponding to an asparagine to aspartic acid substitution, produced virulence indistinguishable from that caused by Italy 309/04. In contrast to the vaccine, the virulent mutant also showed increased viability at temperatures typical of turkey core tissues. The notion that increased viral virulence resulted from enhanced ability to replicate in tissues away from the cool respiratory tract, cannot be discounted.