The seal H3N8 virus from the 2011 outbreak has naturally acquired this D701N substitution (Table 1), which was confirmed by clonal sequencing directly from infected tissue (50 clones/animal) to be the only phenotype present in all five animals.
The above comment is from the newly released mBio paper “Emergence of Fatal Avian Influenza in New England Harbor Seals” on a Columbia University analysis of sequences from 5 fatal infections in 2011 of New England habor seals by an avian H3N8. As noted above, all five PB2 sequences had D701N whih has been previously found in human H5N1 cases as well as lab experiments associating the change with increased transmission in mammals.
PB2 H5N1 changes have been of interest since E627K was identified in a sub-set of 1997 Hong Kong cased, which was followed by lab experiments demonstrating increased virulence in mice. The body temperature of birds is significantly higher than mammals, and position 627 can impact polymerase activity in a temperature sensitive manner. E627K is present in seasonal flu and enzymatic activity is optimal at 33 C, the temperature of a human nose in the winter. In contrast, PB2 with an E at position 627 has optimal activity at 41 C, the temperature of an avian intestine.
Recent H5N1 transmission experiments used H5N1 PB2 sequences with E627K, or H1N1pdm09 PB2 which has G590S and Q591R, which are known changes that can substitute for E627K to optimize polymerase activity in mammals.
In addition to G590S/Q591R substitution for E627K, another change, D701N also can substitute for E627K to increase transmission efficiencies in mammals. The PB2 sequence from the fatally infected seals had avian polymorphisms at positions 590, 591, and 627, but position 701 had D710N which is associated with mammalian transmission.
This change raises concerns that the avian H3N8 in the harbor seals has adapted to more efficient transmission and a higher viral load leading the the high mortality noted in 2011 for seals in the waters off the New England coast.
Further analysis of the PB2 sequences identified new acquisitions, including D710N found in H3N8 sequences from dogs and horses.