30.08.2014

Wong et al (2014) Virology 468-470 (2014) 72–80

Avian influenza viruses (AIVs) are classified by the World Organization of Animal Health (OIE) into two pathogenicity groups: Low-pathogenic avian influenza (LPAI) viruses cause mild or asymptomatic infections in chickens, and highly pathogenic avian influenza (HPAI) viruses cause severe morbidity and mortality. Of the 16 influenza virus subtypes isolated from avian species to date, only some strains of H5 and H7 are classified as HPAI viruses.
The virulence of HPAI viruses has been largely attributed to the presence of additional, often basic amino acids at the hemagglutinin (HA) protein cleavage site. During virus replication, the precursor HA protein (HA0) is cleaved by proteases into functional subunits HA1 and HA2 to allow for fusion with host membranes. LPAI strains possess a monobasic XXR/G cleavage motif and are cleaved by tissue-restricted proteases while, HPAI strains have inserted peptides or at least 4 basic amino acids RX (K/R)R/G (known as multibasic cleavage site - MBCS) at the cleavage site which enables cleavage by ubiquitous furin-like proteases, leading to increased tissue tropism and multi-organ involvement in infected hosts. In laboratory, these HPAI viruses do not require exogenous trypsin for in vitro growth. Importantly, HPAI strains are believed to emerge when LPAI strains are introduced into domestic poultry and acquire additional amino acids in the HA cleavage site during virus replication.
While MBCS is a prerequisite to a highly pathogenic phenotype, it is not the only contributing factor. Non-H5/H7 subtypes (including H4) influenza viruses can support a MBCS but enhanced pathogenicity also depended on the internal gene constellation.
However, with the exception of a canine H3N2 virus isolated from in 2009 in China, non-H5/H7 subtypes with MBCS-like motifs are not isolated in the field. In August 2012, Wong et al isolated an H4N2 virus from a farm in California that reported a 1.6% mortality in its quail flock. Sequence analyses revealed that A/Quail/California/D113023808/2012 (Quail/CA12) contained a multibasic amino acid motif in the cleavage site of HA and was most closely related to a duck-origin H4N2 virus, A/Pekin Duck/California/P30/2006 (PD/CA06), isolated in 2006, with a typical monobasic cleavage motif. The researchers found that Quail/CA12 is a reassortant virus with the HA and neuraminidase (NA) gene most similar to a duck-isolated H4N2 virus, PD/CA06 with a monobasic HA cleavagesite. Quail/CA12 required exogenous trypsin for efficient growth in culture and caused no clinical illness in infected chickens. Quail/CA12 had high binding preference for α2,6-linked sialic acids and showed higher replication and transmission ability in chickens and quails than PD/CA06. Although the H4N2 virus remained low pathogenic, these data suggests that the acquisition of MBCS in the field is not restricted to H5 or H7 subtypes.