A Low Pathogenic H5N2 Influenza Virus Isolated in Taiwan Acquired High Pathogenicity by Consecutive Passages in Chickens

概要

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H5N2 viruses were isolated from cloacal swab samples of the apparently healthy chickens in Taiwan in 2003 and 2008 on the surveillance of avian influenza. Each of the viruses was eradicated by stamping out. Official diagnosis reported that Intravenous Pathogenicity Index (IVPI) of the isolates were 0.00 and 0.89, respectively, indicating that these were low pathogenic strains although the hemagglutinin of the strain isolated in 2008 (Taiwan08) had multi-basic amino acid residues at the cleavage site (PQRKKR/G). In the present study, these H5N2 viruses were assessed for their intravenous and intranasal pathogenicity for chicken. It was examined whether Taiwan08 acquires pathogenicity through consecutive passages in chickens. Intravenous pathogenicity of Taiwan08 depended upon age of chickens used for IVPI test; all of the eight-week-old chickens intravenously inoculated with Taiwan08 showed clinical signs, but survived for ten days post inoculation (IVPI=0.68) whereas all the six-week-old chickens died (IVPI=1.86). Taiwan08-P8, which were passaged in chickens for eight times, killed all the eight-week-old chickens (IVPI=2.36). The four-week-old chickens died by intranasal inoculation of Taiwan08-P8, indicating that Taiwan08 should have become highly pathogenic during the circulation in chicken flocks. These results emphasize the importance of stamping out policy against avian influenza even if the IVPI of the causal virus is low.