Pathogenicity evaluation of neuraminidase-negative H5 and H7 viruses in day-old chicks and adult chicken

We evaluated the virulence of replication-competent NA-negative (no functional neuraminidase) avian influenza viruses in chicken of different ages. Interestingly, the virulence of a previously described NA-negative H5-virus (Kalthoff et al., J Virol 2013;87:13556) is very much age-dependent. Day-old-chicks succumb to a systemic infection, while chicken one week of age do not show any clinical signs at all. Therefore, the day-old-chick organism is most likely not able to restrict replication of the virus as older chicken do. In addition to virulence dependency on host age, the exchange of the H5 HA by an H7 HA for an H7NA-deleted HA reassortant remarkably led to a highly pathogenic phenotype even in adult chicken. This provides evidence, that the virulence of the H7-subtype is less dependent on the NA-protein than that of the H5-virus. These striking observations suggest that the HA/NA interplay might be governed by other mechanisms in HPAIV with the different serotypes H5 or H7.