Characterising viable virus from air exhaled by H1N1 influenza-infected ferrets reveals the importance of haemagglutinin stability for airborne infectivity

Abstract

The transmissibility and pandemic potential of influenza viruses depends on their ability to efficiently replicate and be released from an infected host, retain viability as they pass through the environment, and then initiate infection in the next host. There is a significant gap in knowledge about viral properties that enable survival of influenza viruses between hosts, due to a lack of experimental methods to reliably isolate viable virus from the air. Using a novel technique, we isolate and characterise infectious virus from droplets emitted by 2009 pandemic H1N1-infected ferrets. We demonstrate that infectious virus is predominantly released early after infection. A virus containing a mutation destabilising the haemagglutinin (HA) surface protein displayed reduced survival in air. Infectious virus recovered from droplets exhaled by ferrets inoculated with this virus contained mutations that conferred restabilisation of HA, indicating the importance of influenza HA stability for betweenhost survival. Using this unique approach can improve knowledge about the determinants and mechanisms of influenza transmissibility and ultimately could be applied to studies of airborne virus exhaled from infected people.

Influenza A virus is transmissible via aerosolized fomites

“…Subsequently, when aerosolized under laboratory conditions, swine influenza viruses that transmit efficiently by air among ferrets were shown to remain infectious in aerosols longer than viruses that transmit less efficiently 43 . When the airborne virus source was an infected ferret, a human influenza virus isolate from the 2009 H1N1 pandemic demonstrated enhanced airborne survival, relative to a variant virus encoding a destabilizing HA mutation that increases its pH of fusion 44 . These data suggest that the stability of the HA protein in the environment is one critical factor in influenza virus transmissibility by airborne routes.…”

Hemagglutinin Stability and Its Impact on Influenza A Virus Infectivity, Pathogenicity, and Transmissibility in Avians, Mice, Swine, Seals, Ferrets, and Humans

“…A/England/195/2009 (H1N1) has also been used as a model to investigate the impact of HA stability on the emission from and transmissibility between ferrets [ 169 ]. While similar amounts of infectious virus were recovered from the nasal washes of ferrets infected with HA1-E31K (HA activation pH 5.3; virus inactivation pH 5.15) and HA1-Y17H (activation pH 5.9; inactivation pH 5.75), the stabilized HA1-E31K variant was recovered from emitted airborne droplets at a substantially higher level (184 plaques compared to 23) [ 153 , 169 ]. Furthermore, inoculated ferrets and air-emitted virus from the HA1-Y17H group contained the stabilizing variants HA1-Y17H, HA2-V55I, HA2-E47K, and HA1-V29I ( Table 5 ) [ 169 ].…”

Interplay between H1N1 influenza a virus infection, extracellular and intracellular respiratory tract pH, and host responses in a mouse model

“…When inoculated intranasally in ferrets, Y17H-infected animals had reduced and delayed nasal titers and were incapable of transmitting by the airborne route; however, selection of a stabilized HA variant (H17H/R106K) that had an activation pH of 5.3 enabled airborne transmission [30]. In the background of A/England/195/2009 (H1N1), airemitted Y17H virus produced significantly fewer plaques than those produced by E31K, which has an activation pH of 5.3 [40]. Circulating pH1N1 viruses isolated in 2009 had HA activation pH values of 5.5 [14,30,39], while those circulating from 2010-2012 had acquired one or more stabilizing mutations that reduced the HA activation pH to 5.2-5.4 [30,[35][36][37].…”