Thermostability of subpopulations of H2N3 influenza virus isolates from mallard ducks.

Maintenance of avian influenza virus in waterfowl populations requires that virions remain infectious while in the environment. Temperature has been shown to negatively correlate with persistence time, which is the duration for which virions are infectious. However, thermostability can vary between isolates regardless of subtype, and it is not known whether this variation occurs when host and geographic location of isolation are controlled. In this study, we analyzed the thermostabilities of 7 H2N3 viruses isolated from mallard ducks in Alberta, Canada. Virus samples were incubated at 37 degrees C and 55 degrees C, and infectivity titers were calculated at different time points. Based on the rate of infectivity inactivation at 37 degrees C, isolates could be grouped into either a thermosensitive or thermostable fraction for both egg- and MDCK-grown virus populations. Titers decreased more rapidly for isolates incubated at 55 degrees C, and this loss of infectivity occurred in a nonlinear, 2-step process, which is in contrast with the consensus on thermostability. This suggests that stock samples contain a mixture of subpopulations with different thermostabilities. The rate of decrease for the sensitive fraction was approximately 14 times higher than that for the stable fraction. The presence of subpopulations is further supported by selection experiments and plaque purification, both of which result in homogenous populations that exhibit linear decreases of infectivity titer. Therefore, variation of thermostability of influenza virus isolates begins at the level of the population. The presence of subpopulations with high thermostability suggests that avian viruses can persist in water longer than previously estimated, thus increasing the probability of transmission to susceptible hosts.