Mallard or chicken? Comparing the isolation of avian influenza A viruses in embryonated Mallard and chicken eggs

BACKGROUND: To date, the most efficient and robust method for isolating avian influenza A viruses (IAVs) is using embryonated chicken eggs (ECEs). It is known that low-pathogenic avian IAVs undergo rapid genetic changes when introduced to poultry holdings, but the factors driving mutagenesis are not well understood. Despite this, there is limited data on the effects of the standard method of virus isolation of avian-derived viruses, that is, whether isolation in ECEs causes adaptive changes in avian IAVs. Eggs from a homologous species could potentially offer an isolation vessel less prone to induce adaptive changes. METHODS: We performed eight serial passages of two avian IAVs isolated from fecal samples of wild Mallards in both ECEs and embryonated Mallard eggs, and hemagglutination assay titers and hemagglutinin sequences were compared. RESULTS: There was no obvious difference in titers between ECEs and embryonated Mallard eggs. Sequence analyses of the isolates showed no apparent difference in the rate of introduction of amino acid substitutions in the hemagglutinin gene (three substitutions in total in embryonated Mallard eggs and two substitutions in ECEs). CONCLUSION: Embryonated Mallard eggs seem to be good isolation vessels for avian IAVs but carry some practical problems such as limited availability and short egg-laying season of Mallards. Our study finds isolation of Mallard-derived avian IAVs in ECEs non-inferior to isolation in embryonated Mallard eggs, but more research in the area may be warranted as this is a small-scale study.