Different incubation temperatures affect viral polymerase activity and yields of low-pathogenic avian influenza viruses in embryonated chicken eggs

Various incubation conditions (35A degrees C-38A degrees C, 2-7 days) have been used in surveillance studies of the prevalence of avian influenza viruses in wild birds. Here, we studied viral polymerase activity and virus growth kinetics of low-pathogenic avian influenza viruses (LPAIVs) isolated from field samples [A/duck/Hong Kong/365/1978 (H4N6) and A/duck/Nanchang/2-0480/2000 (H9N2)] during incubation at different temperatures (35A degrees C, 37A degrees C, and 39A degrees C) in the allantoic cavity of 10-day-old embryonated chicken eggs (ECE). The higher incubation temperatures (37A degrees C and 39A degrees C) resulted in a significantly higher rate of virus growth, which is most likely a result of increased viral polymerase activity (20%-60%), than was observed at 35A degrees C, and as much as a 100% greater virus yield (as measured by hemagglutination assay) was observed two days after inoculation. Our findings revealed that the optimal activity of the viral polymerase complex, resulting in the highest yield of LPAIV field isolates, could be obtained by incubation for two days in ECE at 37A degrees C and 39A degrees C.