Figure 4 shows that there was a substantial decrease in virus
infectivity over time for the three types of environmental water,
which was observed using both methods (microtiter endpoint
titration and qRT-PCR using CRBCs). In contrast, no
apparent decrease in the total number of HA copies over time
was detected when quantification was performed by qRT-PCR
without CRBCs. In ddw, the virus infectivity, as determined by
any of the three testing methods, remained relatively constant
during the 17-day period.