Virus samples were quantified by plaque assay as described previously [12]. Briefly, 10-fold serial dilutions of the collected samples were prepared. Fifty microliters of the serial dilutions, 450 μL of liquid culture of bacterial host, and 4.5 mL of soft agar were mixed and poured over agar plates. Plates were incubated at the bacterial host’s growth temperature for 24 h. The number of plaques on plates was counted, and the virus concentration in the samples were calculated as shown in Eq 1.