Application of Capsid Integrity (RT-)qPCR to Assessing Occurrence of Intact Viruses in Surface Water and Tap Water in Japan

Capsid integrity (RT-)qPCR has recently been developed to discriminate between intact forms from inactivated forms of viruses, but its applicability to identifying integrity of viruses in drinking water has remained limited. In this study, we investigated the application of capsid integrity (RT-)qPCR using cis-dichlorodiammineplatinum (CDDP) with sodium deoxycholate (SD) pretreatment (SD-CDDP-(RT-)qPCR) to detect intact viruses in surface water and tap water. A total of 63 water samples (surface water, n = 20; tap water, n = 43) were collected in the Kanto region in Japan and quantified by conventional (RT)-qPCR and SD-CDDP-(RT-)qPCR for pepper mild mottle virus (PMMoV) and seven other viruses pathogenic to humans (Aichivirus (AiV), noroviruses of genotypes I and II, enterovirus, adenovirus type 40 and 41, and JC and BK polyomaviruses). In surface water, PMMoV (100%) was more frequently detected than other human pathogenic viruses (30%-60%), as determined by conventional (RT-)qPCR. SD-CDDP-(RT-)qPCR also revealed that intact PMMoV (95%) was more common than intact human pathogenic viruses (20%-45%). In the tap water samples, most of the target viruses were not detected by conventional (RT-)qPCR, except for PMMoV (9%) and AiV (5%). PMMoV remained positive (5%), whereas no AiV was detected when tested by SD-CDDP-(RT-)qPCR, indicating that some PMMoV had an intact capsid, whereas AiV had damaged capsids. The presence of AiV in the absence of PMMoV in tap water produced from groundwater may demonstrate the limitation of PMMoV as a viral indicator in groundwater. In addition to being abundant in surface water, PMMoV was detected in tap water, including PMMoV with intact capsids. Thus, the absence of intact PMMoV may be used to guarantee the viral safety of tap water produced from surface water.