Purpose: This study evaluated the efficacy of chlorine and heat for inactivation of human norovirus and the cultivable surrogate, Tulane virus (TuV), with and without the presence of organic matter.
Methods: Suspension assays were carried out according to American Society for Testing and Materials protocol E1052-11 for a two minute exposure of viruses to sodium hypochlorite (household bleach) and heat. A 20% suspension of human stool confirmed positive for GII.4 Sydney and ultracentrifuged, TuV tested in purified form or suspened in 20% human feces, served as inocula. Virus inactivation was determined using RNase RT-qPCR on both viruses, and by infectivity (plaque) assay for TuV.
Results: There was a >4-log reduction in purified TuV infectivity at 5.0 ppm free available chlorine (FAC), but 300 ppm FAC was needed for the same reduction when TuV was suspended in 20% feces. However, it took 500 and 300 ppm FAC to achieve >4-log reduction in TuV and human norovirus genome copies, respectively, under the elevated organic load. For heat treatment, the infectious titer of purified TuV was reduced by >4 log after 65°C for 2 min, and at 72.5°C for 2 min in fecal suspension. By RT-qPCR, higher temperatures (>87.5°C) were needed to achieve >4-log reduction in purified TuV genome copy, and with organic matter; 100°C was insufficient to reach the same reduction for either GII.4 Sydney or TuV.
Significance: Organic load was protective of viruses exposed to heat or chlorine. Measurement of residual genome copies, especially when the virus is suspended in organic matter, may represent an extreme measure of virucidal efficacy.