Norovirus infections spread rapidly particularly in senior residences, hospitals, hotels, schools and even cruise ships. Apparently, contaminated surfaces may play an important role in the spread of human NoV [12
]. In a rehabilitation centre, the protracted course of norovirus-induced gastroenteritis was also connected to an environmental contamination [16
]. Therefore, measures for the prevention and control of NoV transmission should include surface disinfection with products highly effective against this viral pathogen.
Many surface cleaners in the household are based on 0.5% hypochlorite. It was shown by reverse transcriptase polymerase chain reaction (RT-PCR) for human NoVs that the cleaning of fecally contaminated surfaces with a cloth soaked in detergents or treatment with 0.5% hypochlorite alone failed to eliminate human NoVs. The virus could still be detected on up to 28% of the surfaces by RT-PCR. Only wiping the surface with a cloth soaked in detergents followed by applying a combined hypochlorite/detergent treatment was successful to achieve a sufficient efficacy against human NoVs [17
Since there is only limited knowledge about human NoV inactivation on surfaces, it was the aim of this study to evaluate the virucidal properties of some often-used chemical ingredients of surface disinfectants in a standardized test procedure. The importance of carrier-based methods to test efficacy of disinfectants against vertebrate viruses has been stressed [18
Meanwhile, carrier test methods including a ring trial launched under the initiative of the Organisation for Economic Cooperation and Development have been published [19
]. In our study the protocol used followed the draft of the European Committee for Standardization TC 216 with 0.03% BSA and 0.3% BSA plus 0.3% sheep erythrocytes as interfering substances without mechanical action [22
The absence of a cell culture assay for human NoV makes it necessary to introduce a reliable surrogate virus when studying the environmental persistence and efficacy of chemical disinfectants. Because FCV can be propagated in cell culture, it has been extensively studied in inactivation studies in the past [5
]. In contrast to FCV, MNV is transmitted by the fecal-oral route, thus making this virus a promising surrogate for human NoVs. Consequently, stability and inactivation by alcohol-based hand rubs and commercial products used in veterinary medicine for surface disinfection have been successfully tested with MNV in suspension and carrier tests [24
In this study, MNV showed a remarkable stability while drying. The difference in the virus titre before and after drying on the carrier did not exceed 0.35 log10
(data not shown in table). In contrast, the poliovirus titre decreased by about 3 log10
during the drying process [27
In general, non-enveloped viruses can persist for approximately up to two months on inanimate surfaces [28
]. For example, reovirus was able to survive on polyvinyl chloride carriers for a period of 30 days dried in organic matrix [29
MNV can persist in various environmental conditions. After 40 days at -20°C and 4°C only a < 2 log10
reduction was observed [30
]. This study also revealed that MNV survived better in a stool suspension than on the surface of gauze or diaper material.
Recently, the stability of MNV mixed with artificial feces on stainless steel coupons was studied [25
]. In this study, only a minimal loss of infectivity was measured at pH 2, whereas FCV was rather unstable at pH values lower than 3.
In this study we show that PAA and GDA were able to reduce the titre of MNV under clean conditions about more than 4 log10
within 5 minutes of exposure time. When testing PAA a concentration of 0.1% (1000 ppm) and when testing GDA a concentration of 0.25% (2500 ppm) was needed to inactivate 99.99% of the viruses. These results were confirmed by the ability of two commercial products based upon GDA and PAA to inactivate MNV [26
In tests with other viruses simulating practical conditions, 2.0% GDA was able to inactivate HIV, rotavirus and HAV on carriers within one minute [31
]. Additionally, reovirus embedded in an artificial test soil was inactivated by GDA within one minute, whereas a concentration of 0.1% took 12 minutes to inactivate this virus completely [29
Generally, besides PAA and GDA, chlorine-based products are often recommended to inactivate non-enveloped viruses on contaminated inanimate surfaces. Testing nonporous and porous surfaces, 20 to 200 ppm of hypochlorous acid solution reduced the titres of human NoV and MNV by 3 log10
within a contact time of 10 minutes [34
In further experiments we analysed the virucidal properties of ethanol, 1-propanol and 2-propanol against MNV in the quantitative carrier test under clean conditions with concentrations from 10 to 60% (v/v). Since results from the suspension test with the MNV where ethanol in a concentration of 60% (v/v) to 80% (v/v) was more effective than 1-propanol followed by 2-propanol (unpublished), we expected ethanol to be the most effective virucidal agent, followed by 1- and 2-propanol. Surprisingly, 1-propanol was the most effective alcohol to inactivate MNV on the carrier under clean conditions. Interestingly, a concentration of only 30% (v/v) 1-propanol was sufficient to inactivate 99.99% of the virus. In comparison, the two other alcohols were completely ineffective at this concentration. Ethanol achieved a similar effectiveness only at 50–55% (v/v), while 2-propanol at a concentration of 60% (v/v) did not reach a 4 log10 reduction. These results are in contrast to data with FCV. Here, 2-propanol inactivating 99% within one minute at 40 to 60% was found to be more effective than ethanol (6).
Approaching practical conditions, the soil load was increased from 0.03% BSA (clean conditions) to 0.3% BSA plus 0.3% sheep erythrocytes (dirty conditions). Previous results of suspension tests or carrier assays had shown that a higher organic load resulted in a decreased effectiveness of the biocide [35
]. Interestingly, our results could not confirm these findings. There was no difference in log10
reduction detectable between clean and dirty conditions. All three alcohols tested showed similar reduction factors at 40% (v/v) and 60% (v/v) with 0.03% BSA as well as with 0.3% BSA plus 0.3% erythrocytes. At the tested concentration of 40% (v/v) 1-propanol was highly active against MNV after an exposure time of 5 minutes by reaching a reduction of 6 log10
, whereas ethanol and 2-propanol could not inactivate the virus sufficiently. The effectiveness of ethanol and 2-propanol could be enhanced by increasing the concentration of alcohol. At an alcohol concentration of 60% (v/v) ethanol reached an efficacy comparable to 1-propanol, whereas 2-propanol was not capable of reducing the virus titre by more than 3 log10
at the same concentration.
In summary, our data indicate that 1-propanol was the most effective alcohol to inactivate MNV on stainless steel discs within 5 minutes of contact time. Independent of the amount of soil load, even a concentration of 30% (v/v) 1-propanol had the capability to decrease the virus titre by ≥ 4 log10 within 5 minutes.
In further studies, it would be interesting to compare the inactivating properties of these biocides against MNV on other carriers often used in hospitals and other medical settings.