[Current topics on inactivation of norovirus]

Human norovirus is the most important foodborne virus in Japan. According to the statistics of food poisoning by the Ministry of Health, Labour, and Welfare (MHLW), the number of patients infected with norovirus has accounted for half of all the patients with food poisoning in recent years. One of the most important measures for the control of infectious diseases is establishing of techniques for inactivating pathogens. For the prevention of food poisoning caused by norovirus, MHLW recommends that foods be subjected to heat treatment at 85 degrees C for 1 min or more; moreover, it recommends the use of sodium hypochlorite to inactivate (disinfect) this virus. However, application of these treatments is not always feasible because heat results in denaturation and sodium hypochlorite can be toxic to the human body and can cause discoloration. Therefore, it is necessary to develop and improve the efficacy of disinfectants and physiochemical treatments against the virus. Human norovirus cannot be propagated in cell culture or in a small animal. This matter is the greatest hindrance for testing the stability of this virus in environments or for evaluating the efficacy of disinfectants, heat treatment, pH treatment, ultraviolet or gamma irradiation, high hydrostatic pressure treatment, and other methods for the inactivation of the virus. Hence, some viruses such as human enterovirus, feline calicivirus, or mouse norovirus have been used as surrogates of human norovirus. The data on inactivation and stability of surrogate viruses are exclusively used as the data of human noroviruses. In recent years, some attempts to distinguish between infectious and noninfectious virus particles by genetic methods such as polymerase chain reaction have been made. These methods include pretreatments by RNase for digesting viral RNAs from non-intact or destroyed virus particles, or addition of a reagent such as ethidium monoazide for inhibiting PCR amplification of viral RNAs from them, before RNA extraction. Non-intact virus particles, which may represent virus particles with some damage (s) in the structural protein(s), are not necessarily synonymous with non-infectious virus particles. However, the results of methods using these treatments, compared to the results of traditional methods without these treatments, seem to be more correlated to the amount of the infectious virus particles. Although many disinfectants or physiochemical treatments have been reported, traditional techniques such as removal of virus particles by washing in running water, heat treatment, or disinfection by sodium hypochlorite are still important control measures. Establishment of control measures for human norovirus and successful propagation of the virus in cell culture are strongly desired.