Advisor at Japan Food Research Laboratories
Norovirus is a small round virus (see Figure1 in my column #2). Linearly-arranged 33,333 noroviruses would measure only about 1mm. This virus causes acute gastroenteritis such as vomiting and diarrhea in humans. Most healthy adults will recover in a few days, but elderly people, infants, or the sick need a special care. Infection of these “high-risk group” people are associated with severe disease. As the symptoms include serious nausea or vomiting, great care is needed to prevent their choking.
Norovirus is transmitted via the oral route, and if just one norovirus reaches the small-intestinal epithelial cell of an eater, it might start proliferating. The eater can also be a norovirus carrier without symstoms, which is called “inapparent infection”
Objects or foods directly / indirectly polluted by feces or vomit of an infected person will be the source of infection. Faucets, doorknobs, or shared towels can serve as a mediator. As norovirus is low specific gravity, it can be transmitted human to human via the airborne route. It has been reported that norovirus infection spread through droplet transmission or airborne transmission within a relatively small space. “Airborne transmission” in this case is more precisely expressed by “dust transmission” as norovirus scatters around due to its levity instead of broad airborne transmission (droplet nuclei infection) of tuberculosis
Other viruses such as rotavirus or sapovirus cause infectious gastroenteritis during the cold season too, but the virus that afflicts the greatest number of people and causes social confusion is norovirus. This virus infects humans in other seasons too, but epidemiologically speaking, its number of patients increases in the cold and dry season.
In January 2014, 1271 people suffered from food poisoning caused by norovirus-contaminated school lunch served at an elementary school in Hamamatsu City. The causative food was bread. The bread was contaminated by norovirus transmitted from a bread factory worker, which made people reaffirm that heated food can cause norovirus food poisoning. Already in 2003, bread with toasted soybean flour served as a school lunch in Hokkaido caused norovirus food poisoning. As norovirus is killed by heat, these causative breads are thought to have been contaminated by norovirus transmitted from bread factory workers after being baked. No worker at these factories was sick at those times, so there must have been workers with inapparent infection or recovered from infectious disease with continued virus excretion. Small number of norovirus transmitted to breads must have reached small intestine of many elementary school students and caused mass food-poisoning cases.
In 2006, a party participant at a hotel had vomiting, and its inappropriate treatment made many people, passed the spot afterwards, infected. Survived noroviruses supposedly floated around, got sucked in, reached small intestine of the passersby and proliferated.
Vaccines or therapeutic agents for norovirus have not been developed yet. One of the reasons for this delay is that cultured norovirus cells for research have not been established. Norovirus proliferates only in small-intestinal epithelial cells of humans. If someone could establish cultured cells of norovirus that bothers many people and brings social confusion, he or she would win the Nobel Prize. Testing methods using genes have been developed, but they can’t tell whether detected norovirus is infective or inactivated. Its preventative measure is hygienic habit of the entire nation; especially promotion of hand-washing and improvements in food sanitation. Food handling facilities must take thorough countermeasures against norovirus. Unless the whole society does not tackle this virus, same as influenza virus, the outcome will be very limited (Figure2).
At food-handling facilities, people in bad physical shape or carrying viruses are required to keep away from the facility for the purpose of hygiene control. Whether rank-and-filer or factory manager, the person is not allowed to enter the facility. It is a shame if you once step outside the norovirus-free facility, you would be surrounded by noroviruses. If a customer had vomiting at convenience stores or restaurants, should they close their stores so that noroviruses would not be scattered around?
I think we should tackle norovirus as an infectious disease by society as a whole in the same manner as influenza. Let us multiply the consideration toward others, “Vitamin I (love)”, and try not to let norovirus multiply in our small intestine.
Aron J. Hall,et.al.,Foodborne Norovirus Outbreaks — United States, 2009–2012,,MMWR,63,1-5(2014).
Ministry of Health, Labour and Welfare: Q&A on Norovirus (Last updated: November 19, 2014)
Tokyo Metropolitan Institute of Public Health: Infectious Gastroenteritis---Focusing on Norovirus (September 25, 2014)