Norovirus-related food poisoning is back this season after Japan saw 416 cases affecting 17,632 people in 2013. The health and welfare ministry says the number of cases has been rising for the past 10 years.

At a Kyoto hospital, four inpatients, 83 to 91 years old, died of the poisoning in mid-December. Norovirus poisoning also hit students at 15 elementary schools in Hamamatsu, Shizuoka Prefecture, in mid-January. On Jan. 16 alone, some 1,000 students were absent from school. Those schools were closed temporarily.

One needs to discard the notion that food poisoning happens only in warmer seasons. Norovirus food poisoning continues into February. Disinfection by alcohol does not work. Frequent hand-washing with soap is the easiest and most effective preventive measure, in particular after using the toilet.

The norovirus is spherical-shaped and relatively small with a diameter of just 30 to 40 nanometers and is spread through fecally contaminated food, aerosolization and person-to-person contact. It is highly infective; fewer than 100 noroviruses can cause illness. One gram of excreta from an infected patient contains 1 million to 1 billion noroviruses. In one reported case, a person showed symptoms more than 10 days after unknowingly picking up norovirus from a carpet.

A person infected by the norovirus usually starts experiencing diarrhea or vomiting after one or two days. Infants and elderly people can suffer dehydration from diarrhea or suffocation caused by vomit getting stuck in their throats. There are no known drugs or vaccines that can weaken or kill noroviruses.

Noroviruses often accumulate in bivalve shellfish such as clams and oysters. If shellfish is not well-cooked, there is high risk of infection. Food should be heated to the core at a temperature of 85 to 90 Celsius for more than 1½ minutes to kill any noroviruses. It is also important to disinfect cookware with high heat or chlorine-based disinfectants as alcohol does not work.

Cleaning up the vomit of norovirus patients demands utmost precaution, including the use of surgical masks and disposable rubber gloves. The vomit must be wiped up with a paper towel and thrown away in a plastic garbage bag. The surface should then be wiped with a paper towel soaked with a 0.1 percent sodium hypochlorite (ji-a-enso-san natoriumu) solution, and then the towel, the gloves and the mask must be disposed of in the garbage bag. Hands should be thoroughly washed with soap.

Attention must be paid to the fact that people who work in the food industry are increasingly becoming sources of norovirus infection. In the poisoning case at the Hamamatsu elementary schools, noroviruses were detected in bread served during the school lunch and in four workers at a bread factory. Food-industry workers must frequently wash their hands with soap and hot water, and use disposable rubber gloves when handling food.

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