Food Poisoning by Oysters in Hiroshima Region
Norwalk Virus (NV)
In recent years the food poisoning that a norwalk virus (NV) is etiologic agent occurs frequently in the Hiroshima region in the winter season and attracts attention. Oysters are doubted as food causing the food poisoning. In this study, we used a questionnaire to examine the outbreak situation of food poisoning by shellfish in the Hiroshima region. As the results of the investigation, it was recognized that food poisoning by shellfish occurred by oysters mainly in the winter season. Concerning the preparation method, food poisoning was most frequent with vinegared oysters. However, in addition to raw oysters, food poisoning occurred by cooked oysters such as fried oysters. Etiologic agent of food poisoning is regarded as norwalk virus and, as for the cause, it is thought that heating in cooking is insufficient. In the case of most, oysters are prepared in home and they are eaten in home. A main symptom of food poisoning by oysters is diarrhea, vomitus, tormina, and this agrees with a characteristic of a symptom of norwalk virus. In addition, we did an experiment to cook an oyster and examined a temperature change for a center of an oyster. When we boiled an oyster in boiling water for 40 seconds, the temperature of a center of an oyster became 39.3 degrees. In addition, the temperature of a center became 80.3 degrees when we fried an oyster in salad oil of 180 degrees for three minutes. By a process of these cooking, the center temperature of an oyster was lower than expectation. It is considerably difficult to let norwalk virus in an oyster completely become extinct by cooking. Recently, contamination of Hiroshima Bay by sewage has been getting worse, and oysters accumulate norwalk virus and pathogenic bacteria which came from excrement in seawater in the mid-gut gland. Therefore, we should avoid taking raw oysters even in the winter season to prevent food poisoning as far as we are possible. In addition, sufficient care to use clean oysters cultivated in a designated sea area should be taken when we cook oysters.