A Cluster of Hepatitis A Infections Presumed to be Related to Asari Clams and Investigation of the Spread of Viral Contamination from Asari Clams

: In a cluster of hepatitis A infections that occurred in Nagano Prefecture in 2017, hepatitis A virus (HAV) was detected in asari clams (reference food) and the patients' fecal samples. Initially, the asari clams were suspected to be the infection source. However, the exact infection route remained unknown because a patient who had not consumed an asari clam dish also developed the disease. Suspecting a secondary infection originating from the asari clams, we investigated the presence of HAV genomes in water used for washing and soaking the frozen asari clams and detected HAV in the soaking water. These results suggest that soaking water is a risk factor for secondary contamination because of the leakage of HAV accumulated in midgut gland of the asari clam. During the asari clam sand removal process, the water used to clean asari clams spread across a wide area in a concentric fashion, raising concerns that this process may aggravate contamination. In addition to HAV, diarrhea viruses, such as norovirus, have often been detected in bivalves, including asari clams. Thus, handling these foodstuffs requires adequate care.