How Wastewater Monitoring is Helping to Investigate a Hepatitis A Outbreak Linked to Oysters' Consumption.

In April 2024, two cases of hepatitis A (HA) were linked to the consumption of oysters produced in the same region, which triggered a multidisciplinary study. A network was quickly set up in a one-health approach including epidemiological and clinical data, as well as analysis of locally collected shellfish and wastewater samples. By the end of the outbreak, up to 17 HA cases met the case definition: all shared a same genotype IA strain and have eaten oysters. The prompt response enabled the identification of oyster samples positive for the HAV genome at the start of the monitoring period; these oyster samples subsequently tested negative during the following two months of the sampling campaign. Monitoring three nearby wastewater treatment plants revealed that the contamination was limited to a single area, allowing for more effective monitoring of shellfish. None of the identified cases lived in the area served by the treatment plant, and no clinical cases were detected among the local population, either before or after the outbreak. The decline of HAV concentration in sewage signed the end of the outbreak. While predicting contamination of shellfish growing areas is complicated, sewage surveillance can help establishing an alert system to prevent shellfish contamination, which is an important issue for silent diseases.