BACKGROUND: The Cholera and Other Vibriosis Illness Surveillance (COVIS) system collects data on domestic culture-confirmed vibriosis infections and works with state health departments and the Food and Drug Administration (FDA) to identify sources of infections. Cases are commonly associated with molluscan shellfish (clams, oysters, mussels, scallops), and traceback investigations can determine implicated shellfish harvest areas. Harvest area data can link single cases from multiple states and facilitate timely interventions to prevent further illnesses. Investigations can link cases to 1 (single source) or >1 harvest area (multisource). Often, multisource cases are excluded from an area’s case counts and do not contribute to area closure decisions. Harvest area data reporting, multisource case frequency, and key traceback findings are summarized.
METHODS: Health departments, often with departments of agriculture or environmental health, investigate Vibrio cases with recent seafood exposure through retail outlets. Harvest areas are identified from shellfish identification tags. We summarize reports to COVIS between 2007 and 2015 associated with molluscan shellfish consumption, and we describe common reporting barriers based on state partner feedback.
RESULTS: Between 2007 and 2015, state health departments reported 3,007 foodborne Vibrio illnesses associated with molluscan shellfish consumption to CDC; 41.5% included harvest area information. Harvest area reporting decreased from 52.4% of cases in 2007 to 44.6% in 2015. Among cases with known harvest area, sources included Pacific states (49.5%), Atlantic states (30.4%), and Gulf states (20.1%). Among investigations that included harvest area information, 18.3% identified >1 harvest area. States identified difficulty following up on exposures across state lines and inadequate shellfish tag recordkeeping at retail as barriers to reporting harvest area information.
CONCLUSIONS: Harvest area information is obtained in fewer than half of Vibrio illnesses associated with molluscan shellfish, and reporting is decreasing. Increased collection of harvest area data can improve identification of Vibrio illnesses sources. Almost half of all traceback is reported from harvest areas in the Pacific. Multisource exposures are common; new methods that address uncertainty associated with multiple possible sources may increase the ability to use data from them. Barriers to reporting harvest area information need to be addressed. Common data sharing platforms may improve our ability to coordinate traceback investigations involving multiple states. Training on procedures for molluscan shellfish tag review and retention at retail establishments, in addition to new methods with which to track this information, could increase reporting of harvest area information, leading to timelier action to prevent further illness.