Researchers at the BC Centre for Disease Control report in the British Medical Journal that between November 2016 and March 2017 more than 400 individuals across Canada developed norovirus gastroenteritis associated with the consumption of BC oysters. Over 100 cases occurred mid-November in participants at a Tofino oyster festival. Six cases occurred in persons attending a December oyster barbecue in Victoria. By March over 300 additional cases of norovirus linked to cultivated BC oysters harvested from multiple sites on both the east and west coasts of Vancouver Island were identified in BC, Alberta, and Ontario consumers.
Norovirus is a highly infectious cause of gastroenteritis typically spread from person to person and is associated with regular community outbreaks in schools, hospitals, day cares, and care facilities. Foodborne outbreaks of norovirus are often linked to ill food handlers. In this recent outbreak, oysters were contaminated in the marine environment where they were farmed. The trace-back of oysters consumed by infected individuals led to the closure of 13 geographically dispersed marine farms in BC and to extensive public outreach.
Genotypic analysis of norovirus isolated from the cases included several variants of genogroup I (GI) early in the outbreak and both genogroups GI and GII later in the outbreak.
Both GI and GII norovirus were detected in oysters from shellfish farms. This suggests that oysters bind and act as a reservoir for community outbreak strains and disseminate those strains to consumers.[1]
Although sewage is often the cause of oyster contamination it remains unclear whether one or many sewage sources contributed to the contamination of shellfish farms. The 2016–17 outbreak was preceded by a wet fall and accompanied an unseasonably cold winter. Wet, cold, and dark winters enhance norovirus survival, allowing for longer retention in ocean sediments and in oysters.[2,3] The infective dose of norovirus is estimated as few as 18 particles.[2] Given the low infective dose and the viability of norovirus in cold water, we postulate that sewage spread by ocean currents may have contaminated geographically dispersed farms. Among potential sources under investigation are sewer overflows, metropolitan and local wastewater treatment plants, municipal raw sewage discharge, and commercial fishing vessels. The BCCDC is leading a collaborative group reviewing pollution sources discharging to BC marine environments that may have contaminated BC oysters.
In this outbreak, both raw and cooked oysters led to illness; oysters were likely insufficiently cooked to inactivate norovirus. In addition to norovirus, pathogens like Vibrio sp., Salmonella sp., and hepatitis A can be transmitted to oyster consumers; cooking oysters to an internal temperature of 90 °C for at least 90 seconds will reduce this risk. The “rule” that shellfish is safe to eat in months with an “r” (September to April) is false. First, bacteria and viruses persist in cold seawater. Second, marine biotoxins (saxitoxin and domoic acid that cause paralytic and amnesic shellfish poisoning) occur year round.
Physicians and laboratories play an important role in controlling foodborne disease. In this outbreak, trace-back of oysters linked to cases was used to close shellfish farms. If you see patients with acute gastroenteritis who recently consumed shellfish, inform your local public health office and submit stool samples for testing.[4]
BC oysters and norovirus: Hundreds of cases in months with an “r”
BCMJ, Vol. 59, No. 6, July, August 2017, page(s) 326,327
BC Centre for Disease Control
Lorraine McIntyre, MSc, Eleni Galanis, MD, MPH, FRCPC, Natalie Prystajecky, PhD, Tom Kosatsky, MD