Using geospatial analysis to identify potential sources of contamination for two norovirus outbreaks associated with commercial oyster harvesting in Tomales Bay, California
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1
California Department of Public Health, United States
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2
California Department of Public Health, United States
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3
California Department of Public Health, United States
Background:
Norovirus is the leading cause of foodborne illness in the United States and causes approximately 21 million illnesses and 800 deaths each year (1). Norovirus spreads quickly and easily, from infected people to others and through contaminated foods, water, and environmental surfaces. Infected people shed billions of virus particles in their feces and vomit but as few as 18 virus particles can cause illness. The low infectious dose of norovirus and ability to persist in the environment for long periods of time means that even a small amount of norovirus in the environment can pose a significant public health risk.
Outbreaks of norovirus are commonly linked to consumption of raw oysters. Oysters obtain food through filtering water and can accumulate norovirus throughout the oyster’s tissues when filtering contaminated water (2). According to recent studies, 3.9% of U.S. oysters are contaminated with norovirus at the retail level (3). Some of these oysters are eaten raw without heat or other treatment to deactivate norovirus or other pathogens. When illnesses linked to consumption of raw oysters are identified, the collection of shellfish tags provides information to trace the oysters to their harvest area.
Objectives:
Two norovirus outbreaks within a four month period, one in January and one in April 2019, were associated with oysters grown in Tomales Bay, California. Collaborative investigations by epidemiologists, laboratorians, and environmental health scientists from local health departments and the California Department of Public Health (CDPH) Infectious Diseases Branch (IDB), Food and Drug Branch (FDB), and Environmental Management Branch (EMB) were conducted to determine the magnitude, characteristics, etiologic agent, and source of both outbreaks. Geospatial analyses were used to enhance the environmental health investigation to understand potential sources of pre- or post-harvest norovirus contamination within the region.
Methods:
During both outbreaks, ill persons were interviewed with a standardized questionnaire to assess the following clinical characteristics and exposure information: symptoms, illness onset dates, duration of symptoms, visits to a healthcare provider, laboratory testing, oyster type, meal date and location, and dining partners. Patient stool specimens were collected and tested for norovirus.
To assess the pre-harvest environmental conditions that may have led to norovirus contamination, CDPH EMB investigated potential sources of human fecal matter, including shoreline surveys, analysis of rainfall events, wind patterns, and tidal flows. Oyster and water samples from Tomales Bay were also collected for laboratory testing. Oysters consumed were traced back to specific harvesting leases in order to identify the common source location and narrow down potentially contaminated areas. Geospatial data including sampling location, lease location, and hazards identified were combined and mapped in ArcGIS to give a more complete picture of any potential risks in the bay.
To evaluate post-harvest sources of contamination, CDPH FDB conducted an onsite assessment at Dealer A’s processing facility. During this assessment, the FDB team evaluated the facility’s water handling, well, and septic systems as well as sanitation and employee health records. The equipment and conditions at Dealer A’s facility were photographed and documented using ESRI’s Collector Application. Samples of oysters from the wet storage tanks at Dealer A were collected and sample data were recorded using ESRI’s Survey123. Photographs and other collected information were mapped in real-time and made available to key decision makers through an ArcGIS Online Web Map.
Results:
The first outbreak, which occurred from December 2018 through January 2019, included 75 ill persons who were interviewed and reported eating raw oysters from Tomales Bay. The outbreak strain, determined by stool samples, was norovirus GI.P4-GI.5. Meal dates ranged from 12/28/18 - 1/5/19 and raw oysters purchased from multiple exposure locations were harvested from Dealer A (n=69) or Dealer B (n=6). The second outbreak occurred in April 2019. As of May 23, 20 ill persons were interviewed and reported consuming raw oysters from multiple exposure locations harvested from Dealer A, with meal dates ranging from 4/19/19 - 4/22/19. The outbreak strain, determined by stool samples, was norovirus GI.Pd-GI.3. CDPH EMB closed the Tomales Bay area, which included seven oyster growers total, to oyster harvesting for 21 days in response to each outbreak.
Using geospatial tools, exposure locations for both outbreaks were mapped. When compared to the retail distribution of Dealer A, the geographic distribution of Dealer A’s product was very similar to case patient exposure locations. This information was combined with geospatial information from the environmental survey and other EMB data to produce a more complete assessment of norovirus risk factors in the growing region. Environmental survey work determined that norovirus in the environment was likely introduced from overboard discharge, faulty septic systems, or runoff from adjacent lands into the bay. These environmental conditions would have likely affected multiple nearby growing areas but illnesses linked to other dealers were not reported. The investigation at Dealer A’s facility found no significant food safety concerns. In both outbreaks, a specific source for pre- or post-harvest norovirus contamination was not identified. However, negative results for oyster and water samples collected during the investigations indicated that there was not an ongoing risk of norovirus contamination from the bay or from Dealer A’s facility.
Conclusions:
Collaborative epidemiologic, laboratory, and environmental health investigations were successful at assessing the magnitude, characteristics, etiologic agent, and potential sources of both outbreaks. However, the use of geospatial data in addition to traditional epidemiologic and environmental work allowed field staff to have a better understanding of environmental conditions affecting the growing, harvesting, and storage of oysters in Tomales Bay and better assess the biologic risks of the area. The use of GIS-based data collection tools enabled the investigation team to conduct real-time field data collection and efficiently and accurately report investigation findings using dynamic maps as a visualization platform for key decision makers. The maps and other geospatial tools facilitated communication between different programs within CDPH and helped produce a more complete public health assessment. GIS and geospatial analysis complimented the more traditional epidemiologic and environmental studies to enhance and improve the public health response to this outbreak.
Acknowledgements
The authors would like to acknowledge the following:
Marin County Environmental Health Department
Marin County Sherriff Department
CDPH Food Safety Section including: Jane Reick, Michael Needham, Duy Trong, and Wendy Reynolds, and Jeff Vidanes
CDPH Infectious Diseases Branch, Disease investigations Section including: Ellora Karmarkar, Karen Click, and Maher Yassin
CDPH Environmental Management Branch including: Joe Christen
Other local and state health departments
References
1. Norovirus Disease in the United States. Hall, Aron J, et al. 8, 2013, Emerging Infectious Diseases, Vol. 19, pp. 1198-1205.
2. Distribution of Norovirus in Oyster tissues. Wang, Dapeng, et al. 6, December 2008, Journal of Applied Microbiology, Vol. 105, pp. 1966-1972.
3. Bacterial and Viral Pathogens in Live Oysters: 2007 United States Market Survey. DePaola, Angelo, et al. 9, 2010, Applied and Environmental Microbiology, Vol. 76, pp. 2754-2768.
4. Final Consumer Options to Control and Prevent Foodborne Norovius Infections. Guix, Susana, Pintó, Rosa M and Bosch, Albert. 4, 2019, Viruses, Vol. 11, p. 333.
5. Norovirus. Robilotti, Elizabeth, Deresinski, Stan and Pinsky, Benjamin A. 1, January 2015, Clinical Micribiology Reviews, Vol. 28, pp. 134-164.
6. Developments of an Emerging Infectious Agent: Norovirus. Tripathi, Manikant and Kumar, Shailendra. [ed.] George Perry. 4, 2019, Annual Research and Review in Biolgoy, Vol. 31, pp. 1-6.
7. National Center for immunization and Respiratory Diseases. Norovirus Symptoms. Norovirus. [Online] April 05, 2019. [Cited: May 15, 2019.] https://www.cdc.gov/norovirus/index.html.
Keywords:
GIS - Geographic Information System,
California, USA,
Norovirus (NoV),
Tomales Bay,
oyster,
Seafood,
foodborne illness
Conference:
GeoVet 2019. Novel spatio-temporal approaches in the era of Big Data, Davis, United States, 8 Oct - 10 Oct, 2019.
Presentation Type:
Poster-session
Topic:
Real-time field data collection and visualization platforms
Citation:
Adcock
BW,
Peralta
VN,
Zubkousky-White
VM,
Fontenot
J,
Rankin
SC and
Grant
C
(2019). Using geospatial analysis to identify potential sources of contamination for two norovirus outbreaks associated with commercial oyster harvesting in Tomales Bay, California.
Front. Vet. Sci.
Conference Abstract:
GeoVet 2019. Novel spatio-temporal approaches in the era of Big Data.
doi: 10.3389/conf.fvets.2019.05.00099
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Received:
30 May 2019;
Published Online:
27 Sep 2019.
*
Correspondence:
Mx. Brandon W Adcock, California Department of Public Health, Sacramento, United States, Brandon.Adcock@cdph.ca.gov