Salmonella and Norovirus in Berries – A Growing Health Concern

In the wake of several disease outbreaks linked with consuming foodstuffs of non-animal origin, the European Food Safety Authority (EFSA) recently published a Scientific Opinion on pathogen risk associated with fresh produce¹. Noting specifically the emergence of Norovirus in frozen berries, the BioHazards panel (BIOHAZ) concentrated on safety issues and the transmission risk of other important human pathogens, including Salmonella, for this food crop. Although they found no epidemiological or disease prevalence data to suggest establishing additional microbiological recommendations for Salmonella in fresh, minimally processed or frozen berries, they strongly advise establishing safety criteria for Norovirus. They used these findings to construct a pathogen risk profile for each berry type and create best practices for production that would minimize contamination.

Production and Processing

Berries, including raspberries, strawberries, redcurrants and cranberries, come from a variety of plant types – low-growing, ground-based bushes, small shrubs or as aerial crops from taller plants. They are grown under a variety of agricultural systems, ranging from open fields to closed greenhouses or polytunnels, on soil or soil-less media, and with or without irrigation. Defined as small, spherical or ovoid fleshy fruits, berries have a high (acidic) juice content and fragility that makes them sensitive to handling and damage. These factors combine to influence berry production. For example, harvesting is usually better by hand than machine and they undergo only minimal processing before presentation to the consumer. Shelf life of fresh berry product is shorter than other similar foods. Most berries consumed in Europe come from the European Union (EU), with strawberries being the predominant crop and raspberries second. Imports account for between 1-5% of total berries sold. Harvesting mostly takes place by hand although mechanical recovery does occur. Producers rarely wash the berries post-harvest to avoid damaging the crop, but may treat them with ozone to retard botrytis rot. From here, berries go straight into final packaging for consumer retail as fresh ready-to-eat produce, or are processed for jam/jelly/juice manufacture or are frozen. The high temperatures associated with jam processing usually kill off pathogens, but studies do show persistence of both Salmonella and Norovirus in frozen product^2-5. Juice production may not include pasteurization which would kill bacteria and viral pathogens.

Contamination Risk Factors

Examining responses from the European Fresh Produce Association, Freshfel, BIOHAZ found risk of contamination at each stage of the berry production chain. Common to all stages were risks posed by cleanliness of the workers involved in handling the berries and the equipment. The main sources of contamination include:

• Water supplies – irrigation sources and flood risk in the fields; use during harvesting procedures; worker hygiene and sanitation; sewage pollution and agricultural run-off; equipment maintenance and disinfection; use in delivery of fungicide and other crop treatments
• Contact with animals or their waste (Salmonella only) – domestic pets, farm animals, or wild animals gaining access to crops; spread of zoonotic infection with manure or fertilizer
• Distribution and retail environment – equipment; packaging; worker hygiene; storage

The working group identified specific risks for Norovirus transmission. Since this pathogen is found only in people, colonization can occur by direct contact with contaminated water for irrigation, sewage outflow and use of human manure, and poor worker/equipment hygiene.

Disease Prevalence

The BioHazards panel considered past history of disease outbreaks associated with non-animal derived foods in an attempt to create realistic advisories for producers. Data from the Centers for Disease Control in the United States showed fruits and nuts, and vine or stalk vegetables as commonly associated with outbreaks. In Europe, only 4.4% of zoonotic outbreaks were attributed directly to foods of non-animal origin. However, a previous EFSA Opinion (2013) showed that berries ranked fourth and sixth in these surveillance records for foodborne human illness associated with Salmonella and Norovirus. Examining public health data collected between 2007 and 2011 for disease attributed to consumption of non-animal origin foods, the group found:

• 1 outbreak of Salmonella (fresh raspberry juice)
• 27 outbreaks of Norovirus (19 attributed to frozen raspberries)
• 9 cases of Norovirus (Finland; attributed to berries)

In the period 2011-2012, records showed 10,952 Norovirus infections linked to ingestion of frozen imported strawberries.The working group noted that although it was possible to attribute cause in a number of cases, they could not pinpoint when the contamination happened i.e. during processing or primary production, for any of these outbreaks.

Minimizing Contamination

Based on the relevant information gathered, the BIOHAZ Panel considered steps to minimize Salmonella and Norovirus contamination in berry crops. Their recommendations include attention to appropriate food safety measures as applicable to berry production, including Good Agricultural Practices, Good Hygiene Practices and Good Manufacturing Practices. Specific risks associated with frozen product:

•          Batch combination combines crops from different producers
•          Use of damaged or crushed fruits
•          Persistence of both Norovirus and Salmonella during freezing
•          High percentage of frozen product comes from imports into EU
•          Increased use of frozen berries as an ingredient in food products for flavoring, color and nutritional value

The BIOHAZ Panel noted that there is no routine testing for Salmonella or Norovirus in berry crops, and that current methods do not determine infectivity/disease risk. Moreover, there are no Process Hygiene criteria for managing microbial surveillance in berries. They strongly advise implementing procedures to avoid contaminating frozen berries, suggesting that monitoring for other indicator species such as Escherichia coli for fecal contamination and detecting genomic Norovirus could be appropriate for investigating compliance with standard hygiene practices. Recommendations to reduce contamination:

•          Improve data recording from non-animal origin foodstuffs
•          Improve standards for Norovirus detection on berries
•          Conduct surveillance on Norovirus prevalence in berries
•          Conduct sanitary audits of berry producers
•          Conduct research into Norovirus and berry production – its prevalence and detection
•          Develop Norovirus detection assays, to quantify, measure infectivity and investigate pathogen internalization
•          Investigate efficacy of decontamination procedures for berry crops
•          Conduct data analysis and risk assessment for development of criteria designed to control Norovirus in berries

In a summary of recommendations, the panel advises that each production enterprise presents its own individual risk profile and as such, must be independently assessed for suitable management practices throughout the whole chain. With the evidence of an emerging public health risk, the BIOHAZ panel firmly concludes that further research into developing official food safety procedures for Norovirus contamination in frozen strawberries and raspberries is a priority. For a Free on-demand webinar on Foodborne Viruses presented by Lee-Ann Jaykus, PhD, Scientific Director, USDA NIFA Food Virology Collaborative visit www.lifetechnologies.com    References 1. EFSA BIOHAZ Panel (EFSA Panel on Biological Hazards), 2014. Scientific Opinion on the risk posed by pathogens in food of non-animal origin. Part 2 (Salmonella and Norovirus in berries). EFSA Journal 2014;12(6):3706, 95 pp. doi:10.2903/j.efsa.2014.3706 Available online: www.efsa.europa.eu/efsajournal © European Food Safety Authority, 2014 2. Knudsen, D.M., et al. (2001) “Survival of Salmonella spp. and Escherichia coli O157 : H7 on fresh and frozen strawberries“, Journal of Food Protection 64 (pp.1483-88) 3. Mäde, D. et al.(2013) “Detection and typing of norovirus from frozen strawberries involved in a large-scale gastroenteritis outbreak in Germany“, Food and Environmental Virology 5 (pp.162-8) 4. Maunula, L. et al. (2009) “Detection of human norovirus from frozen raspberries in a cluster of gastroenteritis outbreaks“, European Surveillance 14 5. Sarvikivi, E. et al (2012) “Multiple norovirus outbreaks linked to imported frozen raspberries“, Epidemiology and Infection 140 (pp.260-7)