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In 2016, we reviewed preventive control measures for secondary transmission of Shiga-toxin producing Escherichia coli (STEC) in humans in European Union (EU)/European Free Trade Association (EEA) countries to inform the revision of the respective Norwegian guidelines which at that time did not accommodate for the varying pathogenic potential of STEC. We interviewed public health experts from EU/EEA institutes, using a semi-structured questionnaire. We revised the Norwegian guidelines using a risk-based approach informed by the new scientific evidence on risk factors for HUS and the survey results. All 13 (42%) participating countries tested STEC for Shiga toxin (stx) 1, stx2 and eae (encoding intimin). Five countries differentiated their control measures based on clinical and/or microbiological case characteristics, but only Denmark based their measures on routinely conducted stx subtyping. In all countries, but Norway, clearance was obtained with ⩽3 negative STEC specimens. After this review, Norway revised the STEC guidelines and recommended only follow-up of cases infected with high-virulent STEC (determined by microbiological and clinical information); clearance is obtained with three negative specimens. Implementation of the revised Norwegian guidelines will lead to a decrease of STEC cases needing follow-up and clearance, and will reduce the burden of unnecessary public health measures and the socioeconomic impact on cases. This review of guidelines could assist other countries in adapting their STEC control measures.

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In Norway, incidence of sporadic domestically acquired salmonellosis is low, and most frequently due to Salmonalla Typhimurium. We investigated the risk factors for sporadic Salmonella infections in Norway to improve control and prevention measures. Surveillance data for all Salmonella infections from 2000 to 2015 were analysed for seasonality and proportion associated with domestic reservoirs, hedgehogs and wild birds. A prospective case-control study was conducted from 2010 to 2012 by recruiting cases from the Norwegian Surveillance System for Communicable Diseases and controls from the Norwegian Population Registry (389 cases and 1500 controls). Univariable analyses using logistic regression were conducted and a multivariable model was developed using regularised/penalised logistic regression. In univariable analysis, eating snow, dirt, sand or playing in a sandbox (aOR 4.14; CI 2.15-7.97) was associated with salmonellosis. This was also the only exposure significantly associated with illness in the multivariable model. Since 2004, 34.2% (n = 354) of S. Typhimuirum cases had an MLVA profile linked to a domestic reservoir. A seasonal trend with a peak in August for all Salmonella types and in February for S. Typhimurium was observed. Indirect exposure to domestic reservoirs remains a source of salmonellosis in Norway, particularly for children. Information to the public about avoiding environmental exposure should be strengthened and initiatives to combat salmonellosis in the food chain should be reinforced.

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In Norway, no published data on seroprevalence of hepatitis E virus (HEV) in humans and swine exists. Serum samples from blood donors, veterinarians, swine farm workers and swine were analysed by ELISA to estimate the seroprevalence of HEV in Norway and to investigate the association between direct contact with swine and HEV seroprevalence in humans. The seroprevalence of HEV IgG antibodies was 30% (24/79) in farm workers, 13% (21/163) in veterinarians, 14% (162/1200) in blood donors and 90% (137/153) in swine. Our results show a high seroprevalence of HEV in humans and swine in Norway. HEV seroprevalence in farm workers and blood donors increased with age, and veterinarians working with swine were twice as likely to be HEV seropositive compared to other veterinarians. High HEV seroprevalence in farm workers and veterinarians working with swine support previous reports suggesting swine as a reservoir for HEV infections in humans in Europe.

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In November 2013, the Norwegian Institute of Public Health was notified of a gastroenteritis outbreak following two meetings held at a conference centre. Identical food and beverages were served during the meetings. We investigated in order to identify the vehicle of infection and implement control measures. Meeting participants completed an online questionnaire on consumption of foods and beverages. We asked symptomatic participants to provide a stool sample. We defined a case as diarrhoea and/or vomiting in a participant who became ill within 3 days after the meeting. We calculated attack rates (AR) and adjusted risk ratios (aRR) with 95% confidence intervals (CI) using binomial regression. We conducted environmental investigations. Overall, 147/168 (88%) participants responded, of which 74 (50%) met the case definition. All five stool samples provided were norovirus positive. No kitchen staff reported being sick. Risk of illness was higher in those who consumed raspberry mousse (aRR 3·4, 95% CI 1·4-8·2) and sliced fresh fruit (aRR 1·9, 95% CI 1·3-2·8). Seventy cases (95%) ate raspberry mousse. Frozen raspberries used for the mousse were imported and not heat-treated before consumption. Non-heat-treated frozen raspberries were the most likely outbreak vehicle. Contamination by a food handler could not be excluded. We recommend heat-treatment of imported frozen berries before consumption.

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We investigated a nationwide outbreak of Salmonella Coeln in Norway, including 26 cases identified between 20 October 2013 and 4 January 2014. We performed a matched case-control study, environmental investigation and detailed traceback of food purchases to identify the source of the outbreak. In the case-control study, cases were found to be more likely than controls to have consumed a ready-to-eat salad mix (matched odds ratio 20, 95% confidence interval 2·7-∞). By traceback of purchases one brand of ready-to-eat salad was indicated, but all environmental samples were negative for Salmonella. This outbreak underlines that pre-washed and bagged salads carry a risk of infection despite thorough cleaning procedures by the importer. To further reduce the risk of infection by consumption of ready-to-eat salads product quality should be ensured by importers. Outbreaks linked to salads reinforce the importance of implementation of appropriate food safety management systems, including good practices in lettuce production.

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In 2005, the Norwegian Institute of Public Health established a web-based outbreak rapid alert system called Vesuv. The system is used for mandatory outbreak alerts from municipal medical officers, healthcare institutions, and food safety authorities. As of 2013, 1426 outbreaks have been reported, involving 32913 cases. More than half of the outbreaks occurred in healthcare institutions (759 outbreaks, 53·2%). A total of 474 (33·2%) outbreaks were associated with food or drinking water. The web-based rapid alert system has proved to be a helpful tool by enhancing reporting and enabling rapid and efficient information sharing between different authorities at both the local and national levels. It is also an important tool for event-based reporting, as required by the International Health Regulations (IHR) 2005. Collecting information from all the outbreak alerts and reports in a national database is also useful for analysing trends, such as occurrence of certain microorganisms, places or sources of infection, or route of transmission. This can facilitate the identification of specific areas where more general preventive measures are needed.

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In May 2013, Italy declared a national outbreak of hepatitis A, which also affected several foreign tourists who had recently visited the country. Molecular investigations identified some cases as infected with an identical strain of hepatitis A virus subgenotype IA. After additional European Union/European Economic Area (EU/EEA) countries reported locally acquired and travel-related cases associated with the same outbreak, an international outbreak investigation team was convened, a European outbreak case definition was issued and harmonisation of the national epidemiological and microbiological investigations was encouraged. From January 2013 to August 2014, 1,589 hepatitis A cases were reported associated with the multistate outbreak; 1,102 (70%) of the cases were hospitalised for a median time of six days; two related deaths were reported. Epidemiological and microbiological investigations implicated mixed frozen berries as the vehicle of infection of the outbreak. In order to control the spread of the outbreak, suspected or contaminated food batches were recalled, the public was recommended to heat-treat berries, and post-exposure prophylaxis of contacts was performed. The outbreak highlighted how large food-borne hepatitis A outbreaks may affect the increasingly susceptible EU/EEA general population and how, with the growing international food trade, frozen berries are a potential high-risk food.

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A total of 175 waterborne outbreaks affecting 85,995 individuals were notified to the national outbreak surveillance systems in Denmark, Finland and Norway from 1998 to 2012, and in Sweden from 1998 to 2011. Between 4 and 18 outbreaks were reported each year during this period. Outbreaks occurred throughout the countries in all seasons, but were most common (n = 75/169, 44%) between June and August. Viruses belonging to the Caliciviridae family and Campylobacter were the pathogens most frequently involved, comprising n = 51 (41%) and n = 36 (29%) of all 123 outbreaks with known aetiology respectively. Although only a few outbreaks were caused by parasites (Giardia and/or Cryptosporidium), they accounted for the largest outbreaks reported during the study period, affecting up to 53,000 persons. Most outbreaks, 124 (76%) of those with a known water source (n = 163) were linked to groundwater. A large proportion of the outbreaks (n = 130/170, 76%) affected a small number of people (less than 100 per outbreak) and were linked to single-household water supplies. However, in 11 (6%) of the outbreaks, more than 1,000 people became ill. Although outbreaks of this size are rare, they highlight the need for increased awareness, particularly of parasites, correct water treatment regimens, and vigilant management and maintenance of the water supply and distribution systems.

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We investigated an outbreak of gastroenteritis following a Christmas buffet served on 4-9 December 2012 to ~1300 hotel guests. More than 300 people were reported ill in initial interviews with hotel guests. To identify possible sources of infection we conducted a cohort investigation through which we identified 214 probable cases. Illness was associated with consumption of scrambled eggs (odds ratio 9·07, 95% confidence interval 5·20-15·84). Imported chives added fresh to the scrambled eggs were the suspected source of the outbreak but were unavailable for testing. Enterotoxigenic Escherichia coli (ETEC) infection was eventually confirmed in 40 hotel guests. This outbreak reinforces that ETEC should be considered in non-endemic countries when the clinical picture is consistent and common gastrointestinal pathogens are not found. Following this outbreak, the Norwegian Food Safety Authority recommended that imported fresh herbs should be heat-treated before use in commercial kitchens.

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Tularaemia has mainly been a sporadic disease in Norway. In 2011, 180 persons (3.7 per 100,000 population) were diagnosed with tularaemia. This article describes the epidemiological and clinical features of tularaemia cases during a year with exceptionally high tularaemia incidence. Data from the national reference laboratory for tularaemia combined with epidemiological data from the Norwegian Surveillance System for Communicable Diseases (MSIS) were used. The incidence of tularaemia varied greatly between counties, but almost every county was involved. The majority (77.8 %) of the cases were diagnosed during the autumn and winter months. The geographic distribution also showed seasonal patterns. Overall, oropharyngeal tularaemia (41.1 %) was the most common clinical presentation, followed by glandular (14.4 %), typhoidal (14.4 %), respiratory (13.3 %) and ulceroglandular (12.8 %) tularaemia. From January to April, oropharyngeal tularaemia dominated, from May to September, ulceroglandular tularaemia was most common, whereas from October to December, there was an almost even distribution between several clinical forms of tularaemia. Eighty-five (47.2 %) of all tularaemia cases were admitted to, or seen as outpatients in, hospitals. An unexpectedly high number (3.9 %) of the patients had positive blood culture with Francisella tularensis. The clinical manifestations of tularaemia in Norway in 2011 were diverse, and changing throughout the year. Classification was sometimes difficult due to uncharacteristic symptoms and unknown mode of transmission. In rodent years, tularaemia is an important differential diagnosis to keep in mind at all times of the year for a variety of clinical symptoms.

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On 7 March 2014, an increase in hepatitis A virus (HAV) infections was identified in Norway. As of 12 April, 19 cases of HAV infection with a virus strain identical to an ongoing European outbreak have been identified. Six probable cases are currently under investigation. On 11 April, a frozen berry mix cake imported from another European country was found as the likely source of the outbreak; the importer has withdrawn the product in Norway.

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In March 2012, a second outbreak of Cryptosporidium parvum affected children following a stay at a holiday farm in Norway; the first outbreak occurred in 2009. We studied a cohort of 145 schoolchildren who had visited the farm, of which 40 (28%) were cases. Cryptosporidium oocysts were detected in faecal samples from humans, goat kids and lambs. Molecular studies revealed C. parvum subtype IIa A19G1R1 in all samples including human samples from the 2009 outbreak. A dose-response relationship was found between the number of optional sessions with animals and illness, increasing from two sessions [risk ratio (RR) 2·7, 95% confidence interval (CI) 0·6-11·5] to six sessions (RR 8·0, 95% CI 1·7-37·7). The occurrence of two outbreaks 3 years apart, with the same subtype of C. parvum, suggests that the parasite is established in the farm's environment. We recommend greater emphasis on hand hygiene and routines related to animal contact.

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On 9 October 2011, the University Hospital of North Norway alerted the Norwegian Institute of Public Health (NIPH) about an increase in Shigella sonnei infections in Tromsø. The isolates had an identical 'multilocus variable-number tandem repeat analysis' (MLVA) profile. Most cases had consumed food provided by delicatessen X. On 14 October, new S. sonnei cases with the same MLVA-profile were reported from Sarpsborg, south-eastern Norway. An outbreak investigation was started to identify the source and prevent further cases. All laboratory-confirmed cases from both clusters were attempted to be interviewed. In addition, a cohort study was performed among the attendees of a banquet in Tromsø where food from delicatessen X had been served and where some people had reported being ill. A trace-back investigation was initiated. In total, 46 cases were confirmed (Tromsø= 42; Sarpsborg= 4). Having eaten basil pesto sauce or fish soup at the banquet in Tromsø were independent risk factors for disease. Basil pesto was the only common food item that had been consumed by confirmed cases occurring in Tromsø and Sarpsborg. The basil had been imported and delivered to both municipalities by the same supplier. No basil from the specific batch was left on the Norwegian market when it was identified as the likely source. As a result of the multidisciplinary investigation, which helped to identify the source, the Norwegian Food Safety Authority, together with NIPH, planned to develop recommendations for food providers on how to handle fresh plant produce prior to consumption.

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In October and November 2013, four cases of wound botulism were confirmed in people who inject drugs (PWID) in Norway. Two additional cases are suspected. Because of the international distribution pathways for heroin ­ the likely source of the outbreak ­ healthcare workers and public health authorities in other countries should remain vigilant for wound botulism in PWID. This outbreak serves as a reminder that countries should ensure access to botulinum antitoxin in case of outbreak situations.

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In 2009, following a bike race, a gastrointestinal illness outbreak affected many participants. A cohort study showed an attack rate of 16·3% with the main risk factor being mud splashes to the face. Considering these findings, in 2010 recommendations to participants in the bike race were issued and environmental control measures were implemented. In 2010, a retrospective cohort study using web-based questionnaires was conducted to measure the use of preventive measures and to assess risk factors associated with gastrointestinal illness. A 69% response rate was achieved and 11721 records analysed, with 572 (attack rate 4·9%) matching the case definition, i.e. participants reporting diarrhoea within 10 days of race. There was a clear increase in the use of mudguards (96·7% reported access to/receiving information on preventive measures) and a significant decrease in gastrointestinal illness. This may indicate that the measures have been effective and should be considered, both in terms of environmental control measures as well as individual measures.

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During a 2009 nationwide outbreak of sorbitolfermenting Escherichia coli O157 in Norway, the Norwegian Institute of Public Health was notified of diarrhoea outbreaks in two nurseries. A link to the nationwide outbreak was suspected and investigated, including retrospective cohort studies. Both nurseries had recently visited farms. Faecal specimens were obtained from symptomatic children as well as from the farm animals and tested for Campylobacter, Salmonella, Yersinia, Shigella and pathogenic E. coli, and isolates were further characterised. Nursery A had 12 symptomatic children, and we found the same strain of C. jejuni in faeces from children and lambs. Nursery B had nine symptomatic children, including one child with bloody diarrhoea carrying enterohaemorrhagic E. coli (EHEC) O26. EHEC O26 with a similar multiple-locus variable number tandem repeat analysis (MLVA)-profile was found in sheep. Five children had enteropathogenic E. coli (EPEC) O76. Animals were not tested for EPEC O76. We found no significant association between illness and risk factors for either nursery. The isolated pathogens differed from the one involved in the nationwide outbreak. In each nursery outbreak, the pathogens isolated from children matched those found in farm animals, implicating animal faeces as the source. Hygiene messages are important to prevent similar outbreaks.

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The Norwegian Surveillance System for Communicable Diseases (MSIS) includes variables related to travel for clinicians to fill when notifying travel-associated infections. We measured the completeness and validated the travel-history information for salmonellosis, campylobacteriosis, giardiasis and shigellosis reported in 2009-2010. Of all 8,978 selected infections in MSIS, 8,122 (91%) were reported with place of infection of which 5,236 (65%) were notified as acquired abroad, including 5,017 with symptoms. Of these, 2,972 (59%) notifications had information on both date of arrival in Norway and date of symptom onset, so time between travel and illness onset could be assessed. Taking in account the incubation period, of the 1,435 infections reported as travel-associated and for which symptom onset occurred after return to Norway, 1,404 (98%) would have indeed been acquired abroad. We found a high level of completeness for the variable 'place of infection'. Our evaluation suggests that the validity of this information is high. However, incomplete data in the variables 'return date to Norway' and 'date of symptoms onset', only allowed assessment of the biological plausibility of being infected abroad for 59% of the cases. We encourage clinicians to report more complete travel information. High quality information on travel-associated gastrointestinal infections is crucial for understanding trends in domestic and imported cases and evaluating implemented control measures.

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