A print friendly PDF version is available from this Communicable Diseases Intelligence issue's table of contents.

Results continued

Gastrointestinal and foodborne disease outbreaks

Risk factors for infection

Food

During 2002, OzFoodNet identified several important risk factors for foodborne illness as a result of outbreak investigations and from preliminary results of case control studies. These included risks due to the following foods and settings for foodborne disease.

Eggs

Sites continue to report outbreaks associated with the consumption of egg-based products, such as mayonnaise and salad dressings. These outbreaks can be large and serious, as highlighted by the outbreak of S. Typhimurium 8 in South Australia. There is a need to review the circumstances of egg-associated outbreaks in detail to identify potential interventions, and whether there is a need for better quality assurance in the industry. The restaurant and catering industries need to be made aware of the risks of using raw unpasteurised eggs in sauces and dressings.

Chicken and poultry

During 2002, OzFoodNet finalised a multi-state case control study into risk factors for Campylobacter infection. Preliminary results indicate that one of the major risk factors for illness is consumption of chicken. Chicken was the only vehicle implicated in Campylobacter outbreaks in 2002, despite it being the most common gastrointestinal disease notified to health departments in Australia. In 2002, outbreaks of poultry-associated salmonellosis continue to occur, including two animal-to-person outbreaks. Poultry is consumed by approximately 80 per cent of people each week. To make our food supply safer, Australia needs to consider ways to reduce the burden of infections due to this source of infection in the community.

Vietnamese pork rolls

There were two outbreaks of Salmonella infection associated with these ethnic specialty dishes during 2002. Health authorities have been aware of the health risks associated with Vietnamese pork rolls for several years. Large outbreaks associated with these rolls have occurred in at least three states due to poor preparation and handling.^17,18 The occurrence of two more outbreaks in 2002 show that they are a particularly high-risk food. Regulatory agencies and restaurants need to urgently improve the safety of these popular foods.

Red meats and meat products

There were several outbreaks associated with red meats during 2002. These were due to a mixture of pathogens and in a variety of settings. The investigation into the multi-state outbreak of Salmonella Typhimurium 170 did not identify a specific food vehicle, but suggested potential links to red meat and poultry consumption. From this investigation is clear that there is a need for better and more timely sharing of data from human, animal and food surveillance systems.

Imported foods

The two outbreaks associated with imported foods during 2002 showed the potential for the spread of foodborne disease internationally. Oysters are known to be at high risk of norovirus contamination. New Zealand reported similar outbreaks at the same time as outbreaks in Australia, although the source of oysters was different. Caterers should follow cooking instructions where provided to prevent foodborne disease. The outbreak of S. Montevideo associated with Egyptian tahini in the Hunter highlighted the potential for contamination of sesame-based products with Salmonella. In this outbreak, the level of contamination was very low (1-2 organisms per gram). However, Salmonella were able to rapidly multiply when the tahini was used to make hommus. Agencies investigating outbreaks of salmonellosis should consider tahini, helva and other sesame-based products as potential sources of contamination. It may help to increase the risk classification of these products and frequency of testing on importation into Australia

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Settings

There were several settings where food was prepared or consumed that were identified as high risk for foodborne disease, which included:

Bakeries

The five outbreaks occurring in bakeries in 2002 revealed the need for better assessment of food safety issues in these premises. Two of the outbreaks were related to Vietnamese pork rolls and the remaining three were associated with cakes filled with cream and/or custard. All outbreaks have been caused by Salmonella sp. Epidemiological investigation of these outbreaks often does not uncover the real source of contamination, as there is a time lag between food consumption and the recognition of the outbreak. There is a need for research to determine what are the critical food safety problems in these facilities that result in foodborne disease.

Restaurants and catered events

Outbreaks due to this sector constituted 57 per cent (47/92) of outbreaks. A variety of pathogens caused these outbreaks, including Salmonella, C. perfringens, norovirus and V. parahaemolyticus. Outbreaks involving restaurants and commercial caterers are more readily recognised, as the meals are often served to large numbers of persons. Clearly there is a need to continue to monitor the causes of outbreaks in this sector to identify gaps in food safety practices. The outbreak of C. perfringens poisoning due to spit roasts highlighted the problems for regulatory agencies operating at the state level dealing with food businesses operating in more than one jurisdiction with poor food practices for preparing food.

Hospitals and aged care

People resident in aged care settings and hospital patients are at particular risk for foodborne disease, which is shown by the five outbreaks that occurred during 2002. Two were due to C. perfringens indicating problems with preparation and handling of foods for residents. The outcomes for patients in these settings are often more adverse, as these sub-populations more susceptible to serious gastrointestinal disease. During 2002, OzFoodNet reviewed listeriosis notifications in Australia, which identified that three out of five listeriosis outbreaks in the years between 1995 and 2000 had occurred in hospital settings. The food supplied to hospital patients and persons in institutions should be readily monitored. There is also a high potential for foodborne transmission of Norovirus when food handlers become infected during the many person-to-person outbreaks that occur each year in these facilities.

Surveillance evaluation and enhancement

Continuous improvement of surveillance is important to ensure that foodborne illness is investigated rapidly and effectively. To improve surveillance it is necessary to evaluate and compare surveillance conducted at different sites.

National information sharing

In 2002, all jurisdictions contributed to a fortnightly national cluster report to identify foodborne illness that was occurring across state and territory boundaries. The cluster report was useful for identifying common events affecting different parts of Australia. The cluster report is useful for tracking the investigation of multi-state clusters, such as S. Typhimurium 170, and S. Potsdam. The cluster report was also important in identifying a single spit roast company as the cause of several outbreaks spread over time and several jurisdictions. The cluster report supplemented information sharing on a closed list server, teleconferences and at quarterly face-to-face meetings.

Outbreak reporting and investigation

During 2002, the Hunter site reported the highest reporting rate of outbreaks of foodborne disease (31.2 per 100,000 population) and foodborne salmonellosis (7.3 per 100,000 population). The rates of other sites reporting foodborne Salmonella outbreaks ranged between 1.3-2.1 outbreaks per 100,000 population. Victoria investigated the largest number of foodborne disease outbreaks (26 outbreaks; 5.4 per 100,000 population) and Salmonella clusters (26 clusters; 5.3 per 100,000 population).

States and territories conducted 52 analytical studies (cohort or case control studies) to investigate foodborne disease outbreaks or clusters of suspected foodborne illness. Investigators used analytical studies for 40 per cent (37/92) of foodborne disease outbreaks, which was similar to 2001. The Hunter had the highest rate for investigations of foodborne disease or potentially foodborne clusters using analytical studies, followed by South Australia. Queensland had one of the lowest rate of analytical investigation despite a high rate of reporting for foodborne outbreaks. This was mainly due to several outbreaks of ciguatera where only descriptive investigation was necessary.

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Completeness of Salmonella serotype and phage type reports

There was considerable improvement in the completeness of Salmonella available on state and territory surveillance databases between the years 2000 to 2002 (Table 10). Overall 96.2 per cent to (273/7,267) of Salmonella notification on databases contained either serotype or phage type, which was an increase of 4.3 per cent from 2000 and 1.7 per cent from 2001.

Only 89.1 per cent (49/55) of phage type information was reported for S. Hadar and 92.7 per cent (114/123) S.  Heidelberg. Phage typing information was available for 95.0 per cent (307/323) of reports for S. Enteritidis in 2002. The largest increase in completeness between 2000 and 2002 was reported for S. Heidelberg (25.4%) and S. Bovismorbificans (16%).

South Australia had the highest proportion of complete Salmonella notification (100%), while four sites reported 98 per cent or higher. New South Wales reported the lowest rate of completeness, but recorded a 10.8 per cent improvement when compared to 2000 figures. Western Australia also reported an increase of 10.7 per cent, as a result of improved case notification to the health department.

Table 10. Number of Salmonella infections notified and proportion of notifications with serotype and phage type information available, Australia, 2000 to 2002, by OzFoodNet site

Information required	Year	Salmonella notifications n	Salmonella with serotype %	Salmonella infections with phage typing information						Salmonella infections with information %
				S. Bovismorbificans %	S. Enteritidis %	S. Hadar %	S. Heidelberg %	S. Typhimurium %	S. Virchow %
ACT	2000	102	96.1	-	100.0	0.0	0.0	100.0	100.0	95.1
	2001	78	98.7	100.0	100.0	100.0	-	100.0	100.0	98.7
	2002	96	97.9	100.0	100.0	-	-	91.1	100.0	93.8
Hunter	2000	86	94.2	100.0	50.0	66.7	100.0	97.4	0.0	87.2
	2001	117	97.4	85.7	50.0	-	50.0	97.1	100.0	92.3
	2002	179	95.5	100.0	100.0	-	0.0	95.1	100.0	92.7
NSW	2000	1,334	92.2	50.0	77.8	52.9	18.2	88.1	38.2	80.5
	2001	1,668	94.2	63.6	80.8	35.3	84.6	95.8	67.2	87.9
	2002	2,147	95.5	92.2	81.5	58.3	75.9	95.7	89.2	91.3
NT	2000	323	91.6	33.3	20.0	50.0	100.0	100.0	50.0	88.9
	2001	390	90.8	100.0	33.3	100.0	-	100.0	100.0	90.3
	2002	329	96.0	100.0	66.7	-	-	100.0	-	95.7
Qld	2000	1,818	97.2	100.0	94.8	100.0	90.6	93.1	97.4	97.7
	2001	2,169	97.0	100.0	91.8	73.3	91.8	95.8	95.0	97.8
	2002	2,722	97.5	100.0	100.0	100.0	97.6	98.1	98.9	99.4
SA	2000	452	99.6	100.0	100.0	100.0	-	100.0	100.0	99.6
	2001	613	99.8	100.0	100.0	100.0	-	100.0	100.0	99.8
	2002	520	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0
Tas.	2000	127	96.9	100.0	100.0	-	0.0	100.0	100.0	96.1
	2001	159	98.7	100.0	100.0	-	-	96.3	-	98.1
	2002	165	99.4	100.0	100.0	100.0	-	100.0	100.0	99.4
Vic.	2000	1,005	97.9	96.3	100.0	90.0	75.0	99.8	99.1	97.4
	2001	1,090	97.7	100.0	100.0	100.0	100.0	99.8	100.0	97.6
	2002	1,207	99.2	90.9	100.0	88.9	100.0	100.0	100.0	99.0
WA	2000	936	92.4	100.0	93.1	85.0	0.0	91.4	80.0	87.7
	2001	858	95.8	85.7	95.7	85.7	0.0	97.0	66.7	93.8
	2002	730	98.6	100.0	100.0	100.0	-	99.2	100.0	98.4
OzFoodNet	2000	6,097	95.4	78.3	90.5	77.8	67.3	94.1	90.4	92.0
	2001	7,025	96.2	87.3	91.4	78.8	88.5	97.5	92.4	94.7
	2002	7,916	97.4	94.3	95.0	89.1	92.7	98.0	97.6	96.9

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Discussion

Each year in Australia, it is estimated that 17.2 million persons experience infectious gastroenteritis and approximately 5.4 million (credible interval 4.0-6.9 million) of these may be due to contaminated food. These estimates are comparable to previous Australian and other international reports and clearly demonstrate the burden that foodborne disease has on Australian society.^3,19,20 The large burden justifies the attention given to foodborne disease surveillance and enhancing the safety of our food supply.

Gastrointestinal infections notified to health departments represent only a small proportion of cases occurring in the community, as most are mild and do not require medical attention. The gastroenteritis survey provides insight into the health seeking behaviour of Australians, with one in five persons with gastroenteritis visiting a doctor and one in 20 providing a faecal specimen. An intervention trial in Melbourne found that persons with acute gastroenteritis who submitted faecal specimens had pathogens identified in only 25 per cent of stools despite intensive testing, which demonstrates that there are many other gastrointestinal pathogens that are unrecognised.²¹ Despite this, notifications of gastrointestinal infections to health departments provide a picture of illness that may potentially be due to food.

In 2002, notifications of selected gastrointestinal infections in Australia were 7.7 per cent higher than the historical mean, which may reflect a true increase in incidence, changing laboratory practices or improving surveillance. Certainly, the increased rates of STEC notification reflected changing patterns of testing faeces and diagnostic tests. This is likely to increase in future, as laboratories screen more specimens and diagnostic tests improve. The crude notification rate of Salmonella infections also increased, while Campylobacter and Listeria infection rates were stable. There were decreases observed for yersiniosis, shigellosis and haemolytic uraemic syndrome.

Australia has similar rates of notified gastrointestinal infections to some other developed countries including Canada, Norway, and the United Kingdom.^22,23,24 Australian rates are lower than rates in neighbouring New Zealand and higher than active surveillance data for salmonellosis in the United States of America (USA).^25,26 Notified Salmonella in the USA affects 16.1 cases per 100,000 population compared to 40.3 cases per 100,000 population in Australia. Even more startling is the difference in Campylobacter notification rates in the USA at 13.4 cases per 100,000 population compared to 110.1 cases per 100,000 population in Australia. The lower rates in the USA may be due to differences in access to healthcare, stool submission rates and testing regimes in laboratories.

The USA reports higher rates of toxigenic E. coli O157:H7 (1.7 cases per 100,000 population) than Australia.²⁶ This organism is easily isolated on routine pathology media and may reflect changes in testing procedures or a true difference in incidence. In 2002, Australian states and territories reported a doubling in the number of E. coli O157 infections, although the total numbers and rates remain small. H typing was not available for the majority of these, but it is likely that the majority are not the H7 subtype as laboratories have rarely isolated it in previous years. The increasing use of molecular detection methods often means that organisms are not cultured for faecal specimens for subsequent serotyping or profiling.

South Australia has conducted enhanced surveillance for STEC for several years, but had not identified any outbreaks until the outbreak at a petting zoo at a regional fair in 2002. Petting zoos have been commonly associated with outbreaks of STEC and other gastrointestinal diseases.^27,28,29 There were two other outbreaks of salmonellosis following poultry hatching programs in two different states. While some Australian states have prepared guidelines for petting zoos it is important that these cover poultry hatching programs and that all zoo operators are aware of the requirements to prevent infections.³⁰

Salmonella caused the most foodborne outbreaks of any agent during 2002. Like many other countries, Salmonella infections are a serious problem for Australia.³¹ Not only do they cause considerable morbidity, but investigations consume much public health effort and resources. For every two Salmonella outbreaks that are attributed to food, there are another three cluster investigations where no source is identified. In addition, there may be as many as 14 cases in the community for every case reported to Australian surveillance systems (OzFoodNet unpublished data). To identify causes of more of these outbreaks, we may need to critically evaluate our current methods of investigation. Investigations are becoming more complicated due to the increasing use of molecular methods for comparing isolates and regular trace back of foods consumed by cases to the source of food supply.^31,32

OzFoodNet identified several risk factors for foodborne infections in 2002 based on the surveillance data and epidemiological studies. Many of these risk factors have been previously recognised, but need to be considered again. The risk posed by raw eggs used in dressings could be easily addressed by the use of pasteurised eggs. While Australia does not have endemic S. Enteritidis 4 that contaminates the internal contents of eggs, there are clearly other subtypes that are associated with eggs. Infections due to poultry, red meats and imported foods continued to occur in 2002. Food handling and preparation practices in bakeries need to be addressed to prevent outbreaks of salmonellosis, which are an increasing problem.^33,34,35

It is important to recognise some of the many limitations of the data that OzFoodNet report. Surveillance data are inherently biased and require careful interpretation. These biases include: the higher likelihood that certain population groups will be tested, and different testing regimes in different states and territories, resulting in different rates of disease. Some of the numbers of notifications are small, as are populations in some jurisdictions. This can make rates of notification unstable and meaningful interpretation difficult. Importantly, some of the most common enteric pathogens are not notifiable, particularly norovirus and enteropathogenic E. coli. These organisms may be notified as the cause of outbreaks, but not individual cases of disease. There can also be considerable variation in assigning causes to outbreaks depending on investigators and circumstances.

There have been consistent improvements to surveillance in recent years, which is shown by the large number of analytical studies used in investigations of outbreaks. The success of national communication through OzFoodNet was highlighted by the identification of 12 separate incidents associated with a single spit roast company operating in several states. We observed a difference in the rate of reporting foodborne disease outbreaks that probably reflects sensitivity of surveillance and differing thresholds for investigation in different jurisdictions.³⁶ There was a significant improvement in the completeness of Salmonella typing information on state and territory databases, which reflects better quality surveillance data. In the future, OzFoodNet aims to regularly compare the timeliness of Salmonella typing reporting to review the effectiveness of data transmission for surveillance systems.

Despite these improvements to surveillance, we need to critically evaluate our efforts in order to prevent foodborne infections. In particular, there is a need to strengthen laboratory-based surveillance using standardised molecular methods for profiling organisms, such as Listeria, outbreak-associated Salmonella and shiga toxin producing E. coli. This information needs to be rapidly communicated to public health investigators to enable more timely investigation of widely spread clusters and prevention of outbreaks. Countries such as the United Kingdom routinely use PFGE as a successful adjunct to traditional Salmonella typing to assist with outbreak investigations.³⁷ Other potential improvements could include: standardised approaches to cluster investigation, and sharing surveillance information from animal and food testing data. None of these new initiatives will occur without appropriate resources, but the burden of diseases clearly requires that we improve to acquire better surveillance data to support control activities.

The burden of foodborne disease is a major concern to the community, industry and government. It is important that foodborne disease surveillance is able to assess whether food safety policies and campaigns are working. National surveillance of foodborne diseases has many benefits and provides long-term data to assist with this task. OzFoodNet needs to consider what kinds of foodborne disease data are useful to evaluate the effect of interventions to make food safer. This will require closer working relationships with food safety professionals, microbiologists, veterinarians and industry. To ensure that control of foodborne disease remains a focus for government, it may be pertinent to set target goals for foodborne diseases similar to other national health priority areas.^26,38

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Acknowledgements

This report is based on the work of epidemiologists in each of the eight OzFoodNet sites during 2002: Rosie Ashbolt, Karen Dempsey, Joy Gregory, Karin Lalor, Geoff Millard, Jennie Musto, Leonie Neville, Jane Raupach, Mohinder Sarna, Russell Stafford, and Leanne Unicomb. It also represents the work of Gillian Hall, Nola Tomaska and Rebecca Hundy from the National Centre for Epidemiology and Population Health, Martyn Kirk, Janet Li and Craig Williams from OzFoodNet. Epidemiologists, project officers, interviewers and research assistants at each of the sites contributed to this report, including: Robert Bell, Barry Combs, Dot Little, Tony Merritt and Lillian Mwanri.

We would like to thank the many people who assisted OzFoodNet in our work during 2002, particularly our international colleagues from the United States of America, Canada, the United Kingdom, Ireland, New Zealand and the World Health Organization. We would also like to thank members of the Communicable Disease Network Australia, the Public Health Laboratory Network, and the Australian Campylobacter subtyping network.

We really appreciated the managerial assistance from staff of the Department of Health and Ageing, Food Standards Australia New Zealand, state and territory health departments and the National Public Health Partnership.

We also acknowledge the hard work of various public health professionals and laboratory staff around Australia who interviewed patients, tested specimens and investigated outbreaks. The high quality of their work is the foundation of this report.

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References

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This article was published in Communicable Diseases Intelligence Volume 27, No 2, June 2003.