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Incidence of foodborne disease, continued

Foodborne disease outbreaks

During 2001, several significant outbreaks occurred and some important themes emerged. This section discusses some of these outbreaks and summarises preliminary data. Common themes from these outbreaks are discussed in the section on 'Information on risk factors'. For a summary list of individual outbreaks associated with food or water reported by OzFoodNet sites see Appendix 2.

In 2001, OzFoodNet sites reported 86 outbreaks that were potentially related to food (Table 2). The 86 outbreaks affected approximately 1,768 people, of whom 4.0 per cent (70/1,768) were hospitalised and one person died. The majority of outbreaks occurred in summer and autumn, although there was a peak in December relating to pre-Christmas functions (Figure 13).

Table 2. Number of outbreaks and clusters, 2001, reported by OzFoodNet site and pathogen

OzFoodNet site	Number of outbreaks, by pathogen type							Vehicles identified		Median number of cases per outbreak (range)	Analytical studies		Clusters investigated||
	Salmonella	Campylobacter	Parasites	Toxin	Viral	Unknown*	Total	Conf†	Susp‡		CC§	Cohort
ACT	0	0	0	0	0	6	6	0	6	46 (19-110)	0	0	3
Vic	7	2	0	3	4	7	23	9	5	22 (3-269)	2	11	26
Hunter	1	1	0	1	0	8	11	5	5	4 (2-20)	0	1	3
Qld	5	2	1	9	1	5	23	15	1	8 (2-87)	2	4	5
WA	3	0	0	0	2	5	10	3	3	20 (4-56)	0	7	1
SA	8	1	0	0	0	0	9	6	0	13 (2-90)	2	4	7
Tas	2	0	0	0	0	1	3	0	1	7 (6-9)	0	2	3
Aust	1	0	0	0	0	0	1	1	0	27	0	0	-
Total	27	6	1	13	9	34	86	39	20	10 (2-269)	6	29	48

* Outbreaks where the aetiology was suspected, but not confirmed have been categorised as 'Unknown'.
† Confirmed vehicle, either microbiologically and/or epidemiologically.
‡ Suspected vehicle, from descriptive epidemiology and biological plausibility, and/or non-specific microbiological indicators.
§ Case control study.
|| The Australian Capital Territory and Tasmania reported investigating only clusters of non-salmonellosis, where Victoria, the Hunter and Queensland reported clusters of Salmonella, and South Australia reported additional investigation of suspected person-to-person transmission.

Figure 13. Foodborne disease outbreaks, 2001, by month and agent



Thirty-one per cent (27/86) of outbreaks were due to Salmonella, while 37 per cent (32/86) were of unknown aetiology. OzFoodNet sites reported that a food vehicle was confirmed for 45 per cent (39/86) of outbreaks and suspected for a further 23 per cent (20/86) of outbreaks. To investigate these outbreaks, health departments conducted 29 cohort studies and 6 case control studies. In addition, sites reported details of 48 investigations into temporal or geographical increases of enteric pathogens, although this number is an underestimate as this information is poorly recorded.

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Significant outbreaks

Australia participated in two international outbreak investigations during 2001. In the first, the Victorian Department of Human Services (DHS) investigated an outbreak of Salmonella Typhimurium Definitive Type 104 associated with helva imported from Turkey.⁷ The Victorian DHS investigated this in conjunction with Sweden, Norway and other European countries. Eighty-seven per cent (20/23) of Australian cases occurred in Victoria, with 2 cases occurring in New South Wales and one in Queensland.

The second international outbreak was due to Salmonella Stanley associated with dried peanuts imported from China.⁸ OzFoodNet coordinated the investigation into this outbreak at the request of CDNA. The investigation was unusual in that it involved small numbers of cases in several Australian jurisdictions. OzFoodNet epidemiologists and health department staff conducted hypothesis-generating interviews, which were collated centrally. The source of the outbreak became clear when the Victorian DHS and the Microbiological Diagnostic Unit, Public Health Laboratory tested dried peanuts nominated by 2 cases. The peanuts subsequently tested positive for three Salmonella serovars: Stanley, Newport and Lexington. This finding triggered an international product recall and assisted health agencies in Canada and the United Kingdom who were investigating similar outbreaks.

The largest community-wide outbreak in 2001 occurred in South Australia and was due to Salmonella Typhimurium phage type 126.⁹ The outbreak lasted for several months, with cases emerging in other jurisdictions later in the epidemic. South Australian investigators conducted a case-control study showing that illness was associated with consumption of chicken. The department also identified corroborating evidence for this link, including descriptive epidemiology and microbiological evidence from samples of raw chicken. The South Australian DHS observed a decrease in human cases of this infection following interventions in the local chicken industry, at the breeder farm, hatchery and processing plant level.

This outbreak raised again the question of the role that contaminated chicken products play in the epidemiology of Salmonella and Campylobacter in humans in Australia. Following the outbreak, a submission to the Food Regulation Standing Committee prompted an examination of this issue by a Development and Implementation Sub-Committee working group.

In June 2001, Queensland investigated a statewide increase in Salmonella Bovismorbificans phage type 32. Investigators suspected that the outbreak was linked to a food product purchased from a fast food restaurant, and conducted a case control study. The study implicated a product containing iceberg lettuce. Environmental investigations identified a mechanical slicer at the processing facility that was positive for Salmonella Bovismorbificans phage type 32.

Agents and vehicles

Thirty-one per cent (27/86) of outbreaks during 2001 were due to Salmonella, with S.   Typhimurium causing 16 outbreaks (Table 3). The proportion of Salmonella outbreaks with good quality evidence for an implicated source was very high, with 52 per cent (14/27) having analytical and/or microbiological evidence. Despite Campylobacter being the most commonly notified pathogen to health authorities, only 6 outbreaks were recorded. Queensland recorded 83 per cent (5/6) of outbreaks of ciguatera poisoning due to fish that were locally caught and consumed. Consumption of fish caused 2 outbreaks of oily diarrhoea due to escolar wax esters and one of histamine poisoning. Norwalk-like viruses were responsible for 8 per cent (7/86) of outbreaks, although it is likely that many outbreaks of unknown aetiology could be caused by these viruses.

Table 3. Number of outbreaks reported, 2001, by aetiological agent, and level of evidence

Agent category	A	A+M	D	D+M	M	Total
C.   perfringens	1	1	2	-	-	4
Norwalk virus	3	-	4	-	-	7
Campylobacteriosis	1	-	4	1	-	6
Ciguatera	-	-	6	-	-	6
Cryptosporidiosis	-	1	-	-	-	1
Escolar wax esters	-	-	-	2	-	2
Salmonella other	-	1	7	1	-	9
Salmonella Typhimurium	3	6	6	1	-	16
Salmonella Virchow	-	-	1	-	1	2
Scombrotoxicosis	-	-	1	-	-	1
Suspected Norwalk virus	-	-	2	-	-	2
Suspected campylobacteriosis	-	-	1	-	-	1
Suspected salmonellosis	-	-	1	-	-	1
Suspected toxin	-	-	8	-	-	8
Unknown	2	-	18	-	-	20
Total	10	9	61	5	1	86

D Descriptive evidence implicating the suspected vehicle or suggesting foodborne transmission.
M Microbiological confirmation of agent in the suspect vehicle and cases.
A Analytical association between illness and one or more foods.

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Outbreak settings

A summary of outbreaks by settings reveals that 29 per cent (25/86) of outbreaks were associated with restaurants, which affected an estimated 381 people (Table 4). Outbreaks at conferences and functions affected the most people (765 cases) and had the largest median outbreak size of 40.5 persons. The hospitalisation rate was very low in this outbreak setting. There were 5 outbreaks in aged care settings affecting 51 people, 10 of whom were hospitalised. Fast food and takeaway businesses were implicated in a smaller number of outbreaks. These outbreaks had a smaller median size of 3 persons.

Table 4. Number of foodborne disease outbreaks, 2001, by settings

Setting	Outbreaks (n)	Affected (n)	Hospitalised (n)	Hospitalisation rate (%)	Median Number affected (Range)
Aged care	5	51	10	19.6	14.5 (3-49)
Camp	6	207	2	1.0	30 (11-87)
Community	5	161	16	9.9	23 (6-88)
Conference/function	15	765	2	0.3	40.5 (2-269)
Home	13	81	24	29.6	7 (2-16)
Hotel	5	36	3	8.3	8 (6-22)
Nationwide	1	27
Restaurant	25	382	8	2.1	8.5 (2-95)
Takeaway	11	50	5	10.0	3 (2-10)
Total	86	1,759	70	4.0	9 (2-269)

It is important to consider when reviewing these data that the setting plays an important role in the recognition and investigation of an outbreak. An outbreak in a conference setting where many people eat common food is easily recognised because many people become ill. Outbreaks associated with takeaway food are difficult to detect. While the volume of food prepared might be very large, it is difficult to identify consumers of contaminated takeaway food as they may be widely dispersed in the community. Some of these outbreaks reported here may have resulted from food safety problems in settings other than those mentioned, as contributing factors have not been taken into account.

No food vehicle was identified in 31 per cent (27/86) of outbreaks in 2001 (Table 5). The most common categories were for meat and poultry, which were responsible for 14 per cent (12/86) and 13 per cent (11/86) of outbreaks respectively. There were 3 outbreaks that were associated with eggs. Fish or shellfish were responsible for, or suspected to have caused 11 outbreaks. The majority of seafood-associated investigations were descriptive, as they were small toxin-related outbreaks where diagnosis was made on clinical grounds. Desserts were responsible for 7 per cent (6/86) of outbreaks, while salads, vegetables or fruits were responsible for 3 per cent (4/86) of outbreaks.

There were 2 outbreaks associated with contaminated drinking water, although investigators only obtained descriptive epidemiological data. One of these was a camp water supply with a high coliform count, and the other was a remote mine site where bore water was suspected as the cause. During 2001, there were 2 outbreaks due to unpasteurised milk. One of these was a small outbreak of cryptosporidiosis associated with milk intended for animal consumption. The other outbreak was suspected to be caused by unpasteurised milk consumed while on a school camp.

Table 5. Outbreaks reported to OzFoodNet sites, 2001, by vehicle category and level of evidence

Vehicle category	Level of evidence					Total
	A	A+M	D	D+M	M
Dessert*	2	4	-	-	-	6
Drinking water†	-	-	2	-	-	2
Eggs	-	1	-	-	-	1
Suspected eggs	-	-	2	-	-	2
Fish/shellfish/seafood	-	-	7	2	-	9
Suspected fish/shellfish	-	-	2	-	-	2
Milk‡	-	1	1	-	-	2
Miscellaneous	-	-	-	1	-	1
Mixed vehicles	2	-	-	-	1	3
Pizza	-	-	5	-	-	5
Poultry	-	1	2	1	-	4
Suspected poultry	1	-	6	-	-	7
Red meat/meat products	3	1	-	1	-	5
Suspected red meat/meat products	1	-	6	-	-	7
Salad/vegetable/fruit§	1	1	1	-	-	3
Unknown	-	-	27	-	-	27
Total	10	9	61	5	1	86

* One outbreak was suspected to be caused by the dessert based on mildly elevated relative risks.
† One outbreak was suspected to be due to drinking water contamination, based on circumstantial evidence.
‡ One outbreak was suspected to be due to unpasteurised milk based on circumstantial evidence, and descriptive epidemiology.
§ One outbreak was suspected to be caused by salads consumed at a barbecue.
D Descriptive evidence implicating the suspected vehicle or suggesting foodborne transmission.
M Microbiological confirmation of agent in the suspect vehicle and cases.
A Analytical association between illness and one or more foods.

The settings in which outbreaks occurred varied with the agent implicated (Table 6). Outbreaks due to salmonellosis occurred in many settings, compared to outbreaks due to C. perfringens where 75 per cent (3/4) of outbreaks occurred in restaurants. Conference or functions, or restaurants were the setting for all of the suspected toxin outbreaks, which would be explained by poor handling of foods. All ciguatera poisoning outbreaks reported in 2001 occurred in homes.
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Table 6. Agents responsible for foodborne disease outbreaks associated with different settings, OzFoodNet sites, 2001

Agent category	Aged care	Camp	Community wide	Conference/function	Home	Hotel	Nationwide	Restaurant	Takeaway	Total
C.   perfringens						1		3		4
Norwalk virus		1		2				4		7
Campylobacteriosis	1			1				3	1	6
Ciguatera					6					6
Cryptosporidiosis			1							1
Escolar				1				1		2
Salmonella other	2	1	2		1	1	1	1		9
Salmonella Typhimurium	1	2	2		4	1		4	2	16
Salmonella Virchow				1	1					2
Scombrotoxicosis								1		1
Suspected Norwalk virus				2						2
Suspected campylobacteriosis		1								1
Suspected salmonellosis									1	1
Suspected toxin				5				3		8
Unknown	1	1		3	1	2		5	7	20
Total	5	6	5	15	13	5	1	25	11	86

It is important to interpret these data cautiously, as we have only reported food vehicles and not sources of infection or cause of contamination (Box 1).

Box 1. Attributing source of Salmonella Typhimurium 64 outbreak

An outbreak in Western Australia of Salmonella Typhimurium 64 was epidemiologically linked to fried ice cream. Fried ice cream has been categorised as a dessert. The cause of this outbreak was related to several potential breaches in food safety, including: using raw eggs to make the batter; using bread crumbs that were also used for crumbing chicken and other meats; and inadequate cooking. The original cause of contamination in this outbreak could have been either raw eggs or cross-contaminated bread crumbs.

This article was published in Communicable Diseases Intelligence Volume 26, No 3, September 2002