Eurosurveillance

ECDC

An outbreak of salmonella typhimurium from unpasteurized milk in Finland

 Rediger
 1 Published: 24.05.05 Updated: 24.05.2005 13:29:56
Kirsi Skogberg, KTL, Paula Hyvönen, EELA, Anja Siitonen, KTL, Pekka Nuorti, KTL
In August 1999, two clusters of gastroenteritis caused by Salmonella typhimurium phage type (PT) 1 were identified in Western Finland. The local authorities were quickly able to link both clusters to raw milk bought from one farm. A large outbreak was observed among participants of a family meeting. A cohort study among participants of the family meeting, identified unpasteurized milk as the vehicle of infection. The same Salmonella phage type was cultured from the specimens taken from the patients, the cows and milk. In Finland, Salmonella typhimurium PT 1 is the most common cause of domestic salmonellosis.

On August 14, 1999, a family meeting was organised in Western Finland. The participants developed gastroenteritis with diarrhea shortly after the occasion. A food handler running a local catering service prepared the food served. Around the same time, 20 residents of the same apartment building in a nearby town became ill with diarrhea and fever. All had eaten home-prepared cheese prepared by one of the residents. According to the preliminary reports, the causative agent in both clusters was Salmonella group B. Both the food handler from the catering service and the person who prepared the cheese had used raw milk obtained from a local farm.

Methods

The local health authorities, the Department of Infectious Disease Epidemiology, National Public Health Institute, National Veterinary and Food Research Institute, and National Food Administration participated in the investigations. Because the source of infection was suspected to be in the local farm, stool cultures were obtained from the personnel and the cows. Milk, feed and water from the farm were also cultured for salmonella. A retrospective cohort study was performed among the participants of the family meeting. A self-administered standard questionnaire was mailed to the participants to obtain information about gastrointestinal symptoms and food and drink consumed during the meeting. Food and drink were specified according to the menu.
Results

Retrospective cohort study

A total of 154 persons participated in the family meeting. Eighty two percent (121 persons) returned the questionnaire. A case was defined as an illness with diarrhea in a participant of the family meeting on August 14, 1999. Sixty one persons (53%) met the case definition. Three food items (home-prepared cheese made of unpasteurized milk, cheese made of unpasteurized butter milk and Italian salad) were significantly associated with gastroenteritis (Table 1). Twenty seven other food items and drinks tested were not associated with illness.

Table 1. Food items significantly assosiated with gastroenteritis

Exposure:

Ate

Did not eat

 

Food item

|||

Toal

Attack rate

|||

Total

Attack rate

Relative risk

95% CI*

p-value

Milk cheese

61

80

76%

0

34

0%

undefined

 

<0.001

Buttermilk cheese

54

76

71%

7

39

18%

4.0

2.0 - 7.9

<0.001

Italian salad

44

72

61%

16

42

38%

1.6

1.0 - 2.5

0.03

*95 percent confidence interval

Since both milk cheese and sour milk cheese were associated with illness, a stratified analysis was performed. None of those who had eaten only cheese made of butter milk (six cases) became ill, while 70 percent (7/10) of those who had eaten cheese made of milk had diarrhea. When persons who had eaten milk cheese were stratified by whether they ate Italian salad or not, none of those who consumed only Italian salad (12 cases) had symptoms, while 80 percent (16/20) of those who had eaten milk cheese had diarrhea. On the basis of stratified analysis, milk cheese was confirmed as the vehicle of the outbreak.

Figure 1. Timing of the outbreaks

Microbiologic results

A total of 120 persons in the two clusters had a positive stool culture for S. Typhimurium PT 1. Identical salmonella strains were isolated from 15 of 20 milk cows, the milk tank and samples taken from the cow feed in the farm. No salmonella was recovered from cows pasturing separately from the milk cows or from the chickens in the farm.

Control measures

Control measures in the farm were implemented according to the guidelines issued by the Finnish Ministry of Agriculture and Forestry. Distribution of milk was only allowed for pasteurisation and the dairy had to be informed about the farm's salmonella situation. Guidelines were also given regarding the use of manure and feed as well as desinfection procedures and use of facilities. These restrictions were implemented until the cattle was confirmed to be free of salmonella in subsequent testing. The infected food-handler was restricted from working.

Conclusion

These outbreaks highlight the importance of monitoring the primary production of foodstuffs of animal origin and pasteurisation of milk. Using raw milk as an ingredient for home made cheese appears particularly risky, because the low cooking temperature (40 C) is optimal for growth of salmonella. However, monitoring and control of small scale trade between a farm and a single caterer is difficult. According to the current regulations, a farm is only allowed to sell unpasteurized milk directly to a single consumer but not for commercial use. If unpasteurized milk products are commercially distributed, the farm has to be approved and registered as a milk producer. This registration entails regular self-check programs and monitoring by the municipal health protection authorities. Distributing unpasteurized milk from a farm where salmonella has been detected is not allowed. The public should be educated about the health risks involved in the consumption of raw milk.


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