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A Salmonella Outbreak in a Nursery School

Between the 5 April, the Epidemiology Unit of the Naples 4  Local Health Agency received notification five cases of Salmonella Group D, all of whom were residents San Giuseppe Vesuviano (26,636 inhabitants). The five cases, who ranged in age from 3 to 5 years and were all females, were reported by hospitals belonging to two other Local Health Agencies in the area. The cases all had onset of symptoms between 30th March and 2nd April, and 4 of the 5 attended the same nursery school in San Giuseppe Vesuviano.

 

On the 9th of April the health unit began an epidemiological investigation to verify and to establish the extent of the outbreak and to identify the source and vehicle of infection. More information was collected on the notified cases, and the investigators then focused their investigation on the school canteen since 4 of the 5 cases had eaten common meals there. The Epidemiology Unit directly contacted the nursery school on April 11th, a few days before it closed for the Easter break. (April 15th). Neither food samples nor ingredients used in the previous 2 weeks at the school were available for analysis. The school management provided absentee records with contact details of the 120 children registered at the school and the menus detailing meals served in the days preceding onset of symptoms. The investigators contacted the mothers of children who were absent for more than 2 days between end of March and the beginning of April in order to identify additional cases.

 

Between the 24th and 26th of April, parents of children from the two classes in which there were confirmed cases were interviewed about their children’s food preferences.  A case was defined as any child resident in the district who, between March 20 and April 8, had symptoms of diarrhoea (at least 3 stools per day) with at least one of the following symptoms: fever (≥ 38°C), abdominal pain, nausea and vomiting.

 

A total of 19 cases were identified with symptoms of diarrhoea (100%), high fever (79%), abdominal pain (58%) and nausea/vomiting (21%) between March 30th and April 2nd. The epidemic curve indicated a point-source of exposure.  Assuming an incubation period for non-typhoid Salmonellosis of 12-72 hours, exposure most likely occurred on the 28th or 29th of March. On the 28th, the menu contained omelettes prepared on the premises using 100 eggs and cooked in the oven, while on the 29th, the children were given commercially prepared pre-cooked chicken strips.

 

The stool cultures from the five original cases could not be typed because the hospital laboratories had not kept the strains after initial tests were completed. Case histories did not provide information implicating any specific suspect food, nor were the results of the interviews with the mothers conclusive. As a result, it was not possible to identify a specific vehicle of infection for the outbreak.

 

The temporal association of cases (all cases ill within hours or days of each other), as well as their similar ages and attendance at the same nursery school, provide some evidence of the possible role of the nursery school kitchen, especially since the only meal the cases had in common on the 28th or 29th of March was at this nursery school. Many factors contributed to the lack of further identification of the source or vehicle of infection and the mode of transmission. These included the delay in the notification of the cases with respect to the onset of illness, which made it difficult to collect accurate information about possible exposures associated with the illness and to obtain possible samples of the foods that had been served. Also, the timing of the outbreak (before Easter break) hampered the investigation, since there was not enough time to obtain all the information from the nursery school before it closed for Easter.  The lack of accurate information on the part of the mothers interviewed was also probably a consequence of this delay. Furthermore, the interviews were conducted several weeks after the end of the outbreak, such that the mothers had little interest in collaboration at that stage.

 

Finally, the hospital microbiology laboratory, which is interested primarily in identifying the micro-organism so appropriate treatment can be initiated, may have underestimated the importance of serotyping of strains for epidemiological surveillance.  For this reason, the samples were not conserved for further typing elsewhere.

 

This investigation identified several gaps between the current organisational operations and the potential control measures that are required during an outbreak situation and provided the opportunity to highlight a few points that require improvement:

1.   Timeliness and sensitivity of the foodborne disease surveillance system of the Campania region need improvement. Timely reporting is needed to make the flow of data more efficient, and family doctors need to be encouraged to notify and to include laboratory-based notifications in order to improve sensitivity.

2.   A regional reference laboratory with the capacity to perform serotyping is needed for the surveillance of Salmonella and other.

3.   Efforts are neededo promote collaboration among doctors and families, health service workers and food companies by providing information on confirmed outbreaks and results of epidemiological investigations.

 

Reported by:
Maria Grazia Panico (Epidemiology Service, Local Health Unit Naples No. 4), Benvon Cotter (ISS, Rome, EPIET Fellow), Emilio Russo (Health District no. 79 Naples No.4)

 

Editorial Note:

Salmonellosis is an important public health problem in industrialised countries and Salmonella is one of the most common pathogens associated with foodborne outbreaks (1). The foods most frequently implicated as vehicles of infection are meat, poultry products, eggs and their derivatives contaminated at the point of origin and not subjected to an adequate heat treatment, thereby creating an ideal environment for these bacteria to multiply. Recently, Salmonella has also been associated with other non-typical vehicles as a result of the increased use of catering services and commercial ready-to-eat foods e.g. nine cases associated with the consumption of packaged, ready-to -eat salad (2).

 

Knowledge of the pathogen and associated risk factors for contamination is essential in order to adopt appropriate control measures and for the prevention of further outbreaks. There are approximately 10,000 cases of non-typhoid Salmonella notified in Italy each year and approximately 700 foodborne outbreaks (3). However, these figures probably grossly underestimate the true burden of foodborne outbreaks, which often occur in family settings or are not investigated microbiologically or epidemiologically.

 

The Naples 4 Local Health Agency detected an outbreak involving 19 children in a nursery school but was not able to identify a particular food nor whether there were unhygienic food preparation methods or inadequate storage in the nursery school premises that could have contributed to the problem.

 

Why investigate such foodborne outbreaks, considering that such investigations represent a heavy demand on time and resources? Is it useful to publish reports such as this on outbreaks even if they are not fully conclusive?  The answer is ‘yes’ for the following reasons:

1)   Salmonellosis is a potentially severe disease and can lead to death and economic losses due to treatment and other indirect costs, including the losses incurred by any commercial company that may be involved;

2)   If the vehicle is a commercial product, the investigation is essential to identify the product in order to stop or recall its production/distribution

3)   Identifying a particular food or problems in the preparation or storage practices provides important information for the food handlers and consumers in order to know what measures need to be taken for particular foods that may need extra attention (e.g. tiramisu, a dessert prepared with raw eggs).

4)   Distinguishing between different strains of Salmonella by serotyping and/or phagetyping can be useful to determine geographic spread of the disease.

 

This outbreak was difficult to investigate because it involved young children and it occurred just prior to the Easter holiday break which was actually prolonged this year until 25th of April as a result of a second national holiday. However, the investigation highlighted many problems with the notification system and outbreak investigation procedure. The timeliness of notification is important but reaction time to these notifications must also be timely if an investigation is to be successful. If a decision is made to carry out an investigation, it is necessary to act quickly. The greater the time elapsed, the harder it is to recover leftover foods, to obtain a list of potential exposures and for people to remember what they have eaten.

 

A second consideration in the investigation of foodborne outbreaks is that “one prepares for war during peacetime”. The organizational machinery required for an outbreak investigation needs to be developed, tested, and maintained in the absence of an epidemic because having an appropriate system in place increases the likelihood of conducting a successful investigation when an epidemic does occur. Communication lines need to be functioning well between the microbiology laboratory, epidemiologists, food hygienists, and veterinarians. In fact, outbreak investigations are particularly useful if they succeed not only in identifying the vehicle but also in reconstructing all the phases in the preparation of the vehicle, from the original producer or supplier of the food to the final preparation of the food by the consumer and in highlighting critical control points where there is a greater risk of contamination.

 

At the end of an investigation, the outbreak report should be distributed to all those interested: food handlers public health personnel, and those who are responsible for disase notification. This is the only way to make clear the importance and usefulness of notification and outbreak investigation.

 

Stefania Salmaso, Laboratory of Epidemiology and Biostatistics, Istituto Superiore di Sanità, Rome.

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