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What is food borne disease?

Food borne disease is an illness caused by a person eating contaminated food. It can be caused by a range of bacteria, viruses, parasites, and toxins.

Depending on the cause, it can result in a range of symptoms, including diarrhoea, vomiting, fever, aches and pains, jaundice, and sometimes can be fatal.

Food borne diseases are very common around the world. More than four million cases are estimated to occur in Australia each year.

What causes food borne disease outbreaks?

Outbreaks occur when multiple people eat contaminated food. This can occur in a range of settings, including:

  • a group of people sharing a meal, for example in the home, at a picnic, a restaurant or a catered event like a wedding, where the food may be undercooked or mishandled
  • a number of individuals purchasing food from the same take-away or restaurant
  • people eating food that is contaminated and sold ready to eat, such as fruit and vegetables, salads, deli meats.

Contamination of food can occur anywhere from when the food is grown or produced, packaged, processed, transported, stored, marketed, or prepared.

Poor storage and handling of contaminated food can result in growth of contaminants, increasing the risk that the food can cause illness if people eat it without cooking or properly decontaminating it.

How are food borne disease outbreaks identified?

Under the Public Health Act, doctors must notify NSW Health of outbreaks of food borne disease . However many people with food borne disease never visit a doctor and the disease is never reported, or doctors only see one sick person and do not realise that others are affected.

Outbreaks may also be indicated when epidemiologists identify an unusual increase in the number of cases of a notifiable disease, such as Salmonella infection, hepatitis A or Listeria infection occurring in the community.

How are food borne disease outbreaks investigated?

Investigation of food borne disease outbreaks is a collaboration between NSW Health and the NSW Food Authority. Outbreaks are investigated in order to identify the likely food source of the outbreak, and take actions to prevent other people eating that food. In order to identify current and potential exposure risks, health authorities will use a range of information, which may include: interview data, hospital records, restaurant booking lists, public-facing social media and shopper loyalty program data.

Investigations follow well-established protocols which can be found at NSW Control Guidelines. Expert panels often oversee more complex investigations.

Where multiple states are affected by an outbreak, a coordinated investigation is carried out according to national guidelines for the epidemiological investigation of multi-jurisdictional outbreaks that are potentially foodborne.

NSW Health is responsible for investigating patients with food borne disease to see if there are common foods they may have eaten, and if so whether these foods may be the source of their illness.

The NSW Food Authority is responsible for regulating the food industry to minimise the risk that food sold is contaminated, and for investigating the source of any food that may be contaminated, and removing it from sale.

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How do the NSW Health and Food Authority investigators determine if a food is a source of an outbreak?

Pinpointing the source of an outbreak requires a range of information. Important factors that can help investigators understand the possible source of an outbreak include:

  • detailed information on what the people who are sick ate during their incubation period – for some infections this incubation period can be a few days, for others it might be months
  • whether laboratory tests show that the sick people had the same strain of the bacteria or virus that caused their illness
  • whether the sick people were more likely to eat a particular food than other people who were not sick
  • the foods that were more commonly eaten by the sick people and the source of those foods (Foods that came from the same farm, wholesaler, or retailer may been more likely to be the cause of an outbreak than foods that came from many different places)
  • whether the food was likely to be have been contaminated at its source or during handling.

Gathering these pieces of information often requires careful investigations, including:

  • detailed interviews with patients (often more than once) about foods eaten, medical history, treatments, and where foods were purchased
  • specialist laboratory testing of samples taken from patients
  • finding and interviewing in details volunteers who are similar to the sick people but were not part of the outbreak to find out what they may have eaten in a similar time period
  • detailed assessment of the shops and restaurants that sold the foods
  • tracing back who supplied the food to those shops and restaurants
  • retrieving and taking samples for testing of suspected food items
  • assessing the production processes involved in the growing and preparation of the foods.

What are some of the challenges in identifying the food that caused an outbreak?

People can eat hundreds of different foods during their incubation periods, so knowing which of them made them sick can be very difficult. People may not remember all the food they ate.

Because people eat a wide range of foods every day, and many people may not readily remember all the foods they ate over a period of several days or even months, obtaining accurate food histories from patients can be challenging. This is particularly true for foods that they may have had only once or twice during their incubation period

To try and tease out the particular food that caused the outbreak, epidemiologists will often compare the food eaten by the sick people with the food eaten by other people who were not part of the outbreak. Those foods more commonly eaten by the sick people in the outbreak give a clue to the possible source. However, this comparison is difficult to make with any confidence when small numbers of patients are involved in an outbreak, because differences found might be because of chance alone.

What are the key dates involved in the investigation of food borne diseases?

The onset of illness of food borne disease is the date the person first develops symptoms.

The incubation period is the time taken between eating the food and onset of illness. This can range from hours to many weeks, depending on the disease

Time to diagnosis is the time between onset of illness and the doctor making a diagnosis of illness. This usually involves special laboratory tests and typically takes several days

Time to notification is the time between onset of illness to the case being notified to NSW Health. As this depends on the person going to the doctor, the doctor taking samples, the laboratory receiving and testing samples, it is typically several days

In addition, specialist laboratory testing to identify whether patients’ infections are linked take several days from the time the laboratory received the sample. For example whole genome sequencing can take at least eight days to provide a result

NSW Health can begin the investigation when it receives notifications of cases.

Why not warn the public to avoid eating all suspect foods?

NSW Health routinely advises vulnerable people to avoid high risks foods. For example, pregnant women, the elderly, and people with immune suppression should avoid eating foods that may be contaminated with listeria, for example pre-cut melons, soft cheeses, deli meats and other ready to eat foods.

Before the source of an outbreak can be publicly named investigators must be reasonably confident that the food source has been identified so that people can take action to avoid eating it, and the Food Authority can recall the food from sale.

Wrongly identifying a food as the source of an outbreak can have dire consequences, including allowing the real source to continue to be consumed, as well as seriously harming the reputation and commercially viability of the producers and retailers of foods that are not implicated.

Because the evidence collected on the source of a food borne disease outbreak is largely circumstantial, public health authorities need to make sure that they have quickly gathered and assessed the above information to inform a public warning and recall.

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Page Updated: Friday 14 September 2018
Contact page owner: Communicable Diseases