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.
Outbreaks occur when multiple people eat contaminated food. This can occur in a range of settings, including:
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.
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.
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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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:
Gathering these pieces of information often requires careful investigations, including:
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.
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.
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.