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Food poisoning

Food poisoning is a common, often mild, but sometimes deadly illness. It is caused by the consumption of food or drink that is contaminated with bacteria, parasites or viruses. Most cases result from bacterial contamination. Food poisoning happens in one of two ways: either in the food (for example in undercooked meat or unpasteurised milk), or on the food (if it is prepared by someone who has not washed their hands). The length of the incubation period (the time between swallowing the bacteria and symptoms appearing) varies from hours to days, depending on the type of bacteria and how many were swallowed. The most common symptoms of food poisoning are sickness, vomiting, abdominal pain and diarrhoea. It’s difficult to know exactly how many people get food poisoning because mild cases often go unreported but the Food Standards Agency estimates that food poisoning affects up to 5.5 million people in the UK each year (NHS Choices, 2013f). It usually lasts for less than three days, but can continue for up to a week. The greatest danger lies in the loss of fluids and salts from prolonged diarrhoea. The results can be deadly in infants and over 60s. Also, in these patients, the bacteria may enter the bloodstream infecting other parts of the body and may cause death unless the person is treated promptly with antibiotics.

Some toxins can cause food poisoning within a much shorter time than described above. In these cases, vomiting is the main symptom. Foods particularly susceptible to contamination if not handled, stored or cooked properly include:

  • Raw meat and poultry
  • Raw eggs
  • Raw shellfish
  • Unpasteurised milk
  • 'Ready to eat' foods, such as cooked sliced meats, pâté, soft cheeses and prepacked sandwiches

Source: NHS Choices, 2013e.

Most cases of food poisoning are related to the consumption of animal products (meat, poultry, eggs, fish and dairy) as plants tend not to harbour the types of bacteria capable of causing food poisoning in humans. Intensive animal husbandry technologies, introduced to minimise production costs, have led to the emergence of new zoonotic diseases; animal diseases that can be transmitted to humans (WHO, 2013c). Escherichia coli (E. coli) O157 was identified for the first time in 1979 and has since caused illness and deaths (especially among children) owing to its presence (in several countries) in minced beef, unpasteurised cider, cow’s milk, manure-contaminated lettuce and alfalfa and manure-contaminated drinking-water (WHO, 2013c). The potential sources of E. coli contamination of fruit (cider apples) are numerous. One possible source may be bird droppings; birds have been shown to spread various food-borne pathogens including Campylobacter, Salmonella, Vibrio cholerae and Listeria species. Another possibility is windfall apples being exposed to animal faeces. The contamination of damaged apples with E. coli O157:H7 can also be spread by fruit flies and then fruit-to-fruit transmission by fruit flies ensures the infection spreads (Janisiewicz et al., 1999). Indeed research shows that flies can transmit foodborne pathogens and that the areas of higher risk are those in closer proximity to animal production sites (Barreiro et al., 2013). In a joint report between the FSA Scotland and the Scottish Executive it was noted that the main source of E. coli O157 is from cattle and sheep, but that more cases of E. coli O157 are now associated with environmental contamination, including contact with animal faeces or contamination by faeces of water supplies, than with food (FSA/SE, 2001). If plants do cause food poisoning it is generally because they have been contaminated with animal excreta, human sewerage or handled with dirty hands during preparation. Safe disposal of manure from largescale animal and poultry production facilities is a growing food safety problem in much of the world (WHO, 2013c).

Food can become contaminated at any stage during production, processing or cooking, for example, food poisoning can be caused by:

  • Not cooking food thoroughly (particularly poultry, pork, burgers, sausages and kebabs)
  • Not storing food that needs to be chilled at below 5°C correctly
  • Leaving cooked food for too long at warm temperatures
  • Someone who is ill or who has dirty hands touching the food
  • Eating food that has passed its ‘use by’ date
  • Cross-contamination (the spread of bacteria, such as E.coli, from contaminated foods)

Source: NHS Choices, 2013e.

The most common cause of food poisoning in the UK is the bacterium Campylobacter, which are usually found on raw or undercooked meat (particularly poultry), unpasteurised milk and untreated water. Undercooked chicken liver and liver pâté are also common sources. The next most common cause is Salmonella, which are often found in raw meat and poultry, they can also be passed into eggs and unpasteurised milk. Listeria bacteria may be found in a range of chilled, ready-to-eat food, including: pre-packed sandwiches, pâté, butter, soft cheeses (such as brie, camembert or others with a similar rind), soft blue cheese, cooked sliced meats and smoked salmon. E. coli are found in the digestive systems of many animals, including humans. Most strains are harmless but some strains can cause serious illness. Most cases of E. coli food poisoning occur after eating undercooked beef (particularly mince, burgers and meatballs) or drinking unpasteurised milk (NHS Choices, 2013f).

The virus most commonly linked to gastrointestinal illness is the norovirus (also known as the vomiting bug). It is easily transmitted from person to person, from contaminated food or water. Raw shellfish, particularly oysters can be a source of viral contamination. A study funded by the Foods Standards Agency found that three-quarters of oysters sampled from harvesting beds within UK waters contained norovirus (albeit at low levels in half the samples). The FSA advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish to reduce their risk of food poisoning (NHS Choices, 2013f).

In the Food Standards Agency’s Annual Report to the Chief Scientist 2012-2013, it was reported that of the five major pathogens monitored by the Agency (campylobacter, Listeria monocytogenes, norovirus, E. coli O157 and salmonella), campylobacter remains the most frequently reported cause of foodborne disease accounting for 60 per cent of reported cases in England and Wales and the highest proportion of hospitalisations (92 per cent). Although foodborne illness due to Listeria monocytogenes is relatively rare (less than one per cent), it is associated with the highest mortality at 30 per cent (FSA, 2013a).

The Food Standards Agency’s current best estimate suggests that there are around one million cases of foodborne illness in the UK each year, resulting in 20,000 hospital admissions and contributing to around 500 deaths. However, many illnesses go unreported and their report states that around 25 per cent of the population suffer from an episode of intestinal infectious disease each year; equivalent to 17 million cases annually. The public health impact of gastrointestinal infection continues to be significant; the estimated cost for England and Wales in 2011 was £1.6 billion (FSA, 2013a).

Listeria is an opportunistic pathogen that can cause severe illness (listeriosis) in vulnerable groups such as pregnant women, babies, the elderly and people with reduced immunity. The Government advises pregnant women to avoid soft mould-ripened cheese, such as Camembert and Brie, blue cheese and all types of meat pâté. Other bacteria that can cause food poisoning include species of Staphylococcus and Clostridium. Certain strains of otherwise normal intestinal bacteria can cause food poisoning. For example, E. coli is usually harmless but the strain E. coli O157 can cause kidney failure and death.

The majority of food poisoning cases in the UK are caused by consuming contaminated meat or dairy products. For example, of the Staphylococcal food poisonings reported in the UK between 1969 and 1990, 53 per cent were due to meat products (especially ham), 22 per cent were due to poultry, eight per cent were due to milk products, seven per cent to fish and shellfish and 3.5 per cent to eggs (Wieneke et al., 1993). While most cases of food poisoning are associated with meat and poultry, the link between milk products and food poisoning should not be discounted: 20 separate outbreaks of food poisoning in England and Wales associated with the consumption of milk and dairy products were reported to the Public Health Laboratory Service Communicable Disease Surveillance Centre between 1992 and 1996 (Djuretic et al., 1997). 600 people were affected and over 45 were admitted to hospital. Salmonella species were responsible for 11 of the outbreaks, Campylobacter species for five, E. coli O157 for three and Cryptosporidium parvum for one. Outbreaks were associated with hotels, a psychogeriatric hospital, schools, a Royal Air Force base, a farm visit, an outdoor festival and milk supplied directly from farms. Milk was implicated in 16 of the outbreaks, 10 of which were associated with unpasteurised milk. Two outbreaks were associated with eating contaminated ice-cream and two with eating contaminated cheese.

In 2010, it was reported that since 2001, an increase in the number of listeriosis cases has been observed in several EU countries, including England and Wales, predominantly in the over-60s population (Little et al., 2010). The main culprits for the overall population and over 60’s were given as follows: mixed sandwiches and pre-packed salads (23.1 and 22 per cent respectively); finfish (16.8 and 14.7 per cent) and beef (15.3 and 11.2 per cent). For pregnancy-associated cases, beef (12.3 per cent), milk and milk products (11.8 per cent), and finfish (11.2 per cent) were more important sources of infection. Food poisoning may result from milk and milk products if they have not been properly heated (pasteurised) or if they have become contaminated following pasteurisation. A report published in the New England Journal of Medicine reported how 142 cases of listeriosis in Los Angeles in 1985 led to 48 deaths (Linnan, 1988). An extensive investigation traced the source to a cheese factory where it was found that a Mexican-style soft cheese had been contaminated with unpasteurised milk.

Bacteria are too small to see and they do not taste or smell of anything so it is difficult to detect their presence. The risk of food poisoning can be minimised by following some basic hygiene rules. This means washing hands before handling food, washing salads thoroughly (to remove contaminating bacteria from manure for example), making sure all food is covered and chilled. If meat is to be consumed it must be thawed and cooked properly to kill harmful bacteria. It is important to keep raw meat (and its juices) away from other foods.

The Centers for Disease Control and Prevention (CDC) in Atlanta, US is a national public health institute that focuses on disease control and prevention. The CDC state that raw (unpasteurised) milk can carry harmful bacteria and other germs and that while it is possible to get foodborne illnesses from many different foods, they say that raw milk is one of the riskiest of all. Bacteria such as E. coli, Campylobacter and Salmonella can contaminate milk from cows, sheep and goats. Animals that carry these germs usually appear healthy. Getting sick from raw milk can mean many days of diarrhoea, stomach cramping, and vomiting. Less commonly, it can mean kidney failure, paralysis, chronic disorders and even death. A person can develop severe or even life-threatening diseases, such as Guillain-Barré syndrome, which can cause paralysis and haemolytic uremic syndrome, which can result in kidney failure and stroke (CDC, 2013).

Milk contamination may occur from:

  • Cow faeces coming into direct contact with the milk
  • Infection of the cow’s udder (mastitis)
  • Cow diseases (eg bovine tuberculosis)
  • Bacteria that live on the skin of cows
  • Environmental contamination (eg faeces, dirt, processing equipment)
  • Insects, rodents and other animal vectors
  • Humans (eg by cross-contamination from soiled clothing or boots)

Source: CDC, 2013.

Good hygienic practices during milking may reduce, but not eliminate, the risk of milk contamination. However, as the CDC state, dairy farms are a reservoir for illness-causing bacteria. No matter what precautions farmers take and even if their raw milk tests come back negative, they cannot guarantee that the milk, or the products made from their milk, are free of harmful germs (CDC, 2013).

Distribution of raw milk is illegal in Scotland. In England it is illegal to sell it from shops or supermarkets but a number of registered producers can sell raw or ‘green top’ milk, directly to consumers, either from a farm or at a farmers’ market or through a delivery service. The number of registered raw cow’s drinking milk producers in England and Wales fell from around 570 in 1997 to 102 in 2009 (FSA, 2009). They must display the warning "this product has not been heat-treated and may contain organisms harmful to health" and the dairy must conform to higher hygiene standards than dairies producing only pasteurised milk. Avoiding unpasteurised milk, raw eggs and undercooked meat further reduces the risk of food poisoning. Of course the safest option is to follow a plant-based diet free of red meat, poultry, fish, milk and eggs. Excluding animal foods from the diet will dramatically decrease the risk of food poisoning.