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Hygiene and Environmental Health Module: 9. Foodborne Diseases and the Investigation of Disease Outbreaks

Study Session 9  Foodborne Diseases and the Investigation of Disease Outbreaks


Foodborne diseases are a major public health problem. They result from eating foods that contain substances which are either infectious or toxic in nature. In the previous session you have learned about microbial and chemical food contamination. In this session you will learn more about the foodborne diseases that are important for public health, their type and classification, their characteristics and their common symptoms. You will also learn how outbreaks of foodborne diseases should be investigated.

Learning Outcomes for Study Session 9

When you have studied this session, you should be able to:

9.1  Define and use correctly all of the key words printed in bold. (SAQ 9.1)

9.2   Describe the main types and classification of foodborne diseases. (SAQs 9.1 and 9.2)

9.3  Describe and give examples of the most common bacterial, viral and parasitic foodborne diseases. (SAQ 9.3)

9.4  Describe how you can conduct investigations of foodborne disease outbreaks. (SAQ 9.4)

9.1  Overview of foodborne diseases

Since as far back as the time when the documentation of human history began, consumption of contaminated food and foodborne diseases have been a major global health problem. Contamination can be with microorganisms, chemicals and physical objects in food (as you learned in Study Session 8), which can lead to a variety of foodborne diseases or ill effects such as poisoning.

Foodborne diseases are still a major public health concern all over the world today. They are responsible for many cases of adult illnesses and some deaths, but more importantly, contaminated food is a source of the acute diarrhoeal diseases that claim the lives of enormous numbers of children every day. Worldwide, about 2 million children under the age of five years die from diarrhoeal diseases every year.

In developing countries like Ethiopia, the problem reaches great proportions for many reasons. Most basic among these are poverty and a lack of public health awareness. The problem of foodborne disease is more serious among rural communities where there tends to be a lower level of awareness about the causes and prevention of foodborne infection.

Well-documented information is lacking regarding the extent of foodborne diseases in Ethiopia because many cases are not properly diagnosed or not reported, and many people who are sick with foodborne diseases do not visit health facilities. This makes it difficult to collect statistical data or even make an estimation of the level of the problem – except that it is certainly huge.

9.2  Transmission of foodborne diseases

In the two previous study sessions you have learned about microorganisms and food contamination. The single method of transmission of foodborne diseases to human beings is through ingestion (eating) of food in the following categories:

  • Raw or undercooked meat and meat products
  • raw milk (that is, milk that has not been pasteurised or sterilised)
  • food items contaminated with human faeces (directly or indirectly)
  • raw vegetables contaminated with soil
  • food contaminated by chemicals, e.g. pesticides such as malathion
  • food prepared using contaminated water, e.g. for washing vegetables
  • food kept in an unsuitable condition for a long time after preparation
  • poisonous plants.
  • Why is it unwise to eat food that has been kept for a long time after it was prepared?

  • It may have been kept in conditions that created a favourable environment for the growth and multiplication of microorganisms in the food, especially if it was exposed to flies, cockroaches, etc., or kept at a warm temperature.

9.3  Classification of foodborne diseases

Foodborne diseases are usually classified on the basis of whatever causes them. Accordingly they are divided into two broad categories: food poisoning and food infections. Each of these categories is further subdivided on the basis of different types of causative agent (see Figure 9.1). We will discuss each of them in turn.

Classification of foodborne diseases
Figure 9.1  Classification of foodborne diseases.

9.3.1  Food poisoning

Food poisoning can be from chemical or biological sources. If we eat food that contains harmful chemicals, or biological toxins (poisons) from plants, animals or microorganisms, that food can make us sick. Some common sources of food poisoning are caused by contaminants already in the food when the raw materials are harvested, for example:

  • Bacterial toxins produced by bacteria such as Clostridium botulinum and Clostridium perfringens, which are commonly found in the natural environment, e.g. in soil.
  • Chemical toxins, e.g. insecticides sprayed onto growing crops.
  • Heavy metals, e.g. lead and mercury, particularly in fish caught near chemical processing facilities.
  • Certain toxic plant tissues, e.g. poisonous mushrooms.
  • Toxic animal tissues, e.g. the poison glands of certain fish, crabs, etc.

Chemical food poisoning can also occur if foodstuffs have been in contact with toxic chemicals during food production, processing, storage and handling.

The symptoms of food poisoning can range from mild headache to severe flu-like symptoms. The most common signs and symptoms are nausea, stomach cramps, diarrhoea, fever, chills and vomiting. A person with food poisoning may have any combination of these symptoms depending on the cause or the agent involved. The illness may begin from 1 to 72 hours after eating the food.

9.3.2  Food infection

The diagnosis, treatment and prevention of all these diseases are covered in more detail in the Module on Communicable Diseases.

Food infection occurs as a result of ingestion of pathogenic microorganisms with food. The ingested microorganisms multiply in the gut and can cause diseases like diarrhoea, typhoid fever and cholera; intestinal parasites can cause diseases such as amoebiasis and taeniasis (tapeworm disease); and zoonotic foodborne diseases (i.e. those that are transmitted to humans from other animals), e.g. anthrax and bovine tuberculosis.

There are many different kinds of foodborne diseases and they may require different treatments, depending on the symptoms they cause. Illnesses that cause acute watery diarrhoea or persistent vomiting lead to dehydration if the person loses more body fluids and salts (electrolytes) than they are able to replace. It is therefore important to rehydrate the person, ideally with oral rehydration salts (ORS), or if this is not available, a simple mixture of clean water with some sugar and salt is advised.

Electrolytes are salts in the body that conduct electricity; they are found in all cells, blood and other body fluids, and are essential for normal functioning.

9.3.3  A catalogue of foodborne diseases

Tables 9.1 and 9.2 in Appendix 9.1 (at the end of this study session) summarise the types of organism which cause food infections and food poisoning (respectively). The tables also show the types of food items that are the main risk factors for the associated foodborne diseases. You are not expected to memorise the details of these tables; use them as references that you can consult for information when you need it.

  • Look at Table 9.1. What do anthrax and tapeworm infection have in common?

  • Raw meat consumption from sick and dying animals (like ox, cow, sheep, goat, camel) is responsible for transmitting anthrax, and raw beef and pork are the source of tapeworm infection.

  • Which foodborne infections in Table 9.1 are commonly associated with consumption of contaminated milk and dairy products?

  • Brucellosis, typhoid fever, non-typhoid salmonellosis, bovine tuberculosis, E.coli infection and listeriosis.

9.4  Selected examples of common foodborne diseases in Ethiopia

In Tables 9.1 and 9.2 we summarised the most widespread foodborne diseases and the different causative agents and types of foods involved. Now you will learn about a few of the most common foodborne diseases in Ethiopia, together with some advice that you can use to inform people in your community on how to avoid these diseases.

9.4.1  Bacterial infections

Many common diarrhoeal diseases are caused by bacterial infections transmitted by ingestion of contaminated food and water. Prevention of these diseases should be focused on good personal hygiene by all food handlers, including the consumer of the food. Some bacterial diseases such as anthrax, bovine tuberculosis and brucellosis are particularly related to foods of animal origin; these are described in detail in Study Session 12.

9.4.2  Viral infections

Several different viruses may be transmitted by contaminated food via the faeco-oral route. Foodborne viral infections usually have an incubation period of between one and three days. They cause illnesses which are self-limited in people who are otherwise healthy (i.e. they recover naturally) but occasionally severe illness and even deaths may also occur.

In the group of viral infections causing viral gastroenteritis (VGE), rotavirus is a common cause of vomiting and watery diarrhoea. Dehydration is the likely consequence unless appropriate rehydration therapy is used. Caliciviruses such as norovirus (also known as Norwalk virus) also cause diarrhoea.

Viral hepatitis caused by Hepatitis A and E viruses is almost exclusively transmitted by the faeco-oral route. Hepatitis A is distinguished from other viral causes by its prolonged (two to six weeks) incubation period and its ability to spread beyond the stomach and intestines into the liver. It often induces jaundice, or yellowing of the skin, and can occasionally lead to chronic liver dysfunction.

9.4.3  Tapeworms

Tapeworms are one of the most common causes of foodborne parasitic diseases in Ethiopia.

Beef tapeworm

You will learn more about beef tapeworm in Study Session 12.

Taenia saginata (the beef tapeworm) is the most common cause of tapeworm disease in Ethiopia. Immature forms of the tapeworm develop in the muscles of animals that have eaten tapeworm eggs while grazing on infected grass. People are infected when they eat raw or undercooked beef (Figure 9.2). The adult tapeworms develop in the person’s small intestine and segments of the worms containing eggs are deposited in the environment when the person defecates. This is how the cycle is continued.

Cutting raw beef
Figure 9.2  Eating raw beef can be dangerous because it might be infected with beef tapeworm. (Photo: Zegeye Hailemariam)
Dog tapeworm

Hydatid disease, caused by dog tapeworm, is transmitted when a person ingests the eggs of Echinococcus granulosus in food contaminated with dog faeces. This disease may cause symptoms in women that resemble ‘false pregnancy’, because its effect is to enlarge the liver and cause the abdomen to swell so the woman may appear to be pregnant. The infection may also lodge in the lung or the brain. The prevention of disease caused by dog tapeworm is through personal hygiene when handling food and thorough washing of raw foods, especially if they have come into contact with soil.

Fish tapeworm

Fish tapeworm (Diphyllobothrium latum) infects people through the consumption of raw fish and is more common in the lake areas of Ethiopia where the diet is highly dependent on fish. The symptoms of infection with the fish tapeworm are similar to those of other tapeworm infections, i.e. abdominal discomfort or pain, nausea, vomiting or diarrhoea, and loss of appetite and weight loss. People should be advised only to eat fish that has been properly cooked.

9.4.4  Bacterial food poisoning

In this section, we describe two of the most common sources of food poisoning in Ethiopia, and the advice you can give to people in your community on how to avoid being poisoned by these bacterial sources of contamination.

Staphylococcal food poisoning

Staphylococcal food poisoning is caused by one of the many species of staphylococcal bacteria and is the most common and major type of food poisoning you are likely to encounter. This type of food poisoning can result from the preparation of food more than half a day in advance of needs, storage at ambient temperature, inadequate cooling or inadequate reheating. It begins with symptoms such as nausea, vomiting, stomach cramping and diarrhoea. These can persist for days and lead to dehydration, loss of electrolytes and even death if not treated promptly. Control measures are promoting and monitoring the personal hygiene of food handlers, safe and hygienic conditions in food preparation areas, and keeping cooked or processed foods covered and in cool conditions until consumed.


Foodborne botulism is a form of food poisoning caused by Clostridium botulinum. It occurs in poorly canned foods, including home-canned foods, and honey. It is advisable not to eat food from deformed or bulging cans and not to give honey to young children.

9.4.5  Chemical food poisoning

There are two main types of chemical poisoning. One is caused by chemical products and the other by heavy metals.


Common sources of chemical contamination of foods are pesticides including insecticides, herbicides and rodenticides, and detergents, or their containers. When these chemical products find their way into food they can cause poisoning. There are also many cases of intentional chemical poisoning in Ethiopia when people drink these chemicals to commit suicide. Many people die from chemical poisoning if they do not go to healthcare facilities in time.

Heavy metals

Metals cause poisoning when foods are stored in faulty or damaged containers made of materials like tin, lead, copper and zinc. These metals can dissolve in acid foods such as fruit juices and produce fast-acting poisons in the body when ingested. Possible sources of contamination include residues migrating into foods from soldered cans, leaching from utensils, contaminated water, glazed pottery, painted glassware and paints.

9.5  General management of foodborne diseases

The management approach to patients with foodborne diseases depends on the identification of the specific causative agent, whether microbial, chemical or other. There are many different kinds of foodborne diseases and they may require different treatments, depending on the symptoms they cause. Many episodes of acute diarrhoeal disease are self-limiting and require only fluid replacement and supportive care. If an antibiotic is required, the choice should be based on the clinical symptoms and signs.

Important!You should refer all patients with acute diarrhoea that are not responding to rehydration and supportive care.

Patients with severe diarrhoea and vomiting may need oral rehydration salts (ORS) and antibiotics. In the most severe cases, for example in a cholera epidemic, intravenous fluids containing glucose and normal saline may have to be given to support rehydration. If the disease is due to food poisoning, there may be a need to give an antitoxin, or other antidote to neutralise the effect of the toxin, if such medicines exist or can be accessed in time. These more specialised interventions can only be done at a health facility. However, the limitations of health facilities in rural areas may restrict the choice of the specific management approach.

As a Health Extension Practitioner, you should educate the members of your community on how to recognise the symptoms of foodborne diseases, and to seek advice and supportive treatment from you. If there is a large number of cases, you should document them and report them as soon as possible to the District Health Office.

9.6  Investigation of foodborne disease outbreaks

The surveillance and management of disease outbreaks is covered in detail in the Communicable Diseases Module.

Foodborne disease outbreaks, i.e. several similar cases occurring at the same time, are not uncommon. To identify the source and prevent reoccurrence of such outbreaks, systematic clinical and laboratory investigations have to be made. The investigation and control of foodborne disease outbreaks are multidisciplinary tasks requiring skills in the areas of clinical medicine, epidemiology, laboratory medicine, food microbiology and chemistry, food safety and food control, and risk communication and management. Many outbreaks of foodborne disease are poorly investigated, if at all, because these skills are unavailable, or because a field investigator is expected to master them all single-handedly without having been fully trained.

If you have an unusually large number of people with symptoms of foodborne diseases in your community, you should follow the general steps and procedures summarised in Box 9.1.

Box 9.1  Steps in investigating a foodborne disease outbreak

  1. Collect initial information about the number of cases and report this to the appropriate person at the nearest Health Centre or District Health Office.
  2. Develop an initial case definition (who is ill, where are they, what are their symptoms, when did they become ill). You should also record the age and gender of all people affected.
  3. Develop an initial questionnaire to determine if there is a common source of contamination (e.g. if everyone affected ate the same food, or food from the same place).
  4. Collect specimens of faeces, vomit, etc. according to the procedures laid down by the Federal Ministry of Health (FMOH) and send them to the Health Centre for transport to the nearest Laboratory of Public Health. You will need to use a sterile container for the samples and store them in an icebox. Wear gloves and wash your hands thoroughly before and after taking samples. Send the samples to the laboratory immediately.
  5. If you identify a particular location or event as the origin of the outbreak you should visit the exposure site for a field inspection and environmental sampling.
  6. Take samples of the foods under investigation according to the procedures laid down by the FMOH. You will need to use sterile bags or containers and ensure that your sample is representative of the food consumed. You may need to act quickly to obtain food samples in case any remainder has been used up or disposed of.
  7. If the source of the outbreak is a workplace, you will need to interview the employers and employees. You should try to find out how many of them ate the same food, when they ate it, how much they ate, and where the food came from.
  8. Data analysis and interpretation. For example, does the data you have recorded indicate that any particular age group or gender is affected more than others?
  9. Report. Summarise the findings from your investigation and compile into a brief report that should be submitted to the Health Centre or public health emergency management centre.

Summary of Study Session 9

In Study Session 9, you have learned that:

  1. Foodborne diseases have long been a major public health problem and continue to be a significant cause of human ill health and death.
  2. Foodborne diseases can be classified into two main types: food infection and food poisoning.
  3. Food infections are classified as bacterial, viral, parasitic or fungal.
  4. Food poisoning is classified according to the type of toxin that causes it which may be natural, bacterial, fungal or chemical.
  5. General advice for the prevention of foodborne disease includes good personal hygiene by food handlers, careful food storage and proper cooking methods.
  6. The management of foodborne disease depends on the particular symptoms and disease. Patients with acute diarrhoea should be treated with oral rehydration therapy and, if they do not respond promptly, they should be referred.
  7. Any outbreak of foodborne disease needs to be investigated following prescribed procedures in order to identify the source of the outbreak.

Self-Assessment Questions (SAQs) for Study Session 9

Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.

SAQ 9.1 (tests Learning Outcomes 9.1 and 9.2)

Explain the difference between foodborne diseases caused by infections and those caused by poisoning.


Diseases caused by food infection result from ingestion of food that has been contaminated by microorganisms including bacteria, viruses, parasitic organisms and fungi.

Diseases caused by food poisoning result from ingestion of food that has been contaminated by a toxin or poison which may be of chemical or biological origin.

SAQ 9.2 (tests Learning Outcome 9.2)

Outline three factors that contribute to the widespread occurrence of foodborne disease in Ethiopia.


There are many possible factors you may have identified including:

  • poor personal hygiene by food handlers
  • dirty kitchen utensils and work surfaces
  • eating food that has been contaminated by washing with unsafe water
  • poor storage of food so flies and other disease vectors can come in contact with it
  • inadequate cooking or reheating of food

SAQ 9.3 (tests Learning Outcome 9.3)

Which of the following foodborne diseases is different from the others and why?

  • typhoid fever
  • amoebiasis
  • shigellosis
  • cholera.

Typhoid fever, shigellosis and cholera are all bacterial foodborne infections. Amoebiasis is a parasitic infection.

SAQ 9.4 (tests Learning Outcome 9.4)

Imagine you received a report that several people had become ill after a recent wedding ceremony. You were told that at least 20 people had developed acute diarrhoea within 24 hours. What would be your first step in investigating this outbreak?


The first step would be to gather information about the outbreak. You would need to find out exactly how many people were affected, what their ages were and whether they were male or female. You would need to confirm that they had all been together at the wedding ceremony and ask what they had eaten and when they each became ill.

You may also need to consider further steps such as taking samples from the patients or samples of food.

Appendix 9.1

Table 9.1  Foodborne infections, causative agents and commonly affected foodstuffs.
Disease categoryDiseaseCausative agent(s) Foods commonly involved
BacterialTyphoid feverSalmonella typhi Raw vegetables and fruits, salads, pastries, unpasteurised milk and milk products, meat
Paratyphoid fever Salmonella paratyphi
ShigellosisShigella species All foods handled by unhygienic workers, potato or egg salad, lettuce, raw vegetables
CholeraVibrio choleraFruits and vegetables washed with contaminated water
Non-typhoid salmonellosisSalmonella species, e.g. Salmonella typhimurium Eggs, poultry, undercooked meals, unpasteurised dairy products, sea foods, sausages
BrucellosisBrucella species, mostly Brucella melitensisMilk and dairy products from infected animals
Anthrax Bacillus anthracisContaminated raw and undercooked meat from sick and dying oxen, cows, sheep, goats, camels, etc
Bovine tuberculosisMycobacterium bovisUnpasteurised milk, dairy products or meat from tuberculosis-infected cows
E.coli infectionEscherichia coliBeef, dairy products, fresh products, raw produce (potatoes, lettuce, sprouts, fallen apples), salads
ListeriosisListeria monocytogenesMilk, cheese, ice cream, poultry, red meat

Viral gastroenteritis (VGE)

Rotavirus, caliciviruses including norovirus, astrovirusAny food contaminated with the virus
Viral hepatitisHepatitis A and E virusesRaw shellfish from polluted water, sandwiches, salad and desserts
PoliomyelitisPolio virusAny food contaminated with the virus
Rift valley feverRift valley fever virusAny food contaminated with blood or aerosols from infected domestic animals or their aborted fetuses
ParasiticTaeniasis (tapeworm infection)Taenia speciesRaw beef, raw pork
Amoebiasis Entamoeba histolytica Any food soiled with faeces
Trichinosis Trichinella spiralis Insufficiently cooked pork and pork products
AscariasisAscaris lumbricoides Foods contaminated with soil, especially foods that are eaten raw, such as salads, vegetables
GiardiasisGiardia lamblia Any contaminated food item
ToxoplasmosisToxoplasma gondiiRaw or undercooked meat and any food contaminated with cat faeces
CryptosporidiosisCryptosporidium parvumAny contaminated food item
ParasiticHydatid diseaseEchinococcus granulosus Any food contaminated with dog faeces
DiphyllobothriasisDiphyllobothrium latumRaw or uncooked fish
Trichuriasis Trichuris trichuria Any food contaminated with soil
FungalFungal infections Aspergillus Penicillium YeastsCereal, grains, flour, bread, cornmeal, popcorn, peanut butter, apples and apple products, mouldy supermarket foods, cheese, dried meats
Table 9.2  Food poisoning, toxin type, causative agents and commonly affected foodstuffs.
Disease categoryDiseasesToxin type and causative agentFoods commonly involved
Natural toxins in foodsNeurolathyrismBeta-oxalyl amino-alanineLathyrus sativus (guaya)
Mushroom poisoning Phalloidine and alkaloids found in some poisonous mushroomsPoisonous mushrooms such as species of Amanita phalloides and Amanita muscaria
Bacterial toxinsStaphylococcal food poisoningEntero-toxins from Staphylococcus aureusMilk and milk products, sliced meat, poultry, legumes
Perfringens food poisoning Strain of Clostridium welchii/ C.perfringensInadequately heated or reheated meat poultry and legumes
Botulism food poisoning Toxin of Clostridium botulinumHome-canned foods, low acid vegetables, corn and peas
Escherichia coli food poisoningEnterohaemorrhagic Escherichia coli O157:H7 Ground beef, dairy products and raw beef
Bacillus cereus food poisoning.Enterotoxins of Bacillus cereusCereals, milk and dairy products vegetables, meats, cooked rice
Fungal toxinsErgotismA toxin (ergot) produced by a group of fungi called Claviceps purpurea Rye, wheat, sorghum, barley
Aflatoxin food poisoningAflatoxin produced by some groups of fungus (e.g. Aspergillus flavus, Aspergillus parasiticus) Cereal grains, groundnuts, peanuts, cottonseed, sorghum
Chemical toxinsChemical poisoningHeavy metals (e.g. lead, mercury, cadmium)Fish, canned food Foods contaminated by utensils or coated with heavy metals
Pesticides and insecticides

Residues on crops, vegetables, fruits

Accidental poisoning where some chemicals may be mistaken for food ingredients

When contaminated containers are used to hold stored foods

AdditivesVarious food items where unauthorised additives may be added as colouring agents, sweeteners, preservatives, flavouring agents, etc.