Content Information
Microbial agents are responsible for the majority of foodborne illnesses and can be categorized into three groups: viruses, bacteria, and parasites. In addition, it is also possible to become ill by ingesting food contaminated with a harmful chemicals, bacteria toxins, or other substances. When food that is contaminated is ingested it can cause either foodborne infection or intoxication. An infection is when the organism is ingested, invades and then multiplies in the intestinal lining or when the ingested organism invades, multiplies and then releases a toxin while in the intestinal tract. Intoxication occurs when the organism produces a toxin in the food and then the food is ingested. (The organism may or may not still be in the food).
Classification of Foodborne Illness
Foodborne infection may occur when:
- Viruses, bacteria, or parasites are ingested, invade and multiply in intestinal cells or other tissues, or
- Bacteria are ingested, invade and release a toxin that damage the surrounding tissues or interfere with normal organ system or tissue functions.
- Foodborne infections generally have a longer incubation period (measured in days) because in order for the illness to occur, first multiplication of the organism must occur in the host. The host will usually experience the most common symptoms such as fever, nausea, vomiting, abdominal cramps, and diarrhea.
Foodborne intoxication may occur when:
- a food is already contaminated by a toxin, such as that made by bacteria in the food, toxins found naturally in plants, animals or fungi or poisonous chemicals
- Foodborne intoxication causes illness quickly (incubation period measured in minutes to hours, rather than days) because the toxin quickly affects the body or the body tries to expel it as quickly as possible. The most common symptom for intoxication is vomiting or diarrhea.
Characteristics of Viruses, Bacteria, and Parasites
Viruses are the smallest of the three organisms being discussed in this chapter and cannot be seen by a conventional microscope and do not easily grow in a lab. Viruses only have the ability to replicate in the host’s living cells, so even if a food item is contaminated with a virus, the virus is not able to replicate in the food. However, the virus may still remain infectious while in the environment and once it is ingested, it will replicate in the intestinal tract cells of the host and then be excreted in the feces. Food usually becomes contaminated by an infected food handler with poor personal hygiene who is excreting the virus in their feces or through coming in contact with raw sewage. Only a small number of virus particles are required for the host to develop an infection and become ill. Viruses can only cause an infection and not intoxication. The most common gastrointestinal viral infections include hepatitis A, Norovirus, and rotavirus.
Bacteria are one-celled organisms that can be seen by a microscope and do grow in labs. They increase in number when one fully-grown cell divides into two and then the process continues when those two cells divide into two again and so on. Unlike viruses, bacteria can multiply in food and in favorable conditions large amounts of bacteria can be present in food. Certain bacteria, like Bacillus cereus, Clostridium botulinum, and Clostridium perfringens can form spores. These spores are much more difficult to kill than the other bacterial forms. Bacteria can cause infection or intoxication.
Parasites are either single-celled or multi-celled organisms that are larger than both viruses and bacteria. Like viruses, parasites are not able to replicate in food, however they can live in the environment in the form of a cyst until they are ingested. Once the host ingests the parasite, they can then begin multiplying to cause an infection. Examples of single-celled parasites are Entamoeba histolytica, Giardia lamblia, and Cryptosporidium parvum. Multi-celled parasites may present as eggs, larvae, or other forms and, once ingested, may hatch to develop into adult parasites. An example of a multi-celled parasite is the tapeworm. Some types include, Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Diphyllobothrium species (fish tapeworm).
Transmission of Pathogens
The fecal-oral transmission route is the most common route of foodborne illness. Fecal-oral transmission occurs when a disease-causing organism is shed in the feces of humans or animals and is deposited on the food and then ingested. Contaminated food may cause an infection when raw food is contaminated with the organism and then is not cooked long enough to kill it. It can also occur when cooking utensils are used on the raw contaminated food where the utensil picks up the agent and then it is also used on food that is not cooked or eaten raw (a knife is used on raw chicken and then is used to cut up lettuce). Transmission may also occur when a food handler fails to wash their hands properly after using the restroom, changing a diaper, or handling an animal, and then handles food items.
The Carrier State
A carrier of a foodborne illness is a person who is asymptomatic but may be able to spread the disease to others. There are many reasons why this may happen:
- the person may still be in the incubation period, yet be excreting the virus,
- the person may be recovering, yet still shedding the virus and therefore is still able to spread it person-to-person, or
- the person may be a chronic carrier of the agent.
This is relatively rare, most contamination comes from actively ill persons.
High Risk Populations
Certain populations are at higher risk for developing a foodborne illness. For example, the young and old may be more susceptible to developing a foodborne illness due to their weaker immune systems and more susceptible to developing complications from diarrheal diseases such as dehydration. Immunocompromised patients, such as those under going cancer treatments, organ transplants, and AIDS patients are at a higher risk for developing a foodborne illness because it may take a smaller amount of the organism to cause illness compared to a healthy person with an intact immune system. Pregnancy is also classified as a high risk factor. Certain foodborne infections such as Listeria monocytogenes can cause spontaneous abortions, stillbirths, or abnormalities in the fetus.
Treatment
Generally, bacterial and viral foodborne illnesses are not treated with antibiotics or antivirals and people will recover on their own. If a person, especially a child or someone who is elderly, has severe diarrhea they should seek medical care. Medication and supportive treatment may be prescribed for severe illness or to ease symptoms. The most common cause of serious illness and hospitalization is dehydration.
[tables]