Foodborne illnesses fall into two general categories: intoxication and infection. Foodborne intoxication is caused by ingestion of foods that contain a toxin that may be naturally present in the food, introduced by contamination with poisonous chemicals, or produced by bacteria or fungi growing on foods. Toxins occur naturally in some plant foods and some mushrooms. Toxins may also be present in some fish and shellfish that have consumed toxin-producing algae.

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Toxic chemical contaminants include cleaning agents, pesticides and herbicides, and heavy metals. Bacteria (and, less often, fungi) can produce toxins when they are allowed to grow in foods that have been improperly handled before being eaten. Even if the bacteria have been killed by heating prior to consumption, the toxin is already present in the food and can cause illness rapidly. The toxin may act as quickly as 30 minutes following the ingestion of the contaminated food.

In the United States, foodborne intoxication is most commonly caused by two strains of bacteria, Staphylococcus aureus and Bacillus cereus.

S. aureus lives on the skin and in the nasal passages of about 25% of the population, and food handlers are the source of food contamination with this bacterium. When contaminated foods are not promptly refrigerated these bacteria grow on the food and release a heat-stable toxin that can withstand boiling (212°F) for as long as two hours. The likelihood of foods being contaminated with S. aureus can be drastically reduced if food handlers practice proper hygiene, particularly adequate hand washing.

B. cereus is a bacterium that is widespread in the environment. When these bacteria grow on contaminated rice products, and other starchy foods such as potatoes and pasta, a heat-stable toxin is produced that causes vomiting. Because B. cereus is abundant in soil and on uncooked produce it is often introduced into cooked foods through cross-contamination in the kitchen when uncooked or undercooked vegetables or herbs come in contact with cooked foods. When those cooked foods are not promptly refrigerated the transferred bacteria can then grow. Because both S. aureus and B. cereus produce heat-stable toxins, reheating contaminated foods that were not properly cooled (or kept hot) will often not prevent intoxication.

The majority of foodborne illnesses are foodborne infections that result when we consume foods that are contaminated with microorganisms that then multiply in the intestines, causing an infection that results in illness. One of the most common sources of pathogens in foods and water is from contamination with either animal or human feces. When we become ill from consuming foods or water contaminated in this manner it is referred to as the fecal-oral transmission of disease. Another way that pathogens are introduced into foods is by cross-contamination, where pathogens from contaminated surfaces, utensils, people, or foods (such as raw meats) are transferred to another object or food. This is of particular concern when contaminated foods are then eaten without being cooked after they are contaminated.

Disease-causing microorganisms (or pathogens) include viruses, parasites, and bacteria. Viruses are infectious agents that consist of genetic material surrounded by a protein shell, and are so small that they cannot be seen with a conventional light microscope. Viruses can only multiply inside the living cells of a host and for this reason they are unable to multiply in foods. Parasites are organisms that live on or in a host, and obtain the nutrients they need from the host. They vary widely in size, from small single-celled organisms (protozoans), to large multicelled organisms such as parasitic worms (helminths) that may exceed 30 feet in length. Like viruses, parasites cannot multiply in foods but they can survive in the environment and enter the body through food.

Bacteria are single-celled microorganisms that can be seen with a conventional microscope. Bacteria increase in number by dividing into two cells, with each cell then growing to full size before they divide again. Unlike either viruses or parasites, bacteria can multiply in food when conditions such as temperature, pH, salt concentrations, moisture content, and oxygen levels are favorable for their growth. The longer these favorable conditions for bacterial growth persist, the more bacteria will be present in the food, and the more severe will be the illness if the contaminated food is ingested.

Foodborne illnesses can have different symptoms, but since the offending pathogens are ingested and enter the gastrointestinal tract, symptoms commonly begin there. Nausea, vomiting, abdominal cramps, and diarrhea are most common. Although foodborne illnesses may sometimes have a rapid onset and may last only hours or days, they can also lead to more serious and potentially life-threatening complications. Different types of foodborne diseases vary in the time of onset, typical symptoms, duration of symptoms, and potential consequences. (INFOGRAPHIC 20.4)

Question 20.4

Multiple factors affect the disease-causing potential of foodborne pathogens. Most important, the strength of a person’s immune system plays a big role. Healthy college-aged students, for example, may not be affected or have minimal symptoms, whereas a young child may experience more drastic consequences. The people most at risk of contracting a foodborne illness, are those whose immune systems are not functioning at full capacity. They are at risk of lengthier illnesses with increased risk of hospitalization and death. Infants and young children whose immune systems are not yet fully developed, older adults, and individuals with diseases such as HIV/AIDS, cancer, and diabetes have weakened immune systems that allow greater opportunity for foodborne pathogens to multiply. Pregnant women are also more susceptible to foodborne illness because pregnancy suppresses the immune system of the mother. As a result, pathogens may cross the placenta and infect the unborn child whose immune system is underdeveloped.

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One particular foodborne pathogen that pregnant women are often warned against is Listeria monocytogenes. This pathogen is the third leading cause of death due to foodborne illness in the United States, and one to which pregnant women and their unborn children are particularly susceptible. It is estimated that pregnant women are 10 times more likely to become infected with Listeria than the general population (adults 65 years and older are 4 times more likely to be infected). Though the symptoms of illness (called listeriosis) experienced by the expectant mother are typically mild (fever, fatigue, and body aches), this infection can have devastating effects on the pregnancy, potentially causing miscarriage, stillbirth, preterm birth, or life-threatening infection in the newborn. Pregnant women should avoid consuming soft cheeses (feta, Brie, Camembert, and some Mexican-style cheeses), refrigerated smoked seafood, pâtés, or meat spreads; and hot dogs (unless they are steaming hot). These foods are some of the most common food sources of Listeria. Individuals in the other high-risk groups should also avoid these foods, as well as other foods that are common sources of foodborne pathogens. (INFOGRAPHIC 20.5)

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The first documented cases of E. coli O157:H7 poisoning occurred in 1982 and were traced to undercooked hamburgers served at McDonald’s restaurants in Oregon and Michigan. These cases went largely unpublicized, however. It was not until the 1993 Jack in the Box outbreak that the deadly bug made national news.

The headline-generating incident highlighted the outdated meat inspection system and provided the impetus for Congress to once again take up the issue of food safety. The response was a 1998 law that established a new system of prevention and monitoring for all meatpacking plants in the United States.

Ironically, the principles on which the new system was based were not themselves new. They had been in use since the 1960s to protect one very valuable group of Americans: astronauts.