Normal physiologic defense mechanisms that exist to prevent disease from foodborne and waterborne pathogens include gastric acid, the normal flora of the gastrointestinal tract, intestinal motility, and mucous glycoproteins of the intestinal mucus. The effectiveness of each of these barriers can be altered by age, concomitant infections, chronic illnesses, medications, and the pathogen itself.
A gastric pH of 3 or less is generally found in most healthy adults and can effectively kill most foodborne pathogens. 14 A reduction in acidity can occur secondary to antacids, medications such as H2 blockers, and proton-pump inhibitors. Chronic underlying medical conditions such as diabetes mellitus, pernicious anemia, and gastric surgery also reduce gastric pH. Hypochlorhydria has also been demonstrated in Helicobacter pylori infections.14
Nonpathogenic, indigenous intestinal bacteria inhibit colonization of pathogens by direct competition for nutrients and by the production of fatty acids or chemicals that are bactericidal. Alterations of intestinal flora occur commonly after antibiotic use, as well as secondary to radiation therapy, chemotherapy, and abdominal surgery.14 Recent antibiotic use lowers resistance to Salmonella infections and has been implicated as a risk factor for infection with E. coli O157:H7.14
The normal motility of the intestinal tract provides an important defense mechanism by minimizing the contact between the mucosal surface and the pathogen. Bowel stasis permits multiplication and overgrowth of foodborne or waterborne pathogens and contributes to retrograde migration from the colon to the small intestine. Antiperistaltic agents, narcotics, and previous radiation therapy theoretically can prolong gut transit time and increase pathogen infectivity. 14
Bacteria, such as enterotoxigenic E. coli, can increase their pathogenicity by adhering to the mucosal surface via surface pili or fimbria that bind to receptors on intestinal epithelial cells.5 Direct invasion of the mucosal epithelial cells occurs with many foodborne pathogens such as Shigella, Salmonella, enteroinvasive E. coli, Campylobacter, and Vibrio parahaemolyticus.5 Intracellular multiplication of these organisms is followed by epithelial cell death. Cytotoxins, such as the Shiga toxin of Shigella dysentariae or Shiga-like toxins produced by enterohemorrhagic E. coli O157:H7, enteropathogenic E. coli, and V. parahaemolyticus also cause cellular membrane disruption and cell lysis.5 Vibrio cholera and enterotoxic E. coli produce protein toxins that alter fluid and electrolyte transfer across epithelial cell membranes and produce large volumes of fluid that exceed the absorptive capacity of the colon. The resultant, excessive diarrhea can lead to rapid dehydration. 5
The most common pathogens causing foodborne illnesses are Salmonella, Campylobacter, E. coli O157, and the Norwalk viruses.15 Viral gastroenteritis, caused by Norwalk viruses, rotaviruses, enteric adenoviruses, astroviruses, and calciviruses, accounts for the majority of cases of infectious diarrhea seen in the emergency department.16 Usually transmitted person to person, food and water transmission is not uncommon and should be considered.17 Viral infections of the gastrointestinal tract cause an enormous amount of diarrhea-related mortality and morbidity and is one of the leading causes of infant death worldwide. 18 Simultaneous infection with more than one viral pathogen is common.
Foodborne viruses are usually transmitted person to person by the fecal-oral route and are infectious in very low doses. Lack of confirmation of a viral etiology leads to an underreporting of viral foodborne outbreaks.16 Viruses do not multiply or produce toxins, and food items simply act as transfer mediums.17 Foods may be contaminated either at the source or at the time and place of preparation. Bivalve mollusks (oysters, clams, mussels, and cockles) have been implicated in the transmission of viruses. Their contamination usually occurs at the source in polluted shallow coastal waters. 1 l9 Viral contaminants remain active when they are inadequately cooked or consumed raw, particularly in oysters.17 The application of raw sewage or sludge to fruit and vegetable crops is another source of primary viral contamination.17
Viral contamination from infected food handlers occurs after harvesting as food items pass through several hands before reaching consumers. It is largely associated with cold food items that require much handling during preparation (salads and sandwiches) or ice. 17 Surfaces used in food preparation may become contaminated, as well. Food handlers may be asymptomatically shedding virus particles prior to the onset of illness or continue to shed virus particles up to 48 h after recovery. 17 Although unusual, airborne transmission of virus particles has been demonstrated in outbreaks of foodborne illness on cruise ships and in schools. 16 As in bacterial infections, waterborne transmission of virus is an effective mechanism of infectivity. Outbreaks have been associated with contamination of private wells, small water systems, and community water systems216 Seasonal outbreaks of Norwalk gastroenteritis have been associated with swimming in lakes and pools.16
Hepatitis A was the fourth leading cause of foodborne disease among outbreak-associated illness compiled by the CDC for 1988 to 1992. 3 It is the only reportable foodborne viral disease, and the true incidence of illness is underreported. 19 When food samples have become unavailable for testing, hepatitis A outbreaks are likely to be recognized weeks after contaminated food has been consumed. Most infections are the result of consumption of infected bivalves, but outbreaks associated with infected strawberries and raspberries have occurred.1617 Diarrhea is not commonly associated with hepatitis A.
Viruses can be eliminated at any step along the food-handling process. Heating is an effective means of inactivating hepatitis A virus particles in mollusks, and their exposure to temperatures of 85° to 90°C for at least 90 s is recommended.1 19 Viruses are a small target for ionizing radiation. Enteric viruses are often acid resistant, and hepatitis A virus is quite resistant to drying. Viruses in water or on surfaces can be inactivated by chlorine, ozone, or ultraviolet light. 19
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