Foodborne illness: is it on the rise?

Abstract

Foodborne illness is a serious public health threat. The Centers for Disease Control and Prevention (CDC) estimates that 76 million foodborne illnesses, including 325,000 hospitalizations and 5,000 deaths, occur in the United States each year. Two recently published Foodborne Diseases Active Surveillance Network (FoodNet) reports showed that Salmonella, Campylobacter, Shigella, Cryptosporidium, and Shiga toxin Escherichia coli (STEC) O157 continue to be leading causes of both the number and incidence of laboratory-confirmed foodborne infections in the United States. According to the United States Department of Agriculture (USDA), foodborne illness costs the US economy $10–83 billion per year. Recent large foodborne outbreaks have led to claims that the number of foodborne disease outbreaks and concomitant illnesses has increased in recent years. However, a comparison of data from the CDC showed very little change in the incidence of foodborne illness caused by common pathogens between 2008 and the preceding 3 years (2005–2007). Nevertheless, despite intensified prevention efforts, foodborne illness remains a persistent problem in the United States. Food can become contaminated at any point in the farm-to-table continuum, as well as in consumers' own kitchens. Therefore, foodborne illness risk reduction and control interventions must be implemented at every step throughout the food preparation process, from farm to table. In addition, more effective food safety education programs for foodhandlers and consumers are needed. Strategies should take into account food safety-related trends including large-scale production and wide distribution of food, globalization of the food supply, eating outside of the home, emergence of new pathogens, and growing population of at-risk consumers.

Escherichia coli, food safety, foodborne illness, outbreak, Salmonella

INTRODUCTION

Over the past few decades, the United States Department of Agriculture–Food Safety and Inspection Service (USDA–FSIS), the US Food and Drug Administration–Center for Food Safety and Applied Nutrition (FDA–CFSAN), and state/city/county departments of health have intensified food safety regulation and enforcement as well as inspection of food production and distribution systems. During this period of time, the USDA–FSIS and FDA–CFSAN have also launched and prescribed many food safety management systems, including the Hazard Analysis Critical Control Points (HACCP) system.^1,2 HACCP is a logical and verifiable approach towards identifying biological, chemical, and physical hazards in food production and putting safety measures in place to prevent, eliminate, or reduce the hazards to safe levels. By instituting preventive controls, food safety hazards are less likely to occur. The effectiveness of HACCP has led USDA–FSIS to publish its final rule on the HACCP system in 1999, making it a requirement for meat and poultry plants.³ In 2001, the FDA–CFSAN followed suit, announcing its final rule, requiring juice processors to use HACCP principles for juice processing.⁴ Seafood processors also are now required to have HACCP plans.⁵ In addition to these regulations, the FDA–CFSAN, USDA–FSIS, other federal agencies such as the Centers for Disease Control and Prevention (CDC) and the US Environmental Protection Agency (EPA), and state/city/county departments of health have published many foodborne illness-prevention guidance documents for food handlers. Furthermore, federal agencies and state/city/county departments of health provide educational information (for different audiences) through printed materials and online for the safe handling of food and reducing the risks associated with foodborne illness. Many public and private organizations such as the Cooperative Extension System, the Partnership for Food Safety Education, the National Restaurant Association Educational Foundation, and the National Registry of Food Safety Professionals have also implemented education programs that address the control of foodborne pathogens from farm to table.

To preserve food and to prevent spoilage as well as the growth of pathogenic microorganisms, food processing companies use a variety of the numerous food processing methods that are currently available. Some of the most effective methods for preserving food include pasteurization, canning, refrigeration, freezing, drying, reduced oxygen packaging, the addition of food preservatives, curing, and food irradiation. As technology advances, newer and more sophisticated techniques (geared toward lengthening food shelf life and destroying or preventing spoilage and/or growth of pathogenic microorganisms) will continue to be developed.

In recent years there has been a growing trend toward the sale of prewashed bagged produce. In 2004, a study by Nielsen found that bagged salads had become the second-fastest-selling item in US grocery stores after bottled water.⁶ They called this rise “dramatic” and attributed it to convenience. There is consensus among members of the scientific community that thorough washing of fruits and vegetables (particularly those consumed raw) is the key to minimizing the risk of foodborne illness associated with fresh produce. Similarly, it is now widely accepted that washing cannot eliminate contamination. Nevertheless, nutrition educators and healthcare professionals believe that the benefits of eating fresh fruits and vegetables outweigh the risk of contracting a foodborne illness by consuming fresh produce.

Despite the foodborne illness prevention efforts noted above, foodborne outbreaks (defined as the occurrence of two or more cases of similar illness resulting from the ingestion of a common food) continue to occur and claim lives in the United States. Although numerous foodborne disease outbreaks occur every year in the United States, only a small fraction of them are recognized and reported to the CDC. Some of the large outbreaks published or reported to the CDC since 1988 are summarized in Table 1.^7,–31

Table 1

Notable outbreaks of human infection due to foodborne pathogens in the United States, 1980–2009.

Year	State(s)	Pathogen	Vehicle of transmission	Number of illnesses	Number of deaths	Reference
2009	Multistate	Salmonella Typhimurium	Peanut butter	714	9	7
2008	Multistate, District of Columbia, and Canada	Saintpaul serotype of Salmonella enterica	Mexican-grown peppers	1,017	2	8
2007	Multistate	Salmonella	Chicken and turkey pot pies	401	–	9
2007	Multistate	Salmonella	ConAgra	272	–	10
2007	Multistate	Salmonella	Peter Pan and Great Value Peanut Butter	425	–	11
2007	Multistate	E. coli O157:H7	Topp's Brand frozen ground beef patties	40	–	12
2008	Multistate	E. coli O157:H7	Ground beef sold at Kroger® stores in Michigan and Ohio	45	–	13
2007	Massachusetts	Listeria monocytogenes	Farm milk or milk-related products	5	3	14
2006	South Plainfield, New Jersey, and Long Island	E. coli O157:H7	Green onions	67	–	15
2006	Multistate and Canada	E. coli O157:H7	Bagged spinach packaged by Natural Selection Foods	199	3	16
2002	Multistate	E. coli O157:H7	Hamburger	18	–	17
2002	Multistate	Listeria monocytogenes	Processed chicken	46	7	18
2000	Multistate	Salmonella	Bean sprouts	23	–	19
2000	Pennsylvania, Washington	E. coli O157:H7	Dairy farms	56	–	20
2000	11 states	Listeria monocytogenes	Delicatessen turkey	30	4 deaths, 3 miscarriages	21
2000	North Carolina	Listeria monocytogenes	Homemade Mexican-style cheese	12	0 deaths, 5 miscarriages	22
1999	15 States, 2 Canadian provinces	Salmonella	Unpasteurized orange juice	91	–	23
1999	Connecticut, Maryland, New York	Listeria monocytogenes	Paté	11	–	24
1998	22 states	Listeria monocytogenes	Processed meats	101	15 deaths	25
1997	Colorado	E. coli O157:H7	Ground beef	15	–	26
1996	California, Colorado, Washington and British Columbia	E. coli O157:H7	Unpasteurized apple juice	45	–	27
1994	Minnesota, South Dakota, Wisconsin	Salmonella	Ice cream	>3,000	0	28
1994	Illinois Michigan, Wisconsin,	Listeria monocytogenes	Pasteurized chocolate milk	69	0	29
1993	Washington, Idaho, California, Nevada	E. coli O157:H7	Hamburgers	>500	4	30
1989	Connecticut	Listeria monocytogenes	Shrimp	10	–	31

Two recently published Foodborne Diseases Active Surveillance Network (FoodNet) reports showed that Salmonella, Campylobacter, Shigella, Cryptosporidium, and Shiga toxin Escherichia coli (STEC) O157 continue to be the leading causes of both the number and incidence of laboratory-confirmed foodborne infections in the United States (Tables 1, 2 and 3).^32,33 According to the CDC, smaller outbreaks, especially those occurring within one state, are unlikely to be reported to public health officials.³⁴ Outbreaks not causing serious illness, hospitalization, or death, or those not resulting in patients showing symptoms quickly are also unlikely to be reported to public health officials.³⁴ Other factors that can determine whether or not an outbreak is recognized include consumer awareness and healthcare providers' ability to diagnose foodborne illness; interest and motivation of the consumer and healthcare provider to report the illness; and the capacity of state and local (city or county) health departments and environmental agencies to conduct foodborne illness surveillance.³⁴

Table 2

Comparison of the numbers of laboratory-confirmed infections caused by specific bacterial pathogens in 2008³³ and 2005,³² as reported at 10 FoodNet Sites in the states of CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN).

Pathogen	2008		2005
	Percent	Cases reported	Percent	Cases reported
Salmonella	38.95	7,444	44.57	6,505
Campylobacter	34.07	5,825	38.44	5,690
Shigella	12.54	3,029	18.14	2,095
Cryptosporidium	7.94	1,036	6.20	1,326
STEC O157	2.83	513	3.07	473
STEC non-O157	0.77	205	1.23	128
Yersinia	0.98	164	0.98	163
Listeria	0.81	135	0.81	136
Vibrio	0.72	131	0.78	121
Cyclospora	0.39	17	0.10	65
Total	100	18,499	100	16,702

†

Parasitic pathogen.

Table 3

Comparison of the incidence of laboratory-confirmed infections caused by specific bacterial pathogens in 2008³³ and 2005,³² as reported at 10 FoodNet sites in the states of CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN.

Pathogen	Incidence
	2008	2005
Salmonella	16.20	14.47
Campylobacter	12.68	12.66
Shigella	6.59	4.66
Cryptosporidium	2.25	2.95
STEC O157	1.12	1.05
STEC non-O157	0.45	0.28
Yersinia	0.36	0.36
Listeria	0.29	0.30
Vibrio	0.29	0.27
Cyclospora	0.04	0.14

†

Cases per 100,000 population.

‡

Parasitic pathogen.

The occurrence of several small and large foodborne outbreaks in recent years and the accompanying broad media coverage, have led to a widespread perception that the numbers of foodborne disease outbreaks and cases are increasing in the United States. In reality, however, there is little evidence that the actual overall prevalence of foodborne illness is increasing.

FACTORS CONTRIBUTING TO THE OCCURRENCE OF FOODBORNE ILLNESS

The trends affecting the occurrence of foodborne illness include, but are not limited to the following: 1) large-scale production and wide distribution of food; 2) globalization of the food supply; 3) eating outside of the home; 4) microbial genomic diversification/emergence of new pathogens; and 5) growing population of at-risk consumers.

Large-scale production and wide distribution of food

In order to meet the increased demand for food due to growing human population worldwide, industrial-scale and centralized production systems including large-scale farming, intensified animal production, and large-scale food processing and distribution have drastically increased over the past several decades. Nearly all food consumed in the United States today is produced using modern intensive farming.³⁵ Although such intensive food production systems are needed in order to meet the increasing demand for food, they have been blamed for the evolution of new pathogens. For example, the emergence of certain Salmonella serotypes over the past several decades is believed to have been caused by intensive animal farming.³⁶

In an effort to combat the obesity epidemic in the United States, nutrition educators and healthcare professionals are encouraging consumers to eat more fruits and vegetables. At the same time as this effort is being made, however, the number of foodborne illness outbreaks linked to fresh produce (especially those eaten without cooking) is rising.³⁷ This increase is likely being offset by a decrease in the number of outbreaks from other sources, since the overall prevalence of foodborne illness is not increasing.

Although food processing, especially that involving large-scale production, increases product value, lengthens product shelf life, and controls hazards, it is thought to also be responsible for food contamination and the spread of foodborne pathogens. The pooling of raw materials, for example, can result in a single adulterated component contaminating the rest and, potentially, the final product. The fact that food or food ingredients are often handled by several people using different equipment in diverse food processing settings can also increase the opportunities for contamination unless there are effective controls. Also, a continuous food production operation provides the conditions microorganisms need to grow. Such conditions include food, moisture, and temperature. Foodborne microorganisms need nutrients (e.g., carbohydrates and proteins) for their growth. Although the majority of foodborne microorganisms grow well between 41°F and 135°F,³⁸ others can grow at refrigeration temperatures (≤41°F) or survive freezing temperatures (≤32°F). The growth of many foodborne microorganisms is significantly slowed down if water activity (a_w), which is defined as the amount of moisture available for microbial growth, is 0.85 or lower.³⁸ The a_w scale ranges from 0 (driest) to 1.00 (wettest).

Failure to monitor and control pathogenic microorganisms such as Listeria monocytogenes in the food processing environment can result in them forming biofilms and thus colonizing certain food and non-food contact surfaces. Consequently, L. monocytogenes can persist in a food processing plant for a long time. For example, a specific outbreak L. monocytogenes subtype has been shown to persist for at least 12 years in a plant processing turkey frankfurters and delicatessen turkey meat in the US state of Oklahoma.^39,40 Once a microbial population forms a biofilm and establishes a colony, it becomes extremely difficult to eliminate.⁴¹ In fact, at that point, the cleaning and sanitizing of colonized surfaces is often counterproductive. Rather than eliminating the microorganism, activities aimed at cleaning and sanitizing may loosen and release the cells from the biofilm, thereby increasing the likelihood of product contamination.⁴² Contamination can be controlled through continuous surveillance (i.e., collection and analysis of environmental samples on a regular basis).⁴³ In this way, contaminated areas, surfaces, or spots can be detected and appropriate control actions can be taken before biofilms or colonies become established.

The impact of food processing on environmental contamination with foodborne pathogens is well known. In a food processing setting, a contaminated food-contact surface or food ingredient can result in contamination of several batches of final product and wide distribution of contaminated products within a short period of time. The recent Salmonella Typhimurium outbreak⁷ linked to a peanut butter plant in the US state of Georgia is a typical example of the severe consequences of a contaminated food processing environment. As of April 29, 2009, this multistate outbreak had sickened 714 people and possibly contributed to 9 deaths.⁷ The Salmonella contamination not only impacted the incriminated plant, it also resulted in the contamination and recall of other manufacturers' products that contained the contaminated peanut butter or paste.³⁵ Cookies, crackers, cereal, candy, ice cream, and pet foods are among the products of other manufacturers that had peanut butter originating from the GA facility as an ingredient and have consequently been recalled due to possible contamination with Salmonella Typhimurium. Although the CDC and the FDA warned consumers to not eat recalled products as soon as the outbreak was detected, more outbreak-related illnesses continued to be reported by consumers who ate recalled products for the next several months, partly because many of the implicated products have a long shelf life. The CDC's consumer advice and recommendations are available at http://www.cdc.gov/salmonella/typhimurium/update.html#recommendations

Dirty food-contact surfaces and cross-contamination are the most common avenues of contamination both on farms and in food processing environments. Cross-contamination is defined as the transfer of microorganisms from one food or surface to another. Food producers can deny pathogens sites for harbor and thus prevent product contamination by developing and implementing standard operating procedures (SOPs) for cleaning and sanitizing equipment and food-contact surfaces. Preventing cross-contamination is another important way to reduce product contamination. Training employees to properly clean and sanitize equipment and food-contact surfaces, as well as to prevent cross-contamination, is important for ensuring food safety.

Globalization of the food supply

According to the USDA's Economic Research Service (USDA–ERS), imported food accounted for 15% (based on volume) of food consumed in the United States in 2005.⁴⁴ When based on value, the proportion of food consumed in the United States that originated from foreign countries was 7% in 2005. Data from the USDA–ERS show that the average annual import share (when based on both volume and value) of food consumed in the United States has increased since 1980. Fresh produce, tree nuts, and fish and shellfish are the most common food imports. Data also indicate that the demand for ethnic food among consumers in the United States is rising.³⁵

If contaminated with pathogens, imported food (especially fresh produce) can result in spread of the pathogens.^45,46 Therefore, the importation of food from other countries (particularly those with inferior food safety standards) can increase the risk of foodborne illness in the United States. Water contamination is a significant public health issue in many developing countries^47,48 and contaminated water is one of the most common sources of contamination in fresh produce.^49,50 Products typically become adulterated when washed with contaminated water. Other unsafe agricultural practices, such as the possible use of untreated human waste and animal manure as fertilizers, can also result in the contamination of fresh produce.^49,51

Eating food outside of the home

The number of Americans eating food prepared away from home has risen between 1977 and 1996.⁵² Nearly 50% of household expenditure on food in the United States goes towards the purchase of food from full-service and fast-food restaurants.⁵² Lifestyle, social, and economic changes, as well as increased travel and tourism, have all contributed to an increase in the number of American consumers eating food prepared in restaurants. Demand for food from street vendors is also on the rise. Eating food prepared away from home has implications for public health. A considerable number of foodborne illnesses occur in food service establishments. For example, according to the CDC, 50% of foodborne outbreaks reported through its surveillance system occurred in restaurants during 1998–2002.³⁴ Based on these data, the risk of foodborne illness associated with eating out is greater than that of eating food prepared at home.

Further, changes in shopping habits in recent years have led to rising demand for ready-to-eat and refrigerated or frozen food products among consumers in the United States.⁵³ Processed foods involve multiple stages of handling compared to unprocessed foods. Since frequent handling increases the chance of product contamination, such products may present a greater risk of foodborne illness. But the increase observed in recent years in both the number and frequency of foodborne illness outbreaks linked to unprocessed foods, such as fresh produce,^37,45 has called this premise into question.

Numerous cases of foodborne illness also occur in the home.^54,55 Practices leading to such cases include poor time-temperature control (e.g., allowing food to remain too long at room temperature), cross-contamination, and poor personal hygiene. Consumers can avoid foodborne illness by handling food safely, following food preparation recommendations, and avoiding foods commonly associated with foodborne illness (e.g., raw or undercooked meat, poultry, and seafood; raw or undercooked eggs; raw sprouts; unpasteurized fruit and vegetable juices; and unpasteurized milk and soft cheese).⁵⁶ Many educational resources for consumers that focus on preventing foodborne illness in home kitchens are available at http://www.foodsafety.gov and http://www.fightbac.org.

Genetic variability and emergence of new pathogens

The ability of microorganisms to adapt to different environmental conditions and to frequently survive unfavorable environmental conditions and changes has been attributed to their large genetic variability.⁵⁷ Researchers are just beginning to understand the mechanisms underlying the evolution and emergence of new bacterial pathogens. Many studies have revealed that organisms can undergo changes in their physiology in a short period of time, resulting in the emergence of new pathogens.⁵⁸ The emergence of new pathogens can have serious implications for public health, especially if the strains can evolve or mutate to become more virulent or drug resistant.³⁶ For example, multidrug-resistant (MDR) strains of Salmonella have recently emerged, which represent a significant threat to food safety. MDR Salmonella strains that are resistant to the antimicrobial agents designed to eradicate typical Salmonella strains render those drugs ineffective for treating MDR Salmonella-related infections.³⁶ The emergence of MDR Salmonella has been blamed on the administration of antimicrobial agents to food animals.³⁶ The emergence of new pathogens is assisted by a variety of factors including environmental changes, changes in food production or processing methods, changes in populations affected by the pathogens, and changes in microbial characteristics and distribution.⁵⁸

Increasing proportion of at-risk consumers

The highest rates of foodborne illness are among young children, adults who have a weakened immune system, older adults, and pregnant women.

According to projections of the United States Census Bureau, the proportion of individuals aged 65 years and older is increasing in the United States.⁵⁹ These projections estimate that approximately 20% of US residents will be aged 65 and older in 2030. In 2050, this subpopulation is expected to rise to 88.5 million, reflecting more than a twofold change from 38.7 million in 2008.

The immunocompromised group is composed of individuals who are chronically ill, such as AIDS patients or persons who take medications that adversely affect their immune system.⁵³ The latter category includes cancer patients on immunosuppressive drugs, people who take antibiotics, and patients treated for transplants and diabetes.

Due to better healthcare and advances in the field of medicine, older adults and people with weakened immune systems, who would otherwise die prematurely, are living longer; hence, these subpopulations are growing in the United States. It is difficult to determine the impact this increase may be having on the overall incidence of foodborne illness in the United States as any increase in the incidence of foodborne illness among these subpopulations is being mitigated by improved healthcare and advances in medical technologies.

THE BURDEN OF FOODBORNE ILLNESS IN THE UNITED STATES

The CDC estimates that foodborne infections cause 76 million illnesses, including 325,000 hospitalizations and 5,000 deaths, each year in the United States.⁶⁰ Further, the cost of foodborne illness to the US economy is $10–83 billion per year.⁶¹ The costs of foodborne illness include lost productivity, medical costs, loss of customers and sales, and lawsuits resulting in compensation for loss of income, medical expenses, legal fees, and other damages.⁶² Other hidden costs include the pain and suffering associated with foodborne illness, time spent by family members caring for ill relatives, and the cost of treatment-related travel for patients and their relatives.

Bacteria and viruses pose the greatest threat to food safety. During 1998–2002, bacteria caused more than half of all reported foodborne disease outbreaks (55%) and cases (55%).³⁴ During this same period, viruses caused 33% of outbreaks and 41% of foodborne illness cases. Other common causes of foodborne illness include chemical agents, parasites, fungi (yeasts and molds), and physical hazards (e.g., bones or bone fragments; stones, leaves, or stalks; and shell fragments from nuts, shellfish, and eggs).³⁸ Although any food can be contaminated and become potentially hazardous, certain foods pose a greater threat to public health than others. High-risk foods include raw or undercooked foods of animal origin (e.g., meat and poultry, eggs, fish, and unpasteurized milk).⁵⁶ Fresh produce, particularly raw fruits and vegetables, have been responsible for many foodborne outbreaks in recent years and, consequently, are currently considered to be high-risk foods. Unpasteurized fruit juices and alfalfa sprouts/other raw sprouts are also regarded as high-risk foods.⁵⁶

FOODBORNE ILLNESS OUTBREAKS REPORTED IN THE UNITED STATES, 1988–2006

The annual numbers of foodborne disease outbreaks (FBDOs) and foodborne illnesses reported to the CDC between 1988 and 2006 are shown in Figures 1 and 2, respectively. However, the numbers of outbreaks reported annually to the CDC^34,–65 represent only a portion of the actual number of outbreaks that occur. According to the CDC, each year, many foodborne outbreaks and illnesses go unreported.^34,60 For example, it has been estimated that for every case of outbreak-associated cases of salmonellosis (foodborne illness caused by Salmonella infection), reported through the Foodborne Disease Outbreak Surveillance System (PulseNet), an extra 38 go undetected or unreported.⁶⁶ PulseNet is a national network designed to help detect foodborne disease outbreaks.^67,–69 The program was launched in four states in 1996 and by 2001 all 50 states and several large cities were participating. PulseNet is comprised of the CDC and state health departments in the United States. Since its launch in 1996, PulseNet's utility for increasing the number of recognized outbreaks (including those caused by STEC O157, Salmonella enterica, Listeria monocytogenes, Shigella spp., and Campylobacter spp.) has been noted by many authors. In PulseNet laboratories, foodborne pathogens are characterized using the pulsed-field gel electrophoresis (PFGE) subtyping technique. Using standardized protocols, PFGE DNA “fingerprints” of isolates of foodborne pathogens are generated^67,–69 and rapidly exchanged among the CDC and participating laboratories through electronic submission of the PFGE “fingerprints” to a CDC database; the database is configured to allow for rapid comparison of the patterns as well as real-time communication among participating laboratories. A close relationship among PFGE banding patterns points to a common source of an outbreak pathogen subtype. PFGE data from the FDA and USDA–FSIS laboratories are also entered into the CDC FoodNet database.

Figure 1

Foodborne disease outbreaks (FBDOs) in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

Figure 2

Foodborne illnesses in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

The number of cases of foodborne illness increased in 1998 compared to previous years (1988–1997). In 1998, 1,314 FBDOs resulted in 27,257 cases of illness. Prior to 1998, the number of illnesses per year ranged from 11,015 to 22,607 (Figures 1 and 2). The CDC has attributed this increase to the enhanced surveillance made possible through implementation of the National Molecular Subtyping Network for Foodborne Disease Surveillance (PulseNet) program in 1996.^34,–69 The number of foodborne disease outbreaks per year (Figure 1) and the number of illnesses per year (Figure 2) remained relatively constant between 1998 and 2006.

Figure 3 shows the average number of foodborne illness cases per outbreak during 1988–2006. Between 1988 and 1995, the average number of illnesses per outbreak per year was generally high, indicating the outbreaks in those years were relatively large. In 1996, the average number of foodborne illness cases per outbreak peaked at approximately 47. One year later, the average number of illnesses per outbreak decreased to approximately 24 and in 1998 it decreased again to approximately 21. The annual average size of outbreaks was smaller between 1998 and 2006 compared to the years 1988–1997 (Figure 3). According to the CDC, this decrease in the average size of reported outbreaks following implementation of enhanced surveillance in 1996 was largely a result of the increased ability of investigators to detect smaller outbreaks that would not otherwise have been reported.⁶⁴

Figure 3

Foodborne illnesses per foodborne disease outbreak (FBDO) in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

Figure 4 depicts the proportion of reported outbreaks and illnesses of known etiology per year between 1988 and 2006. During 1999–2006, the proportion of outbreaks with a known etiology steadily and consistently increased each year, from 27.6% in 1999 to 50.0% in 2006 (Fig. 4).

Figure 4

Proportion of foodborne disease outbreaks (FBDOs) and illnesses of known etiology in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
 FBDOs (◆), illnesses ()
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

During 1988–2006, bacteria caused the majority of foodborne outbreaks and illnesses (Figures 5 and 6). The number of outbreaks and the number illnesses caused by viruses remained low between 1988 and 1997, but they sharply increased during 1997 and 2000, and they remained high during 2000–2006. It is noteworthy that the number of outbreaks caused by viruses was higher than that caused by bacteria in 2004 and 2006 (Figure 6). Likewise, viruses caused more illnesses compared to bacteria during 2004–2006 (Figure 6). The CDC has largely attributed the changes in the proportion of outbreaks and illnesses caused by viral pathogens noted above to better specimen collection and diagnostic/detection protocols for viral infections.³⁴ Chemical hazards and parasites generally caused only a small proportion of outbreaks and illnesses (Figures 5 and 6).

Figure 5

Foodborne disease outbreaks (FBDOs) by etiology in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
 Bacterial (◆), viral (), chemical (▴), and parasitic (●)
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

Figure 6

Foodborne illnesses by etiology in the United States, 1988–2006. Data from the CDC (1996),^63, (2000),^64, (2006),^34, and (2009).⁶⁵ 
 Bacterial (◆), viral (), chemical (▴), and parasitic (●)
*All data for 2001–2006 were collected electronically through the Electronic Foodborne Outbreak Reporting System (eFORS) without confirmation of etiology by CDC staff; all etiologies are as reported by the state. Etiologies of outbreaks prior to 2001 were confirmed by CDC staff, so some differences may exist between 2001–2006 data and data from prior years.

CONCLUSION

Foodborne illness continues to be a significant public health threat in the United States despite intensified efforts to prevent it. Several trends including large-scale production and wide distribution of food, globalization of the food supply, eating outside of the home, emergence of new pathogens, and increasing proportion of at-risk consumers are facilitating outbreaks of foodborne illness and making it difficult to control foodborne pathogens and decrease the incidence of the infections they cause.

Despite the occurrence of many large outbreaks of foodborne illness in recent years, there is insufficient evidence to support claims that the numbers of actual foodborne illness outbreaks and cases are climbing. According to the CDC, the increase in both reported outbreaks and foodborne illness cases in recent years is likely a reflection of enhanced surveillance rather than a true increase in incidence.³⁴ The media's coverage of some recent outbreaks, especially those linked to peanut butter,⁷ Mexican-grown peppers,⁸ and bagged spinach,¹⁶ has also increased awareness, which may help explain the widespread notion that the numbers of foodborne disease outbreaks and concomitant illnesses have increased in recent years. Furthermore, increased awareness of foodborne illness and its symptoms has likely resulted in increased reporting and better diagnosing of foodborne illnesses by patients and physicians, respectively.

In a recently published report, the Foodborne Diseases Active Surveillance Network (FoodNet), a partnership surveillance project of the CDC, the USDA-FSIS, the FDA, and 10 US states, found there was very little change in the incidence of foodborne illness caused by common pathogens when the 2008 data was compared to the preceding 3 years (2005–2007).³³ While there has been little change in the overall incidence of foodborne illness, a recent HealthyPeople 2010⁷⁰ progress review indicates that the numbers of certain specific foodborne infections have, in fact, gone down. For example, during 1997–2005, the incidence of infections caused by Campylobacter spp., E. coli O157:H7, and L. monocytogenes decreased from 24.6 to 12.7 cases per 100,000 population, 2.1 to 1.1 per 100,000, and 0.47 to 0.30 per 100,000, respectively.⁷¹ The 2010 targets for Campylobacter spp., E. coli O157:H7, and L. monocytogenes are 12.3 cases per 100,000, 1.0 per 100,000, and 0.24 per 100,000, respectively.⁷¹

The continued occurrence of food-related infections calls for better foodborne illness risk reduction and control interventions. Since food can become contaminated at any point during the food processing and preparation chain, including in the consumers' own kitchens, interventions must be implemented at every step, from farm to table.

In order to meet the increased demand for food associated with the growing human population worldwide, industrial-scale and centralized production systems, including large-scale farming, intensified animal production, and large-scale food processing and distribution, have drastically increased over the past several decades. The fact that these systems are needed in order to meet the demand for food is undisputable. However, these kinds of systems have also been blamed for increasing the risk of foodborne illness. Strategies for controlling potential microbial food safety hazards include the use of good agricultural practices; the HACCP program; good manufacturing practices, and standard operating procedures for sanitation. Training employees to avoid food contamination by properly cleaning and sanitizing equipment and food-contact surfaces, preventing cross-contamination, and practicing good personal hygiene is also important for ensuring food safety.

In the home, consumers can avoid foodborne illness by handling food safely, following food preparation recommendations, and avoiding foods commonly associated with foodborne illness (e.g., raw or undercooked meat, poultry, and seafood; raw or undercooked eggs; raw sprouts; unpasteurized fruit and vegetable juices; and unpasteurized milk and soft cheese).⁵⁶ Many educational resources for consumers that focus on preventing foodborne illness in home kitchens are available at http://www.foodsafety.gov and http://www.fightbac.org. To further improve education of the general public about food safety, closer ties between food safety educators and the media are encouraged.

Finally, more research into understanding the biology, ecology, and spread of foodborne pathogens is necessary in order to identify better pathogen-control strategies. To more effectively control foodborne pathogens, more research on the mechanisms underlying the evolution and emergence of new bacterial pathogens is needed.

Declaration of interest

The author has no relevant interests to declare.