Preliminary FoodNet Data on the Incidence of Infection with Pathogens Transmitted Commonly Through Food --- 10 States, 2006

Foodborne illnesses are a substantial health burden in the United States (1). The Foodborne Diseases Active Surveillance Network (FoodNet) of CDC's Emerging Infections Program collects data from 10 U.S. states* regarding diseases caused by enteric pathogens transmitted commonly through food. FoodNet quantifies and monitors the incidence of these infections by conducting active, population-based surveillance for laboratory-confirmed illnesses (1). This report describes preliminary surveillance data for 2006 and compares them with baseline data from the period 1996--1998. Incidence of infections caused by Campylobacter, Listeria, Shigella, and Yersinia has declined since the baseline period. Incidence of infections caused by Shiga toxin-producing Escherichia coli O157 (STEC O157) and Salmonella, however, did not decrease significantly, and Vibrio infections have increased, indicating that further measures are needed to prevent foodborne illness and achieve national health objectives.

In 1996, FoodNet began active, population-based surveillance for laboratory-confirmed cases of infection caused by Campylobacter, Listeria, Salmonella, STEC O157, Shigella, Vibrio, and Yersinia. FoodNet personnel ascertain cases through contact with all clinical laboratories serving their surveillance areas. FoodNet added surveillance for cases of Cryptosporidium and Cyclospora infection in 1997 and STEC non-O157 infection in 2000. In 2004, FoodNet began collecting data on which laboratory-confirmed infections were associated with outbreaks.

Hemolytic uremic syndrome (HUS) surveillance, which began in 2000, is conducted in nine states through a network of pediatric nephrologists and infection-control practitioners and is validated with a review of hospital discharge data. Because of the length of time required for review of hospital records, this report contains preliminary HUS data for 2005.

During 1996--2006, the FoodNet surveillance population increased from 14.2 million persons (5% of the U.S. population) in five states to 44.9 million persons (15% of the U.S. population) in 10 states. Preliminary incidence for 2006 was calculated by dividing the number of laboratory-confirmed infections by 2005 population estimates. Final incidence for 2006 will be reported when 2006 population estimates are available from the U.S. Census Bureau. In previous reports, the final incidence has been similar to the preliminary incidence.

Surveillance

In 2006, a total of 17,252 laboratory-confirmed cases of infections in FoodNet surveillance areas were identified: Salmonella (6,655 cases), Campylobacter (5,712), Shigella (2,736), Cryptosporidium (859), STEC O157 (590), STEC non-O157 (209), Yersinia (158), Vibrio (154), Listeria (138), and Cyclospora (41). The overall incidence per 100,000 population was 14.81 for Salmonella, 12.71 for Campylobacter, 6.09 for Shigella, 1.91 for Cryptosporidium, 1.31 for STEC O157, 0.46 for STEC non-O157, 0.35 for Yersinia, 0.34 for Vibrio, 0.31 for Listeria, and 0.09 for Cyclospora. Substantial variation occurred among surveillance sites (Table). In 2005, FoodNet identified 71 cases of HUS in children aged <18 years (rate: 0.68 per 100,000 children); 47 (66%) cases occurred in children aged <5 years (rate: 1.63).

Of the 5,957 (90%) Salmonella isolates serotyped, seven serotypes accounted for 64% of infections: Typhimurium, 1,157 (19%); Enteritidis, 1,109 (19%); Newport, 531 (9%); Javiana, 292 (5%); Montevideo, 250 (4%); Heidelberg, 239 (4%); and a monophasic serotype identified as Salmonella I 4,[5],12:i:-, 239 (4%). Among 147 (95%) Vibrio isolates for which the species was identified, 94 (64%) were V. parahaemolyticus, and 18 (12%) were V. vulnificus. FoodNet also collected data on 209 STEC non-O157 isolates that were tested for O antigen determination; 188 (90%) had an identifiable O antigen, including O26 (53 isolates [28%]), O103 (46 [24%]), and O111 (29 [15%]); for 21 (10%) isolates, no reaction occurred with the typing antisera used by CDC, or O antigen information was not available.

Comparison with Baseline Period

A main-effects, log-linear Poisson regression model (negative binomial) was used to estimate statistically significant changes in incidence. This model accounts for the increase in the number of FoodNet sites and surveillance population and for variations in incidence among sites (1). For laboratory-confirmed infections, the average annual incidence for 1996--1998 (1997--1998 for Cryptosporidium) was used as the baseline. For HUS surveillance, 2000--2001 was used as the baseline. Estimated changes in incidence (relative rate) between the baseline period and 2006 and 95% confidence intervals (CIs) were calculated. Partly because of concerns that changes in clinical laboratory practices affected incidence, a baseline has not been set for non-O157 STEC (2) or Salmonella I 4,[5],12:i:-.

The estimated annual incidence of several infections changed significantly from baseline to 2006 (Figure 1). The estimated incidence of infection with Yersinia decreased 50% (CI = 37%--60%), Shigella decreased 35% (CI = 8%--54%), Listeria decreased 34% (CI = 17%--47%), Campylobacter decreased 30% (CI = 24%--35%), and Vibrio increased 78% (CI = 34%--138%). The estimated incidence of Cryptosporidium, Salmonella, and STEC O157 did not change significantly compared with the baseline. Although Salmonella incidence did not decrease significantly overall, the incidence of S. Typhimurium decreased significantly (41% [CI = 34%--48%]). In contrast, significant increases in incidence compared with baseline occurred for S. Enteritidis (28%, CI = 4%--57%), S. Newport (42%, CI = 7%--87%), and S. Javiana (92%, CI = 22%--202%). The estimated incidence of S. Heidelberg and S. Montevideo did not change significantly compared with baseline (Figure 2). The estimated incidence of HUS in children aged <5 years also did not change significantly.^†

Outbreak-Associated Cases of Infection

Outbreak-associated cases of infection accounted for at least 88 (15%) of 590 STEC O157 cases in 2006, compared with 36 (9.0%) of 402 cases in 2004 and 107 (23%) of 473 cases in 2005. Three large, high-profile multistate outbreaks of STEC O157 infections associated with produce affected FoodNet sites in 2006. Of the 88 outbreak-associated STEC O157 cases ascertained in FoodNet in 2006, one outbreak associated with bagged fresh spinach (3) accounted for 32 (36%), and two outbreaks associated with lettuce in two fast-food chains accounted for 14 (16%).

Outbreak-associated cases accounted for at least 404 (6.1%) of 6,655 Salmonella cases ascertained in FoodNet in 2006, compared with 352 (5.4%) of 6,498 cases in 2004 and 296 (4.6%) of 6,505 cases in 2005. A multistate outbreak of S. Typhimurium infections associated with tomatoes accounted for 58 (14%) outbreak-associated Salmonella cases ascertained in FoodNet in 2006, and an outbreak of S. Newport infections associated with tomatoes accounted for 37 (9.2%).

Reported by: D Vugia, MD, California Dept of Health Svcs. A Cronquist, MPH, Colorado Dept of Public Health and Environment. J Hadler, MD, Connecticut Dept of Public Health. M Tobin-D'Angelo, MD, Div of Public Health, Georgia Dept of Human Resources. D Blythe, MD, Maryland Dept of Health and Mental Hygiene. K Smith, DVM, Minnesota Dept of Health. S Lathrop, PhD, New Mexico Dept of Health. D Morse, MD, New York State Dept of Health. P Cieslak, MD, Oregon State Public Health Div. T Jones, MD, Tennessee Dept of Health. KG Holt, DVM, Food Safety and Inspection Svc, US Dept of Agriculture. JJ Guzewich, MPH, Center for Food Safety and Applied Nutrition, Food and Drug Admin. OL Henao, PhD, E Scallan, PhD, FJ Angulo, DVM, PM Griffin, MD, RV Tauxe, MD, Div of Foodborne, Bacterial, and Mycotic Diseases, National Center for Zoonotic, Vector-Borne, and Enteric Diseases; SK Greene, PhD, EIS Officer, CDC.

Editorial Note:

In 2006, compared with the 1996--1998 baseline period, significant declines occurred in the estimated incidence of Campylobacter, Listeria, Shigella, and Yersinia infections. However, most of these declines occurred before 2006. Most of the decline in Campylobacter incidence occurred by 2001. In 2006, the incidence of Listeria infections remained higher than at its lowest point in 2002.

After substantial declines in 2003 and 2004, the incidence of STEC O157 infections increased in 2005 (4) and again in 2006. The earlier decline in incidence was temporally associated with measures by the U.S. Department of Agriculture's Food Safety and Inspection Service (USDA-FSIS) and the beef-processing industry to reduce the contamination of ground beef. These measures were accompanied by a decline in the frequency of isolation of STEC O157 from ground beef in 2003 and 2004 (5). In 2005 and 2006, however, the frequency of isolation of STEC O157 in ground beef remained at the same level as 2004.^§ Reasons for the increases in human STEC O157 infections in 2005 and 2006 are not known. However, STEC O157 outbreaks caused by contaminated spinach and lettuce in 2006 highlight the need to more effectively prevent contamination of produce that is consumed raw. In a measure to reduce the risk for illness attributed to fresh produce, the Food and Drug Administration recently published draft guidance advising processors on how to minimize microbial food-safety hazards common to the processing of most fresh-cut fruits and vegetables (6).

Of the six most common Salmonella serotypes in 2006, only Typhimurium has declined since the baseline, and its incidence since 2003 has been stable. Transmission of Salmonella to humans can occur via many vehicles, including produce, eggs, poultry and other meat, and direct contact with animals and their environments. The two outbreaks of salmonellosis associated with tomatoes in 2006 underscore the need to more effectively prevent contamination of produce that is consumed raw. Poultry is an important source of human Salmonella infections. USDA-FSIS reported an increase in the frequency of isolation of Salmonella, particularly S. Enteriditis, in chicken-broiler carcasses during 2000--2005 (7,8). The predominant S. Enteriditis phage type strains isolated from chickens matched those associated with eating chicken in a FoodNet case-control study (7,9), indicating that chicken is an important source of human S. Enteriditis infections. In early 2006, USDA-FSIS launched an initiative to reduce Salmonella in poultry and other meat (10). For the period 2001--2006, a USDA-FSIS testing program identified 2006 as the year with the lowest percentage of chickens that tested positive for Salmonella (8).

The incidence of Vibrio infections has increased to the highest level since FoodNet began conducting surveillance. These infections are most often associated with the consumption of raw seafood, particularly oysters. Additional measures to reduce contamination of seafood more effectively are warranted. Consumers, especially persons who are immunocompromised, should be informed they are at increased risk for Vibrio infections when they consume raw seafood.

Much remains to be done to reach the national health objectives for foodborne illnesses.^¶ Enhanced measures are needed to control pathogens in animals and plants; to reduce or prevent contamination during growing, harvesting, and processing; and to educate consumers more effectively about risks and prevention measures. Such measures can be better focused when the source of human infections (i.e., animal reservoir species and transmission route) is known. In particular, further research is needed to understand how contamination of fresh produce occurs so that new measures to reduce such contamination can be developed and implemented.

Consumers can reduce their risk for foodborne illness by following safe food-handling recommendations and by avoiding consumption of unpasteurized milk, raw or undercooked oysters, raw or undercooked eggs, raw or undercooked ground beef, and undercooked poultry. The risk for foodborne illness also can be decreased by choosing in-shell pasteurized eggs, irradiated ground meat, and high-pressure--treated oysters. Additional information on food safety for consumers is available at http://www.foodsafety.gov.

References

1. Scallan E. Activities, achievements, and lessons learned during the first 10 years of the Foodborne Diseases Active Surveillance Network: 1996-2005. Clin Infect Dis 2007;44:718--25.
2. CDC. Laboratory-confirmed non-O157 Shiga toxin-producing Escherichia coli---Connecticut, 2000--2005. MMWR 2007;56:29--31.
3. CDC. Ongoing multistate outbreak of Escherichia coli serotype O157:H7 infections associated with consumption of fresh spinach---United States, September 2006. MMWR 2006;55:1045--6.
4. CDC. Preliminary FoodNet data on the incidence of infection with pathogens transmitted commonly through food---10 states, United States, 2005. MMWR 2006;55:392--5.
5. Naugle AL, Holt KG, Levine P, Eckel R. Sustained decrease in the rate of Escherichia coli O157:H7-positive raw ground beef samples tested by the Food Safety and Inspection Service. J Food Prot 2006;69: 480--1.
6. US Food and Drug Administration. Draft final guidance for industry: guide to minimize food safety hazards for fresh-cut fruits and vegetables. Fed Regist 2007;72:11364--8. Available at http://www.cfsan.fda.gov/~dms/prodgui3.html.
7. Altekruse SF, Bauer N, Chanlongbutra A, et al. Salmonella Enteritidis in broiler chickens, United States, 2000--2005. Emerg Infect Dis 2006;12:1848--52.
8. US Department of Agriculture, Food Safety and Inspection Service. Progress report on Salmonella testing of raw meat and poultry products, 1998--2006. Washington, DC: US Department of Agriculture; 2007. Available at http://www.fsis.usda.gov/science/progress_report_salmonella_testing/index.asp.
9. Marcus R, Varma JK, Medus C, et al. Re-assessment of risk factors for sporadic Salmonella serotype Enteritidis infections: a case-control study in five FoodNet sites, 2002--2003. Epidemiol Infect 2007;135:84--92.
10. US Department of Agriculture, Food Safety and Inspection Service. Salmonella verification sample result reporting: agency policy and use in public health protection. Fed Regist 2006;71:9772--7. Available at http://www.fsis.usda.gov/oppde/rdad/frpubs/04-026n.pdf.

* Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado, and New York.

^† Additional information, including data on age-specific trends and trends of HUS, is available at http://www.cdc.gov/foodnet.

^§ Additional information is available at http://www.fsis.usda.gov/science/ground_beef_e.coli_testing_results/index.asp.

^¶ Additional information is available at http://www.health.gov/healthypeople.

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Date last reviewed: 4/12/2007