Surveillance for Foodborne Disease Outbreaks --- United States, 2007

Surveillance for Foodborne Disease Outbreaks --- United States, 2007
Weekly
August 13, 2010 / 59(31);973-979


Foodborne agents cause an estimated 76 million illnesses annually in the United States (1). Outbreak surveillance provides insights into the causes of foodborne illness, types of implicated foods, and settings of foodborne infections that can be used in food safety strategies to prevent and control foodborne disease. CDC collects data on foodborne disease outbreaks submitted from all states and territories. This report summarizes epidemiologic data for the 1,097 reported outbreaks occurring during 2007 (the most recent finalized data), which resulted in 21,244 cases of foodborne illness and 18 deaths. Among the 497 foodborne outbreaks with a laboratory-confirmed single etiologic agent reported, norovirus was the most common cause, followed by Salmonella. Among the 18 reported deaths, 11 were attributed to bacterial etiologies (five Salmonella, three Listeria monocytogenes, two Escherichia coli O157:H7, and one Clostridium botulinum), two to viral etiologies (norovirus), and one to a chemical (mushroom toxin). Four deaths occurred in outbreaks with unknown etiologies. Among the 235 outbreaks attributed to a single food commodity, poultry (17%), beef (16%), and leafy vegetables (14%) were most often the cause of illness. Public health, regulatory, and agricultural professionals can use this information when creating targeted control strategies and to support efforts to promote safe food preparation practices among food employees and the public.

A foodborne disease outbreak is defined as the occurrence of two or more similar illnesses resulting from ingestion of a common food. State, local, and territorial health departments use a standard, Internet-based form to voluntarily submit reports of foodborne outbreaks to the Foodborne Disease Outbreak Surveillance System, and a toolkit for investigation and reporting of outbreaks is used to guide reporting officials.*

This report includes outbreaks occurring in 2007 and reported to CDC by May 3, 2010. Population-based rates of reported outbreaks were calculated for each state using U.S. Census estimates of the 2007 state populations.† Reported outbreak data include the number of illnesses, hospitalizations, and deaths associated with each outbreak; the etiologic agent, either confirmed or suspected§; and the implicated food vehicle. CDC classifies implicated foods into the following 17 food commodities: finfish, crustaceans, mollusks, dairy, eggs, beef, game, pork, poultry, grains-beans, oils-sugars, fruits-nuts, fungi, leafy vegetables, root vegetables, sprouts, and vegetables from a vine or stalk (2). Outbreaks in which the reported food vehicle contained ingredients from only one commodity were assigned to that commodity; those in which the reported food vehicle contained ingredients from more than one commodity, could not be grouped in one of the 17 commodities (e.g., coffee, alcohol), or contained insufficient information for commodity assignment were not attributed to any commodity.

Public health officials from 48 states, Puerto Rico, and the District of Columbia reported 1,097 foodborne disease outbreaks; multistate outbreaks involving two additional states (Montana and Nevada) were reported indirectly (Figure). The number of foodborne disease outbreaks (1,097) reported to CDC in 2007 was 8% lower than the annual average (1,193) reported for 2002--2006, and the number of outbreak-related illnesses (21,244 versus 25,079) was 15% lower. The number of outbreaks reported by each state or territory during 2007 varied from 0 to 149 (median: 0.30 outbreaks per 100,000 population; range: 0.03--1.90). A confirmed or suspected single etiologic agent was identified in 698 (64%) outbreaks (497 confirmed, 201 suspected), resulting in 15,477 (73%) illnesses (Table 1). Among the 363 outbreaks with an unknown etiology (5,122 illnesses), 257 outbreaks (71%) with 3,904 illnesses (76%) also had an unknown food vehicle. Outbreaks in which few persons became ill were more likely to have an unknown etiology. Among the 146 outbreaks in which no more than two persons became ill, 51% had no confirmed or suspected etiology. In contrast, no confirmed or suspected etiology was identified for 40% of 346 outbreaks involving three to seven illnesses, 30% of the 89 outbreaks involving eight or nine illnesses, and 24% of the 519 outbreaks involving 10 or more illnesses. The most common reasons reported for not identifying an etiology or food vehicle were 1) delayed reporting of illnesses to the health department, 2) too many food items were consumed by ill persons to identify a single food as the contaminated vehicle, and 3) human or food sample test results were unavailable, either because samples could not be obtained or because tests were negative for the pathogens evaluated.

Among the 497 outbreaks (12,767 illnesses) with a confirmed single etiologic agent, bacteria caused 259 (52%) outbreaks with 6,441 (50%) illnesses, viruses caused 199 (40%) outbreaks with 6,120 (48%) illnesses, chemical agents caused 34 (7%) outbreaks with 141 (1%) illnesses, and parasites caused five (1%) outbreaks with 65 (1%) illnesses. Norovirus was the most common cause of illness, accounting for 193 (39%) of the confirmed single-etiology outbreaks and 97% of those caused by viruses. Salmonella was the second most common, causing 136 (27%) confirmed single-etiology outbreaks and 53% of those attributed to bacteria. Among Salmonella serotypes identified, Enteritidis was the most common, causing 28 confirmed single-etiology outbreaks with 555 illnesses. Shiga toxin-producing E. coli (STEC) caused 40 of the confirmed single-etiology outbreaks (15% of those attributed to bacteria), of which 39 were caused by serogroup O157.

Among the 18 multistate foodborne disease outbreaks (i.e., outbreaks in which exposure to the etiologic agent or agents occurred in more than one state), 10 were attributed to Salmonella, six to E. coli O157:H7, one to C. botulinum, and one to norovirus. Foods associated with multistate Salmonella outbreaks included commercially-processed frozen pot pies (401 illnesses, three deaths), commercially-processed vegetable snacks (87 illnesses), eggs (81 illnesses), spinach/lettuce (76 illnesses), beefsteak tomatoes (65 illnesses), raw tuna (44 illnesses), ground beef (43 illnesses), cheese (20 illnesses), alfalfa sprouts (15 illnesses), and raw fresh basil (11 illnesses). Of the six multistate outbreaks of E. coli O157:H7 infection, the vehicle for five was ground beef (117 illnesses) and the vehicle for one was commercially-processed frozen pepperoni pizzas (27 illnesses). The vehicle for the C. botulinum toxin outbreak (eight illnesses) was commercially canned hotdog chili sauce. The one multistate outbreak caused by norovirus was associated with raw oysters (40 illnesses).

A food vehicle was identified in 470 (43%) outbreaks associated with 9,818 illnesses, of which 235 (50%) with 4,119 (42%) illnesses were linked to a food vehicle with ingredients limited to only one of the 17 commodities (Table 2). The commodities most commonly implicated in outbreaks were finfish (41 outbreaks), poultry (40 outbreaks), and beef (33 outbreaks); the commodities associated with the most illnesses were poultry (691 illnesses), beef (667 illnesses), and leafy vegetables (590 illnesses). The pathogen-commodity pairs responsible for the most outbreak-related illnesses were norovirus in leafy vegetables (315 illnesses), E. coli O157:H7 in beef (298 illnesses), and Clostridium perfringens in poultry (281 illnesses).¶

Two of the three largest reported outbreaks in 2007 were caused by Salmonella. The vehicles were hummus (802 illnesses) and commercially-processed frozen pot pies (401 illnesses and three deaths). The second largest outbreak was caused by norovirus at a conference hotel (526 illnesses); several shared food items were the suspected vehicles. The largest outbreaks assigned to a single food commodity were caused be a chicken dish contaminated with C. perfringens (132 illnesses), leafy vegetable salad contaminated with norovirus (128 illnesses), chili beans contaminated with C. perfringens (125 illnesses), and beef contaminated with E. coli O157:H7 (124 illnesses).

Reported by
A Boore, PhD, KM Herman, MSPH, AS Perez, MPH, CC Chen, MPH, DJ Cole, DVM, PhD, BE Mahon, MD, PM Griffin, MD, IT Williams, PhD, Enteric Diseases Epidemiology Br, Div of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases; AJ Hall, DVM, Epidemiology Br, Div of Viral Diseases, National Center for Immunization and Respiratory Diseases, CDC.

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Surveillance for Foodborne Disease Outbreaks --- United States, 2007