Volume 25-03
February 1, 1999

[Table of Contents]

INCIDENCE OF FOODBORNE ILLNESSES - FOODNET, 1997

Each year, millions of persons become ill from foodborne diseases, though many cases are not reported. The Foodborne Diseases Active Surveillance Network (FoodNet), the primary foodborne diseases component of the United States Centers for Disease Control and Prevention (CDC) Emerging Infections Program⁽¹⁾, was developed to better characterize, understand, and respond to foodborne illnesses in the United States. This report describes FoodNet surveillance data from 1997, the second year of surveillance, and compares findings with data from 1996. The findings demonstrate regional and seasonal differences in the reported incidence of certain bacterial and parasitic diseases, and that substantial changes occurred in the incidence of illnesses caused by some pathogens (e.g. Vibrio and Escherichia coli O157:H7) but the overall incidence of illness caused by the seven diseases under surveillance in both years changed little.

Active bacterial surveillance for laboratory-confirmed cases of Campylobacter, E. coli O157:H7, Listeria, Salmonella, Shigella, Vibrio, and Yersinia infections was initiated on 1 January 1996 in Minnesota, Oregon, and two counties in California, three in Connecticut, and eight in Georgia (expanding to 20 counties in 1997). In 1997, surveillance for laboratory-confirmed cases of Cryptosporidium and Cyclospora infections was added statewide in Minnesota, Connecticut, and eight counties (including the two counties with bacterial surveillance) in California. To identify cases, surveillance personnel contacted each clinical laboratory in their catchment areas either weekly or monthly, depending on the size of the clinical laboratory. Annual incidence was calculated using the number of laboratory-confirmed cases ascertained in the catchment area as the numerator and 1997 postcensus estimates in the same areas as the denominator⁽²⁾. Monthly incidence was calculated based on date of specimen collection.

1997 surveillance data

In 1997, 8,576 laboratory-confirmed cases were identified: 3,974 of campylobacteriosis, 2,205 of salmonellosis, 1,273 of shigellosis, 468 of cryptosporidiosis, 340 of E. coli O157:H7 infections, 139 of yersiniosis, 77 of listeriosis, 51 of Vibrio infections, and 49 of cyclosporiasis. Seasonal variation in isolation rates was seen for several pathogens; 52% of E. coli O157:H7, 35% of Campylobacter, and 32% of Salmonella were isolated in summer months (June to August) (Figure 1). Organisms were isolated from normally sterile sites, including blood and cerebrospinal fluid, in 99% of reported Listeria cases, 7% of Salmonella cases, 3% of Yersinia cases, and 1% of Shigella and Campylobacter cases. Overall, 1,270 (15%) of 8,576 patients with laboratory-confirmed infections were hospitalized; the proportion of persons with cases hospitalized was highest for listeriosis (88%), E. coli O157:H7 infections (29%), and salmonellosis (21%). Thirty-six patients with laboratory-confirmed infections died: 15 with Listeria, 13 with Salmonella, four with E. coli O157:H7, two with Cryptosporidium, one with Campylobacter, and one with Shigella.

Figure 1 Monthly incidence* of selected pathogens - FoodNet**, 1996-1997

* Per 100,000 population.
** Laboratory-confirmed cases of Campylobacter, Escherichia coli O157:H7, and Salmonella infections were identified in Minnesota, Oregon, and selected counties in California (two), Connecticut (three), and Georgia (eight in 1996 and 20 in 1997).

All-site incidence was highest for campylobacteriosis (24.7 per 100,000 population), salmonellosis (13.7), and shigellosis (7.8). The incidence of campylobacteriosis varied from 13.7 in Georgia to 49.3 in California. Although overall salmonellosis incidence was similar among the sites, the incidence of infections with Salmonella serotype Enteritidis varied, from 0.6 in Georgia to 5.8 in Connecticut. Shigellosis incidence varied from 2.9 in Minnesota to 15.9 in Georgia. Incidence differed by site for E. coli O157:H7 infections and yersiniosis; E. coli O157:H7 infections varied from 0.2 in Georgia to 4.2 in Minnesota, and yersiniosis varied from 0.5 in Oregon to 1.2 in Georgia.

Annual incidence also varied by age; for example, the incidence among children aged < 1 year was 56 per 100,000 for campylobacteriosis (range: 18 in Georgia to 159 in California) and 111 per 100,000 for salmonellosis (range: 66 in Oregon to 174 in California).

Comparison with 1996 surveillance data

Overall, incidence of illness caused by the pathogens under surveillance changed little from 1996 to 1997 (Table 1). The largest percentage change occurred in cases of illness caused by Vibrio (from 0.1 in 1996 to 0.3 in 1997). E. coli O157:H7 showed the next largest percentage change (from 2.7 to 2.1, a decrease of 27%). From 1996 to 1997, Minnesota and Oregon reported an overall increase in the incidence of illnesses caused by the pathogens under surveillance; California, Connecticut, and Georgia reported decreases.

Table 1 Incidence* of selected pathogens, by year - FoodNet^**

References

1. CDC. The Foodborne Diseases Active Surveillance Network, 1996. MMWR 1007;46:258-61.

2. Bureau of the Census, Economics and Statistics Administration, US Department of Commerce Population estimates. URL: <http://www.census.gov/population/www/estimates/popest.html>. Date of access: August 1998.

Source: Morbidity and Mortality Weekly Report, Vol 47, No 37, 1998.

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