Epidemiology SLP 1

Descriptive Epidemiology and Hypothesis Generation

INTRODUCTION TO EPIDEMIOLOGY: THE EPIDEMIOLOGIC APPROACH

The incubation period for E. coli O157:H7 ranges from 3-8 days with a median of 3-4 days. The infection often causes severe bloody diarrhea and abdominal cramps, but can also cause a non-bloody diarrhea or result in no symptoms. In some persons, particularly children under 5 years of age and the elderly, the infection can cause a complication called hemolytic uremic syndrome, in which the red blood cells are destroyed and the kidneys fail. About 2-7% of infections lead to this complication.

For this outbreak investigation, a case was defined as diarrhea (at least 3 loose bowel movements a day) and/or abdominal cramps in a resident of the state with onset of symptoms between June 15 and July 15 and a stool culture yielding E.coli O157:H7 with the outbreak strain PFGE pattern.

Of the initial 38 persons who met the case definition, 26 (68%) were female with a median age of 31 years. (Table 1)

Table 1. Age group and gender distribution for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, June 15 - July 15, 1997. (N=38)

Age group (years)	Gender	TOTAL
	Male	Female
0-9	2 (17%)*	2 (8%)	4 (11%)
10-19	2 (17%)	3 (12%)	5 (13%)
20-39	3 (25%)	9 (35%)	12 (32%)
40-59	2 (17%)	8 (31%)	10 (26%)
60+	3 (25%)	4 (15%)	7 (18%)
TOTAL	12 (101%)	26 (101%)	38 (100%)

*percentages refer to column totals.

The 38 cases of E. coli O157:H7 infection meeting the investigation case definition were reported from 10 counties. Onset of illness occurred from mid-June to mid-July, peaking on June 22.

From July 16 - 19, hypothesis-generating interviews were undertaken with seven patients. These patients lived in four different counties and ranged in age from 5-69 years. Three of the patients were female.

Hypothesis-generating interviews revealed that most cases had consumed lettuce and alfalfa sprouts in the week before they became ill. No single restaurant or social event was identified in common.

APPENDIX 1. Distribution of E. coli O 157:H7 cases reported to FoodNetSites* by age group and gender, United States, 1997. (N=340)

Age group (years)	Gender	TOTAL
	Male	Female
0-<1	5 (3%)	5 (3%)	10 (3%)
1-9	77 (48%)	77 (43%)	154 (45%)
10-19	36 (22%)	18 (10%)	54 (16%)
20-29	10 (6%)	20 (11%)	30 (9%)
30-39	6 (4%)	12 (7%)	18 (5%)
40-49	7 (4%)	5 (3%)	12 (4%)
50-59	7 (4%)	17 (10%)	24 (7%)
60+	14 (9%)	24 (13%)	38 (11%)
TOTAL	162 (100%)	178 (100%)	340 (100%)

*Foodborne Diseases Active Surveillance Network (FoodNet) is a collaborative project between CDC, the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), and selected state and local health departments. In 1997, FoodNet conducted population-based active surveillance for confirmed cases of Campylobacter, Escherichia coli O157, Listeria, Salmonella, Shigella, Vibrio, and Yersinia infections in Minnesota, Oregon, and selected counties in California, Connecticut, and Georgia (total population: 16.1 million).