Timeliness of Enteric Disease Surveillance in 6 US States

We reviewed timeline information for a sample of Salmonella spp., Shigella spp., Campylobacter spp., and Escherichia coli O157:H7 cases and all confirmed foodborne outbreaks reported in 6 states during 2002. Increasing the timeliness of case follow-up, molecular subtyping, and linkage of results are critical to reducing delays in the investigation of foodborne outbreaks.

We reviewed timeline information for a sample of Salmonella spp., Shigella spp., Campylobacter spp., and Escherichia coli O157:H7 cases and all confi rmed foodborne outbreaks reported in 6 states during 2002. Increasing the timeliness of case follow-up, molecular subtyping, and linkage of results are critical to reducing delays in the investigation of foodborne outbreaks.
T imely reporting of foodborne diseases is necessary to identify persons at risk for exposure and to prevent additional cases in outbreak settings (1). The present study assesses time intervals for surveillance of foodborne diseases and investigation of outbreaks. Results establish baseline measures to evaluate foodborne disease surveillance systems and identify strategies for improvement (2)(3)(4).

The Study
Data on case investigation timelines in 2002 were collected from records at state and local health departments and public health laboratories in each of 6 states for <100 Salmonella spp. isolates, <50 Shigella spp., Escherichia coli O157:H7, and Campylobacter spp. isolates, and for all foodborne outbreaks. Participating states included 1 with a large population (>6 million), 3 with a medium-sized population, and 2 with a small (<2 million) population from 5 different geographic regions. Two states received supplemental funding through FoodNet. Rules mandated reporting of diagnosed cases from physicians or clinical laboratories to local health departments (2 states), to the state health department (2 states), or to both (2 states). Cases were selected by systematically choosing every nth record on the basis of the number of cases reported and the number sampled. The median intervals from onset of symptoms to surveillance milestone events for individual cases were as follows ( Table 1): collection of stool samples, 2-4 days; initial stool culture results, 5-8 days; case report to health department, 7-9 days; isolate submission to public health laboratory, 8-10 days. For case-patients who were interviewed, the median interval from onset of symptoms to interview was 12 days for E. coli O157:H7 cases, 14 days for Salmonella spp. and Shigella spp. cases, and 18 days for Campylobacter spp. cases. For isolates that were subtyped by PFGE, the median intervals from onset of symptoms to subtyping were 15 days for E. coli O157:H7, 18 days for Salmonella spp., and 21 days for Shigella spp.
A higher percentage of isolates were submitted to the public health laboratory in states where submission was required (98% for Salmonella spp. isolates, 100% for E. coli O157:H7) compared to states where submission was not required (75% for Salmonella spp. isolates, 80%  (5). Of 29 outbreaks that were not confi rmed, norovirus was the suspected cause in 17 (59%) outbreaks, and toxigenic bacteria were suspected in 7 (24%) outbreaks. Median intervals from onset of symptoms to outbreak complaint or recognition were 1 day for bacterial toxins, 3 days for norovirus, 8 days for E. coli O157:H7 and Campylobacter spp., and 16 days for Salmonella spp. (Table 2). Overall, 83 (74%) outbreaks were detected by a consumer complaint, 12 (11%) were detected by a healthcare provider, 11 (10%) were detected by PFGE cluster evaluation, and 6 (5%) were identifi ed through an interview with an individual case-patient. Intervals from onset of symptoms to consumer complaint (median 3 days, range 0-21 days) or to report by healthcare provider (median 3 days, range 0-11 days) were similar. Outbreaks identifi ed by case interview (median 11 days, range 6-16 days) or PFGE cluster evaluation (median 23 days, range 7-83 days) followed case surveillance timelines described above. The median interval from detection of the outbreak to the initiation of the fi rst outbreak investigation step was 0 days (range 0-41 days) for all outbreaks.

Conclusions
The multiple steps between onset of a foodborne illness and its investigation by a public health agency result in delayed recognition of outbreaks caused by reportable enteric diseases. One important way to speed the detection of outbreaks is to encourage clinicians to immediately notify health departments when they suspect a patient is part of an outbreak. Since many outbreaks caused by E. coli O157:H7 and Salmonella spp. last multiple days, physician reporting concurrent with stool collection may provide opportunities for a public health intervention that could prevent outbreak-associated cases.
The speed with which clinical laboratories receive, process specimens, and report results varies by setting, agent, and location. The lack of detail available about these steps is an important limitation of this study. However, health departments generally receive reports from clinicians a median of 2 days after the culture result, and isolates are submitted to public health laboratories within 2-3 days of the initial culture result. These data suggest that improving physician and laboratory reporting practices and logistics could shorten the reporting timeline by 1 or 2 days for most cases.
Timeline elements directly under control of public health agencies include the interval from case report to interview and from submission of the isolate to subtyping by PFGE. Our results demonstrate more variability for these intervals than for earlier steps in enteric disease surveillance. In particular, E. coli O157:H7 infections appear to receive a higher priority than Salmonella spp., Shigella spp., or Campylobacter spp. infections. Half of E. coli O157:H7 cases but less than one fourth of Salmonella spp. cases were contacted by a local health department on the same day the report was received. In addition, outbreaks caused by E. coli O157:H7 were detected a median of 8 days sooner than outbreaks caused by Salmonella spp. Given the risk for hemolytic uremic syndrome after E. coli O157:H7 infections and the potential for person-to person transmission, such attention is warranted. Even so, the intervals from onset of symptoms to PFGE subtyping documented in the nationwide outbreak of E. coli O157:H7 infections associated with spinach demonstrated that little has changed across the public health system from 2002 to 2006 (6). This and other widespread outbreaks of Salmonella spp. infection reinforce the need to increase the timeliness of case follow-up, molecular subtyping, and the linkage of results between them that can reduce delays in the investigation of foodborne outbreaks (7).