Improving waterborne disease outbreak investigations

This article is a summary of discussions held and recommendations made at a workshop for the investigation of waterborne disease outbreaks in Chapel Hill, North Carolina, December 7-8, 1998. Suspected waterborne outbreaks in the United States are primarily investigated by state and local public health officials who may infrequently conduct enteric disease outbreak investigations. Thus, it is important that officials have a formal plan to ensure that epidemiological studies are methodologically sound and that effective collaboration occurs among the epidemiologists, scientists, and engineers who will conduct the investigations. Laboratory support to analyze water samples and clinical specimens should be arranged well in advance of when services may be needed. Enhanced surveillance activities can help officials recognize additional outbreaks and initiate investigations in a timely manner. Epidemiologists should pay more attention early in the investigation to study design, questionnaire development, and sources of bias, especially recall bias, that may affect the interpretation of observed associations. Improved investigations can increase our knowledge about important etiological agents, water systems deficiencies, and sources of water contamination so that waterborne outbreaks can be more effectively prevented.

An outbreak of Escherichia coli O157:H7 infections and haemolytic uraemic syndrome associated with consumption of unpasteurized apple cider

During October 1996, an outbreak of Escherichia coli O157:H7 infections among Connecticut residents occurred. An epidemiologic investigation included enhanced surveillance and a case-control study. Clinical isolates of Escherichia coli O157:H7 were typed by pulsed-field gel electrophoresis (PFGE). Implicated cider samples were analysed by culture and polymerase chain reaction (PCR). Consumption of implicated cider was associated with illness; (matched odds ratio = undefined, 95 % confidence interval = 3.5-infinity). Ultimately, a total of 14 outbreak-associated patients were identified. All isolates analysed by PFGE yielded the outbreak-associated subtype. Escherichia coli O157:H7 was not cultured from three cider samples; PCR analysis detected DNA fragments consistent with Escherichia coli O157:H7 in one. This outbreak was associated with drinking one brand of unpasteurized apple cider. PFGE subtyping supported the epidemiologic association. PCR analysis detected microbial contaminants in the absence of live organisms. Washing and brushing apples did not prevent cider contamination.

A multistate outbreak of Escherichia coli O157:H7 infections associated with consumption of mesclun lettuce

BACKGROUND

An outbreak of Escherichia coli O157:H7 infections in Connecticut and Illinois during May 28 to June 27, 1996, was investigated to determine the source of infections.

METHODS

Independent case-control studies were performed in both states. Pulsed-field gel electrophoresis (PFGE) was performed on E. coli O157:H7 isolates. A case-patient was defined as a Connecticut or northern Illinois resident with diarrhea whose stool culture yielded E. coli O157:H7 of the outbreak-associated PFGE subtype. Controls were town-, age-, and sex-matched to case-patients. We traced implicated lettuce to the farm level and performed environmental investigations to identify unsafe lettuce production practices.

RESULTS

In Connecticut and Illinois, infection was associated with consumption of mesclun lettuce (Connecticut matched odds ratio [MOR], undefined; 95% confidence interval [CI], 3.4 to infinity; and Illinois MOR, undefined; 95% CI, 1.4 to infinity). We traced implicated lettuce to a single grower-processor. Cattle, a known E. coli O157:H7 reservoir, were found near the lettuce fields. Escherichia coli (an indicator of fecal contamination) was cultured from wash water and finished lettuce. A trace-forward investigation identified 3 additional states that received implicated lettuce; E. coli O157:H7 isolates from patients in 1 of these states matched the outbreak-associated PFGE subtype.

CONCLUSIONS

This multistate outbreak of E. coli O157:H7 infections was associated with consumption of mesclun lettuce from a single producer. Molecular subtyping facilitated the epidemiological investigation. This investigation increased the knowledge about current production practices that may contribute to the contamination of lettuce by microbial pathogens. Lettuce production practices should be monitored for microbiological safety.