METHODS: Stool specimens from ill children with outbreak cases of STEC were submitted to the Kansas Health and Environmental Laboratories, where isolation, biochemical identification, toxin testing, serotyping, and pulsed-field gel electrophoresis (PFGE) were performed. A confirmed outbreak case was defined as isolation of STEC O157 with the outbreak PFGE pattern from a clinical specimen.
RESULTS: The daycare attendee involved in the primary outbreak, who had a mild case of illness, submitted a specimen collected after illness reportedly subsided; antibiotics were prescribed and initiated but were discontinued. This specimen initially tested Shiga toxin negative by rapid assay. Further testing was pursued because of the presence of suspect colonies on STEC O157-specific media. Four days following the initial negative toxin test, the culture was confirmed as STEC positive and identified as the outbreak strain of O157. After collection but prior to final laboratory results, exposure occurred in the daycare resulting in secondary transmission of STEC. Although the first child’s illness was relatively mild, the child with the secondary case developed hemolytic uremic syndrome and required hospitalization.
CONCLUSIONS: This child with mild illness caused by STEC tested Shiga toxin negative, but later was confirmed by culture and was likely infectious while attending daycare. Secondary transmission in the daycare resulted in a case of severe illness. Kansas regulation requires children with STEC have two negative stool cultures prior to returning to daycare, but negative results from toxin assays or multiplex PCR panels from clinical laboratories where culture is not always performed are sometimes utilized to allow children to return to daycare. However, various factors including intermittent shedding of bacteria in stool, antibiotic treatment, improper shipment of specimens, and sensitivity of assays may result in false negative results; non-culture based methods may be insufficient for confirmatory negative results. To prevent transmission in daycare settings, utilizing culture for convalescent testing may be the most accurate determination of infectiousness.