This outbreak of E. coli infections reinforces that daycare centres are conducive to the spread of gastrointestinal illness among children. The source of the outbreak is unknown but it is likely that the STEC O103:H2 and STEC O91:H- serotypes were introduced to the daycare centre by separate index cases and then transmitted between the children, staff and family members. This is supported by symptom onset dates among confirmed cases being staggered over a two-month period. In addition, there were no indications from the trawling questionnaires or cohort investigation of any common sources of infection.
At the time of the described outbreak alert, it appeared that multiple children attending the daycare center could have STEC infections linked to STEC O103. Local health officials deemed it appropriate to close the daycare temporarily and screen all children. This control measure is not recommended for all daycare outbreaks but can be considered in severe circumstances. Ultimately no cases developed serious symptoms and the outbreak was interrupted, which may have been attributable to the implementation of this control measure. However, several children attending the daycare that were positive for STEC were asymptomatic. As screening can reveal unexpected results, the consequences of implementing screening must be thoroughly considered, particularly in terms of how children that are asymptomatic but positive for pathogenic E. coli will be managed.
The role of asymptomatic children in transmission of the infections in this outbreak is unknown. As several confirmed STEC cases were only identified through screening, it was not possible to determine the point at which they were infected, the duration of bacterial shedding prior to screening or whether the outbreak was propagated by asymptomatic shedders, particularly as the background level of diarrhea among children with negative STEC test results was high. Exclusion policies are based on the presumption that children can easily transmit STEC infection to other children when asymptomatic, and that ensuring children are no longer shedding the bacteria before they return to daycare will prevent spread of disease. However, the duration of the resulting exclusion period can be extensive, and keeping asymptomatic children home while waiting for multiple consecutive negative test results can have a substantial socioeconomic impact. Parents will often be required to miss work and children will be isolated from their peers for the duration of shedding, which has been reported to last up to 140 days [10]. In this case, nine children were excluded from daycare for a total of 459 days, with a median exclusion period of 53 days per child. This was longer than a previous Norwegian childcare outbreak of E. coli O145, where the median duration was 20 days (range 0-71 days). Although exclusion policies vary considerably from country to country, Norway has perhaps the most restrictive approach in the world with the recommendation to have up to five consecutive negative results before returning to daycare. These were developed in response to two recent serious outbreaks of STEC affecting children that lead to several HUS cases and at least one death [17].
Other Nordic countries [18], parts of Canada, the United States, Australia and the United Kingdom generally require two or three negative tests before returning to daycare. The difference between requiring two negative tests and five negative tests may not ultimately result in a substantially longer exclusion period, unless two negative tests are followed by a positive test. In this outbreak, the children that shed the bacteria for long periods of time were consistently positive until the first negative test, after which there was little variation between negative and positive test results. The difference between two and five tests only increased the exclusion period by 3 – 4 days, which is negligible after more than 100 days of positive tests. In this outbreak, specimens were initially tested by PCR and confirmed by culture. However, the increasing use of culture independent diagnostics may complicate the application of control measures, which in Norway are currently based on knowing the virulence profile and serogroup. As this information is not always available with PCR alone, more rigorous control measures than are necessary may be implemented pending further information and the time before the sufficient number of consecutive tests are negative may be extended. The need molecular characteristics information to inform appropriate control measures in outbreak situations should not be underestimated.
More research is needed regarding the risk of long-term asymptomatic shedders transmitting illness in different settings, including childcare facilities [19]. As further testing of the EPEC specimens was not conducted, it is unknown whether the EPEC infections found through screening are epidemiologically linked to the STEC cases. EPEC is also a cause of diarrhea, especially among children, but is also found in asymptomatic children and generally does not constitute a large problem in industrialized countries. EPEC has been notifiable in Norway since 1994, with more than 1200 cases reported between 1994 and 2011. Serotype O103:H2 has not been found in typical EPEC in Norway but has previously been found in five notified cases of atypical EPEC (aEPEC). For some serotypes, such as O26:H11, it has been shown that aEPEC and STEC are phylogenetically and genetically closely related and that they live in a dynamic relationship in which stx genes might be lost or gained [20],[21]. The substantial number of children positive for EPEC, and the children who were initially positive for STEC and subsequently positive for EPEC, may indicate that there was a relationship between STEC and EPEC in this outbreak, although this cannot be confirmed as analysis of the EPEC specimens was not conducted. However, the observed prevalence of aEPEC found through screening in a Norwegian daycare following an outbreak of STEC O145 in 2009 [7], and studies among healthy children and children with mild gastrointestinal symptoms indicate that aEPEC infection is frequently asymptomatic and that the endemic level of aEPEC is high in Norway [22],[23]. It is therefore not known whether the EPEC results identified through screening are attributable to isolates that have lost their stx1-encoding bacteriophage, a concomitant outbreak of EPEC, sporadic cases of different types of EPEC, or merely an expression of normal fecal flora.
The attack rate for confirmed and probable cases of STEC was highest among the groups for younger children, many of whom were still in diapers, while the outdoor group had no confirmed cases of STEC and the fewest children with either EPEC or reported diarrhea. The children in this group would have limited contact with other children attending the daycare, and therefore less likely to be infected through person-to-person contact. This reinforces that children who have gastrointestinal symptoms, particularly those still wearing diapers, may pose a greater risk to other children and staff members, than asymptomatic children who are carriers. Regardless of the etiology of gastrointestinal symptoms, children should be kept home for the recommended 48 hours following the cease of symptoms. Parents’ reports of duration of symptoms and number of days home from daycare in this outbreak suggest that children are not always kept home for a full 48 hours after symptoms subside, as is required by the daycare’s policy which is based on recommendations from the NIPH. Children that normally have loose stool are not required to stay home from daycare but it is the parents’ responsibility to assess whether a child has an unusual change in stool frequency or consistency. As the results of this investigation indicate that some parents find it challenging to determine when it is appropriate to keep children home from daycare, procedures at daycare centres should be reinforced regularly and parents should be encouraged to have a low threshold for keeping children at home. Following the outbreak, the daycare centre did clarify their protocols among staff and parents, but vigilance should be maintained during non-outbreak periods.
There are some limitations to this investigation. As a sensitive definition for possible cases of STEC infection was used, it is conceivable that cases of gastroenteritis of differing etiology, such as norovirus, occurred during the same period. This cannot be confirmed as except for the two initial cases of STEC, none of the children were tested for pathogens other than E. coli.
