1. IntroductionEscherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Most people normally carry harmless strains of E coli in their intestine. Both the harmless strains and those that cause diarrhea are acquired primarily through ingestion of contaminated food or water . Person-to-person and animal-to-human transmission is through the oral-fecal route. However, several strains of E coli have usually caused diarrhea/gastroenteritis in human which settles within several days without specific treatment .The varieties of E coli can be grouped into six pathotypes according to their virulence determinants whose specific naturemakes each pathotype different both clinical presentation and distinctive epidemiologic characteristics: enteroaggregative (EAEC), enterohemorrhagic (EHEC), enteroinvasive (EIEC), enteropathogenic (EPEC), enterotoxigenic (ETEC), and diffuse adherent (DAEC) .EPEC is one of the oldest recognized diarrheagenic E coli. EPEC cause either a watery or bloody diarrhea . EPEC has been well known to be highly related to infant diarrhea in developing countries through the pioneering work of Bray  who established the importance of EPEC as a cause of outbreaks of infantile gastroenteritis in the UK in the 1940s. These continued until the early 1970s, but since then outbreaks caused by ‘classical’ EPEC strains have become very rare . Infections caused by EPEC are difficult to differentiate from those with other causes; symptoms include watery diarrhea, sometimes accompanied by low-grade fever and vomiting. However, EPEC infection may be severe, vomiting may make oral rehydration difficult and life-threatening dehydration may ensue. Furthermore, disease caused by EPEC may be protracted, resulting in weight loss, malnutrition, and death . Its characteristics are relatively well defined. However, there are some controversies about the incubation period both it can be as short as 6 hours or can be as long as 6 to 10 days [7,8]. As a result, it is known that EPEC has just ‘variable’ incubation time .In South Korea, all outbreaks by water-borne and food-borne disease with two or more persons should be reported to a local public health center for epidemic investigation. When there is an outbreak of which source infectious agent is thought as EPEC, Korea Centers for Disease Control and Prevention (KCDC) epidemic intelligence service (EIS) officers estimate the causal relationship between exposure and onset of outbreak in consideration of two factors, laboratory result and epidemic nature, especially during the incubation period. Following the WHO guideline, KCDC has suggested the incubation period of EPEC as 1 to 6 days [10,11]. Nevertheless, it has caused some difficulties in presuming first exposure time during epidemic investigations when EPEC was confirmed as causative organism by laboratory result, but incubation period of the epidemic was very short, e.g., less than 6 hours.This study is dedicated to clarifying whether the incubation period of EPEC is truly long over 1 day or whether it is shorter than several hours by analyzing recent EPEC outbreaks of South Korea.
2. MethodsThe incubation period, which is the amount of time between infection with a virus or bacteria and the start of symptoms, can vary from one case to another according to the route by which the person was exposed, the dose of bacteria received, and other factors, including immune status. Estimates of the incubation period are further complicated by the fact that some patients have had opportunities for multiple exposures to the pathogens. The particular exposure that caused disease may prove impossible to determine .For these reasons, the most reliable estimates of the incubation period are based on a study of cases having a single documented exposure to a known case. We reviewed all water and food-borne outbreaks data reported to KCDC from 2009 to 2010 and determined whether these outbreaks were truly defined vehicles of transmission and single exposure. With this process, we can presume the incubation period of EPEC among outbreaks in South Korea during 2009–2010.
3. ResultsFrom January 1, 2009, to December 31, 2010, a total of 497 water and food-borne outbreaks were investigated and reported to KCDC and 66 (13.3%) were defined as E coli-origin outbreaks. The distribution of subtype is shown in Table 1. EPEC was the most common subtype of E coli, being confirmed as a causative organism in 26 outbreaks, followed by 20 of ETEC, seven of EHEC, five of EAEC, and eight of mutual or other subtypes.EPEC outbreaks had, as a whole, 11,302 population at risk and 1791 cases were defined as EPEC related cases. Its overall attack rate was 15.9% (range 0.9–100.0). This wide range of attack rate mainly comes from the variety of outbreaks; from an outbreak of small group less than five, but all are defined cases to that of group food service in a school over 1,000 students but had small cases.Among these 26 outbreaks, to presume a more precise incubation period of EPEC, we selected 13 outbreaks that had an obvious identified single-point
4. DiscussionIn 2010, EPEC was one of the three most common causative organisms of water- and food-borne outbreaks in Korea, next to norovirus and salmonella. When an outbreak has occurred, sophisticated epidemiologic investigation is important to determine the source of infection, the exact pathogen, and how it is transmitted through the persons. Prompt and exact response upon investigation is crucial to prevent spread of disease to local community. In this process, to identify correct pathogen is a key point to layout a whole outbreak. However, laboratory results are not the only concern when determining the cause; epidemiologic characteristics should always be considered together.Appropriate standards or guidelines for epidemiologic features of certain pathogens are important when the field outbreak investigation begins. Incubation period is a core-determining factor in these guidelines for inferring the causative organism of outbreak with epidemic curve. Nevertheless, the incubation period of EPEC is not well established in guidelines, including that of KCDC. The WHO guideline states that the EPEC incubation period is 1–6 days (as short as 12–36 hours) . However, it is just ‘variable’ in the guideline of CDC . Moreover, in the past reports, Nakamura and colleagues said it can be as long as two to 20 days, though it can be chiefly 1–5 days . However, in Korea, when there is an EPEC outbreak, it is often as short as 6 hours or less.Our results suggest that MIP of EPEC is not just ‘variable’. ‘Outbreak with clear single exposure history’
5. ConclusionThe clinical and other epidemiologic characteristics of EPEC are well known. However, its incubation period is not well-established in guidelines for epidemiologic investigation. We estimated roughly the incubation period of EPEC with limited data in 2 years for 2009 and 2010. Nevertheless, this serial analysis suggests that EPEC has a relatively shorter incubation period, i.e., as much as about 12 hours. When this period is 1 day or more, then EIS officer should consider the chance of repeated or continuous exposure and focus his or her investigation on determining whether the cases truly have a single-point exposure. It has not yet been discovered whether this difference comes from the type of EPEC (typical vs. atypical), so further investigation through laboratory support should be performed in the near future.