After four students in Germany were sickened by E. coli O104:H4 infections, preliminary tests indicated that 22 out of 30 children at the same school were also infected with the outbreak strain but had no symptoms, health officials reported this week.

Asymptomatic E. coli O104:H4 infection was also found in three kitchen workers at the school in Kreis Paderborn, four child care employees at four different day-care centers in the district and three workers at the catering company that supplied school food, the European Centre for Disease Prevention and Control (ECDC) said in its latest risk-assessment report  on the massive outbreak that began in Germany in May.

The children’s infections “resulted most likely from foodborne” transmission rather than from person-to-person transmission, the ECDC said, but such a “significant proportion of asymptomatic carriers” of the pathogen “represent a risk for new foodborne outbreaks,” in particular if those carriers are food handlers.

“Considering the large number of summer festivals and mass gatherings in the EU, with sometimes inadequate food hygiene standards, targeted public health measures for such events could be of value to prevent further spread,” the ECDC suggested, adding ” … information to the public should stress the need for proper hand washing.”

European health authorities have previously noted that the estimated eight-day incubation period for the O104:H4 strain is longer than the typical three to eight days for most Shiga toxin-producing E. coli infections, and that “may indicate a low infectious dose, which may influence the likelihood of person-to-person and foodborne transmissions by infected persons.” 

However, so far person-to-person transmission does not appear to have been a major factor in the spread of the epidemic, the ECDC report stated. While some secondary infections have been confirmed, there isn’t much evidence of a large occurrence of secondary person-to-person exposure. In particular, no cases of O104:H4 illness in day-care centers, schools or nursing homes have been through secondary infection.

“Another important factor in monitoring the future epidemiology is whether or not the strain has established — or could establish — itself within an animal reservoir,” the ECDC said.

Although the ECDC reported “a dramatic decline” in the number of O104 cases in Germany in the last two weeks, it noted that new cases and clusters are continuing to be reported, despite the identification of sprouts as the most likely source of the outbreak that has caused more than 3,800 illnesses and at least 45 deaths.  

Since the European Food Safety Authority implicated contaminated fenugreek seeds imported from Egypt, the European Commission recalled and temporarily banned the import of certain types of seeds used for sprouting. It also has warned consumers not to grow their own sprouts for consumption or to eat sprouts unless they are cooked thoroughly.

While new cases may arise as the result of other contaminated foods or contaminated seeds   still in circulation, the “main reason for concern at this stage” in the outbreak, the ECDC wrote, is evidence from clusters like the one at the Kreis Paderborn school of “a substantial proportion of subclinical infections.”