Prolonged incubation period of salmonellosis associated with low bacterial doses

In gastroenteritis outbreaks caused by Salmonella-contaminated lunches at elementary, junior high, and nursery schools, outbreaks with long median incubation periods (i.e., 60 to 120 h) were observed frequently between 1990 and 1999 in Japan. We analyzed epidemiological data on 185 outbreaks of Salmonella Enteritidis infection to study the factors underlying the long incubation period. These survey results showed that the median incubation period for Salmonella Enteritidis infection from contaminated school and nursery school lunches was significantly longer than that from other types of cooking facilities. In addition, we analyzed the relationship between the median incubation period and the bacterial dose ingested per person in nine outbreaks of Salmonella Enteritidis infection; the bacterial dose was estimated with reference to the bacterial concentration in the causative foods. A significant negative correlation between the bacterial dose ingested per person and the median incubation period is clearly shown. The time elapsed from the start of the cooking process to the consumption of school and nursery school lunches was significantly shorter than at other cooking facilities, suggesting limited bacterial growth, which in turn is thought to lead to a long incubation period.