Epidemiological studies of coincidental outbreaks of enterohemorrhagic Escherichia coli O157 : H7 infection and infectious gastroenteritis in Niimi City

A sharp rise in the number of patients with infectious gastroenteritis was observed in the 25 th week of year 1996 in the Takahashi-Ashin district by researchers with the Infectious Disease Surveillance Program for tuberculos is and other infectious diseases in the Okayama Prefecture. This sharp rise occurred coincidentally with an outbreak of enterohemorrhagic Escherichia coil O157:H7 (EHEC O157) infection in Niimi City of the Takahashi-Ashin dis trict. However, this phenomenon of coincidental outbreaks was not observed during the outbreak of EHEC O157 infection in Oku Cho. By reviewing outpati ents' charts in a sentinel hospital in Niimi City for the Infectious Diseas e Surveillance Program, it was noted that patients with acute gastrointesti nal infection visiting the hospital during the increased incidence of infec tious gastroenteritis may have been included as misclassified cases of EHEC O157 infection. On the other hand, the exponential probability plotting of symptomatic patients with EHEC O157 infection in Niimi City revealed a bre aking point which suggested a dual exposure to contaminated food or an over lap with other acute gastrointestinal infections. The latter possibility wa s discounted, because stool culture-positive patients with EHEC O157 infect ion also exhibited a similar breaking point, and furthermore, the coinciden tal increase in infectious gastroenteritis in the same area was attributabl e to the EHEC O157 infection. The present study demonstrates the associatio n between the sharp rise in gastroenteritis and the outbreak of EHEC O157 i n the Takahashi-Ashin district. A careful analysis of the cases of infectio us gastroenteritis by the Infectious Disease Surveillance Program would hav e predicted the outbreak of EHEC O157.