Listeriosis following shigellosis.

In 1987 an outbreak of illness due to Listeria monocytogenes occurred in Philadelphia. In contrast to previously studied outbreaks, no source of infection or vehicle of transmission could be identified, and several listerial strains were found to be involved. A hypothesis that was developed and eventually published suggests that clinical listeriosis may occur when individuals who are asymptomatic for listerial infection but whose gastrointestinal tract has been colonized by Listeria organisms become infected with another pathogen. The case of a farmer who developed a brain stem abscess due to L. monocytogenes following an episode of acute enteritis due to Shigella sonnei lends support to this hypothesis.