Multistate outbreak of listeriosis--United States, 1998.

Since early August 1998, 40 illnesses caused by a single strain of Listeria monocytogenes (LM) have been identified in 10 states: Ohio (13 cases); New York; Tennessee, Massachusetts, and West Virginia (three each); Michigan (two); and Connecticut, Oregon, Vermont, and Georgia (one each). Dates of illness onset or LM isolation ranged from August 2 through December 2. All LM isolates from these cases are serotype 4b and share an unusual pattern when subtyped either by pulsed-field gel electrophoresis or by ribotyping methods. Historically, this pattern is rare among LM isolates from humans.