A multistate outbreak of Listeria monocytogenes infections involving 5 distinct strains was associated with consumption of cantaloupe grown at Jensen Farms' production fields in Granada, Colorado. A total of 147 persons were reported to the CDC. Thirty three persons died, and one pregnant woman miscarried. Among persons for whom information is available, reported illness onset ranged from July 31, 2011 through October 27, 2011. Ages of ill persons ranged from less than 1 year of age to 96 years, with the median age of 78 years old. Most ill persons were over 60 years old or had health conditions that weakened their immune systems. Seven of the illnesses were related to pregnancy(three newborns; four pregnant women). Among the 145 ill persons with available information on whether they were hospitalized, 143(99%) were hospitalized. Among the 144 ill persons with available information on what they ate, 134 (93%) reported consuming cantaloupes in the month before illness onset. Several ill persons remembered the type of cantaloupe they had eaten and said they were Rocky Ford cantaloupes, which are grown in the Rocky Ford region of southeastern Colorado. Source tracing of the cantaloupes indicated that they came from Jensen Farms, and were marketed as being from the Rocky Ford region. These cantaloupes were shipped between July 29 through September 10 to at least 24 states with possible further distribution. Laboratory testing by the Colorado Department of Public Health and Environment identified Listeria monocytogenes bacteria on cantaloupes collected from grocery stores and from ill persons' homes. Laboratory testing by FDA identified L. monocytogenes matching outbreak strains in samples from equipment and cantaloupe at the Jensen Farms' packing facility in Granada, Colorado. The processing equipment and the decision not to chlorinate the water used to wash the cantaloupes were two probable causes of the contamination.

This outbreak had several unusual features. This was the first listeriosis outbreak associated with melon. Five widely differing pulse field gel electrophoresis (PFGE) pattern combinations and two serotypes (1/2a and 1/2b) were associated with the outbreak. This outbreak was unusually large and resulted in the highest number of deaths of any U.S. foodborne outbreak since a listeriosis outbreak in 1998(See Bil Mar Foods Ready-to-eat Meats 1998).