CDC collaborated with public health and regulatory officials in several states and the U.S. Food and Drug Administration (FDA) to investigate a multistate outbreak of Listeria monocytogenes infections (listeriosis). Joint investigation efforts indicated that ricotta salata cheese was the likely source.

Public health investigators used DNA “fingerprints” of Listeria obtained through diagnostic testing with pulsed-field gel electrophoresis, or PFGE, to identify cases of illness that were part of this outbreak. They used data from PulseNet, the national subtyping network made up of state and local public health laboratories and federal food regulatory laboratories that performs molecular surveillance of foodborne infections.

A total of 22 persons infected with the outbreak strain of Listeria monocytogenes were reported from 13 states and the District of Columbia. The number of ill people identified in each location was as follows: California (3), Colorado (1), District of Columbia (1), Maryland (3), Massachusetts (1), Minnesota (1), Nebraska (1), New Jersey (3), New Mexico (1), New York (1), Ohio (1), Pennsylvania (2), Virginia (2), and Washington (1).

Among persons for whom information is available, dates that illness was diagnosed ranged from March 28, 2012 to October 6, 2012. Twenty ill persons were hospitalized. Nine of the illnesses were related to a pregnancy; three of these were diagnosed in newborns. The other 13 ill persons ranged in age from 30 years to 87 years, with a median age of 77 years, and 54% were female. Four deaths were reported, one each from Minnesota, New York, Nebraska, and California. In Nebraska and California, public health officials determined that the deaths were related to listeriosis. In Minnesota and New York, public health officials did not report listeriosis as a cause of death because it was not listed as such on the death certificates. One fetal loss also was reported.

Investigation of the Outbreak

Epidemiologic, laboratory, and traceback investigations conducted by officials in local, state, and federal public health, agriculture, and regulatory agencies indicated that Frescolina Marte brand ricotta salata cheese imported from Italy and distributed by Forever Cheese, Inc. was the likely source of this outbreak of listeriosis.

In interviews, ill persons answered questions about foods consumed and other exposures in the month before becoming ill. Twelve (80%) of 15 ill persons interviewed using the Listeria Initiative questionnaire reported consuming a soft cheese. Ten (91%) of 11 ill persons who could provide information about packaging of cheeses reported consuming cheese that had been cut and repackaged at a retail location. Seven cases were linked to consumption of Frescolina Marte brand ricotta salata.

The investigation was complex because ill persons reported consuming many different cheese brands and types from many different retail locations. No one cheese was initially reported by the majority of ill persons, suggesting that cross-contamination of other cheeses through cutting boards or utensils played a role. The investigation focused on identifying intact cheeses shipped to multiple retail locations where, in the cutting process, they could have cross-contaminated other cheeses purchased by ill persons.

FDA isolated the outbreak strain of Listeria from a sample of uncut Frescolina Marte brand ricotta salata cheese, which was imported from Italy and distributed by Forever Cheese, Inc. The outbreak strain was also isolated from other types of soft cheese that had already been cut and repackaged.