The Study
Health departments in the United States electronically submit reports of foodborne disease outbreaks to the Foodborne Disease Outbreak Surveillance System (FDOSS). FDOSS captures information on etiology; implicated food; number of illnesses, hospitalizations, and deaths; and other features. We queried FDOSS for L. monocytogenes outbreaks (≥2 cases) in the United States from 1998, when pulsed-field gel electrophoresis was first used to investigate listeriosis outbreaks, through 2014. We obtained information on fetal losses; deaths; number of cheese types; pasteurization status of milk used to make the cheese; recall issuance; and isolate subtyping from published reports,[5–11] unpublished data, and food recall announcements. We considered infections in pregnant women or infants ≤28 days of age to be pregnancy-associated. We considered outbreaks multistate if exposure to the implicated food occurred in >1 state.
Of 58 listeriosis outbreaks reported during 1998–2014, a total of 17 (30%) were associated with soft cheese (Figure), and resulted in 180 illnesses, 14 fetal losses, and 17 deaths (Technical Appendix Table). Most patients (146, 88%) were hospitalized. Of 116 patients for whom we had information on ethnicity, 38 (33%) were Hispanic. Of 140 cases with available data, 62 (44%) were pregnancy-associated. Median outbreak size was 8 cases (range 2–34 cases). Ten outbreaks were multistate, and 16 were associated with commercial products, of which 14 involved cheeses produced in the United States. The proportion of listeriosis outbreaks linked to soft cheese made from pasteurized milk (12 outbreaks, 33%) was significantly higher during 2007–2014 than during 1998–2006 (1 outbreak, 5%; p = 0.009). Clinical isolates from soft-cheese outbreaks predominantly fell in lineage I (14 outbreaks, 82%). We found 2 sequence type (ST) and clonal complex (CC) combinations in multiple outbreaks (ST5/CC5, 5 outbreaks; ST6/CC6, 2 outbreaks), whereas other ST and CC combinations appeared in single outbreaks (e.g., ST663 or ST558).
Figure.
Listeriosis outbreaks associated with soft cheeses and other foods, United States, 1998–2014. The Centers for Disease Control and Prevention began pulsed-field gel electrophoresis subtyping of clinical Listeria monocytogenes isolates in 1998 and launched the use of standardized interview questions in 2004; the routine use of whole-genome sequencing was introduced in 2013.
Latin-style cheeses were implicated in 11/17 (65%) outbreaks, accounting for 98 (54%) cases of listeriosis. The remaining outbreaks involved sheep's-milk cheese, Middle Eastern– or Eastern European–style cheeses, Middle Eastern–style cheese, Italian-style cheese, blue-veined cheese, and soft-ripened cheeses (1 outbreak each). Nearly all outbreaks (13/17) resulted in recalls.
FDA inspections of cheese-making facilities associated with outbreaks found sanitation and hygiene deficiencies (e.g., roof leaks over manufacturing equipment, an open sewer vent in a manufacturing room, and food-contact aprons stored in restrooms);[7–9] pest infestations (e.g., cockroaches, flying insects);[8] failure to hold food at proper temperature;[8] and presence[8,11,12] or persistence of L. monocytogenes in environmental niches of processing plants.[9]
Emerging Infectious Diseases. 2018;24(6):1116-1118. © 2018 Centers for Disease Control and Prevention (CDC)
