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Increase in US Listeriosis Outbreaks Linked to Cheeses Made Under Unsanitary Conditions

Since 2006, the number of reported US listeriosis outbreaks associated with cheese made under unsanitary conditions has increased.

A new dispatch paper from the US Centers for Disease Control and Prevention (CDC) reports an increase in the number of reported listeriosis outbreaks associated with soft cheeses in the United States since 2006, with pregnant Hispanic women and their newborns most affected.

The CDC stresses that this information underscores the need to improve sanitation measures and boost adherence to pasteurization protocols in order to avoid contamination and thus, decrease the risk of future outbreaks.

Historically, listeriosis outbreaks have been associated with many refrigerated, ready-to-eat foods. For example, outbreaks have been linked back to hot dogs, deli meats, milk and other dairy products, and, of course, soft cheeses.

The CDC reports that the earliest listeriosis outbreak reported in the United States was traced back to Latin-style cheese (queso fresco and cotija) in 1985, where a total of 142 individuals fell ill and 28 died from their infections; twenty fetal losses were also reported.

“The issue of listeriosis outbreaks linked to Latin-style cheese, which is also known as Hispanic-style cheese, is not new,” Kelly Jackson, MPH, a surveillance coordinator at the CDC, said in a recent CDC Emerging Infectious Diseases podcast. “Since the [1985 outbreak], many improvements in food safety have been made. However, we continued to find Listeria outbreaks associated with Hispanic-style soft cheese during our study period…including soft cheese made in the home and commercial settings.”

For the study, investigators looked at data submitted by health departments to the Foodborne Disease Outbreak Surveillance System (FDOSS). Of 58 listeriosis outbreaks reported from 1998 to 2014, 17 (30%) were identified to have been linked with soft cheese. In total, the 17 outbreaks resulted in 180 illnesses, 14 fetal losses, and 17 deaths, with most individuals requiring hospitalization (146, 88%).

Eleven of the 17 outbreaks were traced back to Latin-style cheese specifically, accounting for 98 (54%) of the total reported cases. The CDC reports that 38 (33%) of those who fell ill in these outbreaks were of Hispanic ethnicity and 62 (44%) were pregnancy-associated.

“The US Hispanic population increased from 11% to 17% during this period, and that may have created an increase in consumer demand for Hispanic-style soft cheeses,” Dr. Jackson explained. The CDC stresses that the outbreaks disproportionately affected Hispanic pregnant women and their neonates, a group that is estimated to be 24 times more likely to get listeriosis compared with the general US population.

Soft cheese made from unpasteurized milk is 50 to 160 times more likely to be contaminated with the bacteria than cheese made from pasteurized milk. However, the CDC reports that the number of listeriosis outbreaks caused by the consumption of soft cheese made from pasteurized milk has increased in that past few years. Reasons for this increase include a 2.5-fold increase in per capita consumption of cheese, an increase in consumer demand for certain types of cheeses, and an increase in the number of small producers.

Upon inspecting the cheese-making facilities associated with the outbreaks, the US Food and Drug Administration reported finding sanitation and hygiene deficiencies, pest infestations, and a failure to hold the cheese products at an appropriate temperature.

“We concluded that eating contaminated soft cheese made in unsanitary conditions continues to be a common cause of listeriosis outbreaks in the United States,” Dr. Jackson said. “We recommend that all cheese manufacturers implement robust sanitation and Listeria monitoring programs to prevent illnesses and outbreaks.”