CDC collaborated with public health officials in several states and the U.S. Food and Drug Administration (FDA) to investigate a multistate outbreak of Listeria monocytogenes infections (listeriosis). Listeria can cause a serious, life-threatening illness.

Public health investigators used the PulseNet system to identify illnesses that were part of the outbreak. PulseNet is the national subtyping network of public health and food regulatory agency laboratories coordinated by CDC. DNA “fingerprinting” is performed on Listeria bacteria isolated from ill people by using techniques called pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS). PulseNet manages a national database of these DNA fingerprints to identify possible outbreaks. WGS gives a more detailed DNA fingerprint than PFGE. This additional detail was important because five rare PFGE fingerprints of Listeria were included in this investigation. The sequencing showed that the Listeria strains with the five rare PFGE fingerprints were closely related genetically.

Thirty people infected with one of the closely related Listeria strainswere reported from 10 states since June 16, 2010. The number of ill people reported from each state was as follows: California (18), Colorado (1), Illinois (2), Massachusetts (2), Michigan (1), New York (2), Ohio (1), Tennessee (1), Virginia (1), and Washington (1).

Dates of Listeria specimen collection ranged from June 16, 2010 to August 24, 2015. The cluster was first identified in August 2015 after investigators saw an increase in one of the five rare PFGE fingerprints reported to PulseNet. WGS found that the four other PFGE fingerprints were closely related genetically to the first PFGE fingerprint. Illnesses associated with those PFGE fingerprints were added to the investigation, including illnesses that occurred over 5 years ago.

Ill people ranged in age from less than 1 year to 92, and the median age was 73. Seventy percent of ill people were female. Twenty-eight (93%) of 30 ill people for whom information was available reported being hospitalized. Six of the illnesses were pregnancy-related, and one illness resulted in a fetal loss. Three deaths were reported from California (2) and Ohio (1).

Investigation of the Outbreak

Epidemiologic and laboratory information indicated that soft cheeses distributed by Karoun Dairies were the likely source of this outbreak.

State and local health departments interviewed ill people about the foods they may have eaten or other exposures in the month before their illness began. Twenty (67%) of 30 people with available information were of Middle Eastern or Eastern European descent or shopped at Middle Eastern or Eastern European-style markets. Of 28 ill people for whom information was available, 21 (75%) consumed soft cheese, and all of those 21 (100%) reported eating Middle Eastern, Eastern European, Mediterranean, or Mexican-style cheeses, including ani, feta (e.g., Bulgarian feta), Middle Eastern-style string cheese, nabulsi, and village cheese. Three of seven ill people who specified the brand of cheese eaten reported brands distributed by Karoun Dairies. An additional ill person was unable to name the brand of cheese eaten, but described the packaging and label of a Karoun product. No other brand of soft cheese was reported more than once. One other ill person reported eating Karoun brand kefir cheese. This person was previously included in the number of people reporting a Karoun brand soft cheese, but has now been excluded since kefir is typically classified as a yogurt product rather than a cheese.

FDA isolated Listeria monocytogenes from two environmental samples collected in September 2015 from the Central Valley Cheese, Inc. manufacturing facility in Turlock, California. Central Valley Cheese, Inc. manufactures cheese for Karoun Dairies. Whole genome sequencing showed that these isolates were closely related genetically to isolates from ill people. In addition, whole genome sequencing showed that five Listeria isolates collected in 2010 from the same facility were also closely related genetically to isolates from ill people.

On September 16, 2015, Karoun Dairies, Inc. voluntarily recalled certain cheeses that the company distributes due to possible contamination with Listeria. The recall includes several brands and types of cheeses that were distributed to retail outlets in the United States. Products were sold under the following brands: Karoun, Arz, Gopi, Queso Del Valle, Central Valley Creamery, and Yanni. Products were vacuum packed, in jars or in pails. Weights vary from 5 ounces to 30 pounds. A full list of cheeses is available on the Advice to Consumers, Restaurants, and Retailers page.

This outbreak investigation is over. However, people could continue to get sick because recalled cheeses may still be in homes, restaurants, or retail locations. Consumers, restaurants, and retailers unaware of the recall could eat, serve, or sell them.