Investigation Details

January 9, 2019

CDC, public health and regulatory officials in several states, Canada, and the FDA investigated a multistate outbreak of Shiga toxin-producing Escherichia coli O157:H7 (E. coli O157:H7) infections.

Public health investigators used the PulseNet system to identify illnesses that might be part of this outbreak. PulseNet is the national subtyping network of public health and food regulatory agency laboratories coordinated by CDC. DNA fingerprinting was performed on E. coli bacteria isolated from ill people by using techniques called pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS). CDC PulseNet manages a national database of these DNA fingerprints to identify possible outbreaks. WGS gives a more detailed DNA fingerprint than PFGE. WGS performed on E. coli bacteria from ill people in this outbreak showed that the strains were closely related genetically. This means that the ill people were more likely to share a common source of infection.

As of January 9, 2019, 62 people infected with the outbreak strain of E. coli O157:H7 were reported from 16 states and the District of Columbia. A list of the states and the number of cases in each can be found on the Map of Reported Cases page.

Illnesses started on dates ranging from October 7, 2018, to December 4, 2018. Ill people ranged in age from 1 to 84 years, with a median age of 25. Sixty-six percent of ill people were female. Of 54 people with information available, 25 (46%) were hospitalized, including two people who developed hemolytic uremic syndrome, a type of kidney failure. No deaths were reported.

WGS analysis did not identify predicted antibiotic resistance in 51 isolates from 53 ill people. Two isolates contained genes for resistance to ampicillin. Antibiotic resistance testing by CDC’s National Antimicrobial Resistance Monitoring System laboratory is currently underway. This finding does not affect treatment guidance since antibiotics are not recommended for patients with E. coli O157 infections.

Investigation of the Outbreak

Epidemiologic, laboratory, and traceback evidence indicated that romaine lettuce from the Central Coastal growing regions of northern and central California was the likely source of this outbreak.

In interviews, ill people answered questions about the foods they ate and other exposures in the week before they became ill. Thirty (83%) of 36 people interviewed reported eating romaine lettuce. This percentage was significantly higher than results from a survey Cdc-pdf [PDF – 787 KB] of healthy people in which 47% reported eating romaine lettuce in the week before they were interviewed. Ill people reported eating different types of romaine lettuce in several restaurants and at home.

Two illness clusters were identified at restaurants where ill people reported eating romaine lettuce. An illness cluster is defined as two or more people who do not live in the same household who report eating at the same restaurant location, attending a common event, or shopping at the same location of a grocery store in the week before becoming ill. In these two clusters, several ill people reported eating at the same restaurant or shopping at the same location of a grocery store. Investigating illness clusters provides critical clues about the source of an outbreak. If several unrelated ill people ate or shopped at the same location of a restaurant or store within several days of each other, it suggests that the contaminated food item was served or sold there.

Traceback information from the FDA indicated that ill people in this outbreak ate romaine lettuce harvested from specific counties in the Central Coastal growing regions of northern and central California. The FDA, along with CDC and state partners, investigated farms and cooling facilities in California that were identified in traceback. CDC analyzed water and sediment samples from an Adam Bros. Farming, Inc. farm in Santa Barbara County, which was one of the farms identified in the traceback investigation. The outbreak strain of E. coli O157:H7 was found in sediment within an agricultural water reservoir on the farm. WGS results showed that the E. coli O157:H7 found in the agricultural water reservoir was closely related genetically to the E. coli O157:H7 isolated from ill people.

WGS results also showed that the E. coli O157:H7 strain isolated from ill people in this outbreak is closely related genetically to the E. coli strain isolated from ill people in a 2017 outbreak linked to leafy greens in the United States and to romaine lettuce in CanadaExternal . The outbreak described here is not related to a spring 2018 multistate outbreak of E. coli O157:H7 infections linked to romaine lettuce. People in the spring outbreak were infected with E. coli O157:H7 bacteria with a different DNA fingerprint.

FDA is continuing to investigate to learn more about how the E. coli bacteria could have entered the agricultural water reservoir and ways romaine lettuce from the farm could have been contaminated. The FDA is able to confirm that Adam Bros. Farming, Inc. has not shipped romaine since November 20, 2018. Romaine lettuce from Adam Bros. Farming, Inc. linked to this outbreak is no longer available for sale.

FDA continues its investigation of farms identified in traceback.

As of January 9, 2019, this outbreak appears to be over.