III. CDC and FDA Updates on Recent Produce Outbreaks

2019 FSMA Annual Report

Cyclosporiasis Outbreaks

During the 2018 cyclosporiasis outbreak season (May–August), there were 2,299 laboratory-confirmed, domestically acquired cases reported to CDC. Many of the cases could not be linked to an outbreak and/or vehicle. Importantly, this was the first time Cyclospora was detected in domestically grown products. Preliminary data analyses revealed several notable produce-associated cyclosporiasis outbreaks including

  • 250 cases linked to Kwik Trip/Del Monte pre-packaged vegetable trays
  • 511 cases linked to McDonald’s salads
  • Two basil-associated clusters including 16 cases in two states
  • Three cilantro-associated cases at Mexican-style restaurants: 53 cases6

Epidemiological Challenges

The lack of a laboratory subtyping method, such as pulsed-field gel electrophoresis (PFGE) or WGS, for Cyclospora significantly hampers epidemiological investigations. Large amounts of case exposure data must be reviewed line by line to find potential clusters of illness resulting in significant delays in identifying an illness cluster and more outbreaks that go unsolved. At present, a subtyping method is not feasible for multiple reasons:

  • Cyclospora cannot be cultured.
  • Most stool samples contain too few oocysts.
  • Two sporocysts from the same oocyst may be genetically distinct because Cyclospora reproduces sexually in the gut of infected humans.

CDC is working to develop a molecular typing technique that can be used to link infections caused by genetically related Cyclospora organisms to each other and to a food vehicle/source; many of these efforts are in collaboration with FDA. In addition to the lack of molecular typing, other epidemiological challenges with cyclosporiasis include the impact of CIDTs on surveillance and reporting and changes in how data are collected and transmitted to CDC.

E. coli O157 Outbreak

In 2018 an outbreak of E. coli O157 infections linked to romaine lettuce occurred and was the largest outbreak of Shiga toxin-producing E. coli since the 2006 outbreak associated with spinach. It involved 210 cases (96 hospitalizations, 27 cases of hemolytic uremic syndrome, and 5 deaths) from 36 states. Infections were linked to romaine lettuce via epidemiological, traceback, and laboratory evidence:

  • A high percentage of ill people reported eating romaine lettuce (87% or 145/166 interviewed).
  • FDA traceback linked consumed lettuce to farms in the Yuma growing region.
  • WGS results showed that the illnesses were all closely related genetically, and water isolates collected and analyzed by CDC from three locations of the irrigation canal in the region of the outbreak were closely related to the clinical isolates.

Public health authorities were successful in rapidly identifying and investigating illness sub-clusters to help confirm the outbreak vehicle, issuing public warnings within 8 days of identifying the multistate outbreak, and demonstrating the value of WGS in an investigation.

Epidemiological Challenges

  • People who eat lettuce eat it often, and many have multiple exposures.
  • Many people do not remember the type of lettuce they ate.
  • Short shelf life limits ability to test leftover product.
  • Lack of growing location on packaging complicated the consumer/retail
Page last reviewed: April 7, 2020