IHP 330 Module 8 Epidemiological Research Study Report example

From Restaurant Salads to an Irrigation Canal: An Epidemiological Study Report on the 2018 Romaine Lettuce E. coli O157 Outbreak

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Southern New Hampshire University

IHP 330: Principles of Epidemiology

Module Eight Final Project

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[Date]

From Restaurant Salads to an Irrigation Canal: An Epidemiological Study Report on the 2018 Romaine Lettuce E. coli O157 Outbreak

The Health Issue

Shiga toxin-producing Escherichia coli O157:H7 causes bloody diarrhea and abdominal cramps, and in a minority of patients hemolytic uremic syndrome, a complication that can cause kidney failure and death. The bacteria live in the intestines of cattle and other animals and reach people through contaminated food and water. Produce eaten raw, such as leafy greens, is a recurring vehicle because it is not cooked before eating. In April 2018, officials in New Jersey and Pennsylvania reported a cluster of infections among people who had eaten at locations of one restaurant chain, and a national investigation led by federal public health officials followed (Bottichio et al., 2020).

Methods

Investigators defined a case as infection with illness onset between March 13 and August 22, 2018, with one of 22 outbreak-associated pulsed-field gel electrophoresis patterns of E. coli O157:H7 or E. coli O61, or with a strain closely related to the main outbreak strain by whole-genome sequencing (Bottichio et al., 2020). The definition is laboratory based, which makes it specific but means that only people who sought care and had a stool culture could be counted. The investigation combined three strands: epidemiological interviews to identify common foods and subclusters of illness at restaurants and other venues; traceback of the romaine supplied to those venues through distributors to growers; and an environmental assessment led by the Food and Drug Administration that tested water, soil, manure, compost and animal scat in the growing region.

Descriptive Epidemiology

The investigation identified 240 case-patients in 37 states. By time, illness onsets ran from mid-March into the summer, with most in April and May. By place, cases were spread across the country rather than concentrated in one community, a pattern typical of a widely distributed food. By person, cases occurred across age groups, and 20 subclusters linked groups of ill people to the same restaurant or venue (Bottichio et al., 2020). A national, dispersed pattern of onset with laboratory-linked strains points toward a food sold across many states, not a single local event.

Measures of Severity

The table below calculates key measures from the reported counts.

Table 1

Calculated Measures for the 2018 Romaine Outbreak

Note. Counts from Bottichio et al. (2020).

Testing the Romaine Hypothesis

Of 179 interviewed case-patients, 152, or 84.9 percent, reported eating romaine lettuce in the week before illness began (Bottichio et al., 2020). To judge whether this is unusually high, investigators compared it with a survey of healthy people, in which 46 percent reported eating romaine in the week before their interview (Centers for Disease Control and Prevention [CDC], 2018). A one-sample test of proportions comparing 84.9 percent among 179 cases with an expected 46 percent gives z of about 10.4, far beyond any conventional threshold. Romaine exposure among cases was much higher than would be expected by chance if romaine were unrelated to illness. The subclusters strengthened the link: people who became ill after eating at the same restaurant had eaten romaine served there.

Tracing the Source

Traceback from the subcluster restaurants identified many different distributors and growers, but all of the implicated romaine came from one growing region around Yuma, Arizona. The environmental assessment found the outbreak strain in water samples from an irrigation canal in the region (Bottichio et al., 2020). The finding points to contaminated agricultural water as a likely route by which the bacteria reached the lettuce, with the canal located near a large animal feeding operation, although investigators could not determine exactly how the water became contaminated.

Limitations and Alternative Explanations

Several limitations affect interpretation. The laboratory-based case definition counts only people who sought care and had stool tested, so the true number of infections was certainly larger, and severity measures based on identified cases overstate severity among all infections, because milder illness is less likely to be tested. Food histories rely on recall of meals eaten a week or more earlier, and people often do not know what type of lettuce was in a salad or sandwich. The comparison with a healthy population survey is useful but imperfect, since the survey population differs in timing and composition from the cases. Finally, the distribution chain made it impossible to trace every product to a specific farm. None of these limitations undermines the core conclusion, which rests on three independent lines of evidence: epidemiological, traceback and environmental.

Recommendations

Recommendations follow from the evidence, and several match the federal action plan for leafy greens that regulators issued after this and related outbreaks (U.S. Food and Drug Administration [FDA], 2020). First, because agricultural water was the likely route, growers in leafy greens regions should assess and control risks to irrigation water, especially near livestock operations, and regulators should support testing and treatment of water used on crops eaten raw. Second, because traceback took weeks and could not pinpoint specific farms, the leafy greens industry should adopt labeling and recordkeeping that identify harvest location and date on each package. Third, public health laboratories should continue using whole-genome sequencing, which linked cases across 37 states with high precision. Finally, consumer advisories should name the growing region when traceback identifies one, which allows unaffected product to stay on the market while contaminated product is removed.

Conclusion

The 2018 romaine lettuce outbreak sickened at least 240 people in 37 states, hospitalized more than two in five identified cases and caused five deaths. Laboratory linkage, interviews showing romaine exposure far above background levels, traceback to a single growing region and the discovery of the outbreak strain in irrigation canal water together identify contaminated romaine as the vehicle and agricultural water as a likely source. The investigation shows how the tools of epidemiology, from case definitions to hypothesis testing, work together to find the source of a dispersed outbreak and to point prevention toward the farm rather than the kitchen.

References

Bottichio, L., Keaton, A., Thomas, D., Fulton, T., Tiffany, A., Frick, A., Mattioli, M., Kahler, A., Murphy, J., Otto, M., Tesfai, A., Fields, A., Kline, K., Fiddner, J., Higa, J., Barnes, A., Arroyo, F., Salvatierra, A., Holland, A., . . . Gieraltowski, L. (2020). Shiga toxin-producing Escherichia coli infections associated with romaine lettuce: United States, 2018. Clinical Infectious Diseases, 71(8), e323-e330. https://doi.org/10.1093/cid/ciz1182

Centers for Disease Control and Prevention. (2018). Multistate outbreak of E. coli O157:H7 infections linked to romaine lettuce (final update). https://www.cdc.gov/ecoli/2018/o157h7-04-18/index.html

U.S. Food and Drug Administration. (2020). Leafy greens Shiga toxin-producing E. coli (STEC) action plan. https://www.fda.gov/food/foodborne-pathogens/leafy-greens-stec-action-plan