The U.S. Food and Drug Administration (FDA) has released a report on its investigation of the Salmonella Typhimurium outbreak that caused 31 reported illnesses and four hospitalizations in the U.S. between June and August 2021.

The FDA worked with the U.S. Centers for Disease Control and Prevention (CDC) and state partners to investigate the outbreak, which was linked through epidemiology and traceback to packaged salad greens during the summer of 2021.

This outbreak is believed to be the FDA’s first domestic investigation of a foodborne illness outbreak associated with leafy greens grown in a Controlled Environment Agriculture (CEA) operation. The CEA operation produces leafy greens using common commercial high-density hydroponic growing techniques with deep water culture and floating raft production methods. The report released today includes an overview of the traceback investigation, investigation results, and various factors that potentially contributed to the contamination of packaged leafy greens with Salmonella.

Although a conclusive root cause was not identified, the agency identified certain conditions and practices that could result in contamination, including the presence of a different serotype of Salmonella in pond water used to grow the leafy greens, growth media storage practices, water management practices, and general sanitation practices at the CEA that were inadequate to prevent the introduction or spread of microorganisms of public health significance into the leafy greens.

FDA isolated the outbreak strain of Salmonella Typhimurium in a stormwater retention basin adjacent to the CEA farm. However, the investigation did not reveal if that stormwater retention basin was the source of the Salmonella that ultimately contaminated the leafy greens. This highlights the importance of assessing all microbial hazards, including those associated with adjacent and nearby land uses.

In light of this report, FDA highlights the following requirements and recommendations applicable to firms, such as the hydroponic operation implicated in this S. Typhimurium outbreak, engaged in CEA. Here are some of the requirements and recommendations:

• Develop a keen understanding of potential sources and routes of contamination including the raw materials and inputs used, as well as possible sources of contamination throughout the operation.
• Implement effective sanitation procedures and sampling plans while also paying close attention to hygienic operations and equipment design, ensuring cleaning procedures do not contribute to the dispersion of microbial contaminants that may be present.
• Assess growing operations to ensure implementation of appropriate science- and risk-based preventive measures, including applicable required provisions of the FDA Food Safety Modernization Act (FSMA) Produce Safety Rule and good agricultural practices (GAPs).
• Implement procedures that are effective in rapidly cooling and cold holding harvested leafy greens after harvest and verify the effectiveness of the cooling and cold holding procedures, including the routine monitoring of processing and storage environments and product temperatures to prevent pathogen growth in harvested leafy greens.
• If employing tools such as pre-harvest and post-harvest sampling and testing of food, water, and the physical environment, seek to identify and inform sampling plans, limits of detection, and mitigation measures that control potential sources and routes of bacterial contamination in the growing and harvesting environment.
• Ensure that all growing pond water is safe and of adequate sanitary quality for its intended use, which includes implementing measures (such as water treatment) necessary to reduce the potential for contamination by known or reasonably foreseeable hazards.
• Perform a root cause analysis when a pathogen is identified in the growing environment, in raw agricultural inputs such as water, or in the agricultural commodity to determine how the contamination likely occurred and implement appropriate prevention and verification measures.
• Assess and mitigate risks associated with adjacent and nearby land uses that may impact CEA operations, in both rural and more urbanized settings.

As of January 10, 2022, a total of 44 people infected with the outbreak strain of Salmonella Uganda have been reported from 25 states. Illnesses started on dates ranging from December 24, 2020, to December 2, 2021.

Sick people range in age from younger than 1 to 84 years, with a median age of 27, and 8 ill people were children under the age of 5. Of 43 people with sex information, 27 (63%) are female. Of 37 people with information available, 15 (41%) have been hospitalized. No deaths have been reported.

Public health investigators are using the PulseNet system to identify illnesses that may be part of this outbreak. CDC PulseNet manages a national database of DNA fingerprints of bacteria that cause gastrointestinal illnesses. DNA fingerprinting is performed on bacteria using a method called whole genome sequencing (WGS). WGS showed that bacteria from sick people’s samples are closely related genetically. This means that people in this outbreak likely became sick from contact with the same type of animal.

On November 19, 2021, Oregon Health Authority collected samples from a bearded dragon in a sick person’s home for testing. WGS showed that the Salmonella on the animal is closely related to bacteria from sick people. This means that people likely got sick from touching bearded dragons.