Why are bacterial outbreaks climbing despite pandemic precautions—and could your dinner plate be at risk? New CDC data uncovers rising animal/environmental contamination and a hidden ‘glove-handoff’ in food safety failures.

Report: Contributing Factors of Foodborne Illness Outbreaks — National Outbreak Reporting System, United States, 2014–2022. Image Credit: Pol Sole / Shutterstock

In a recent report published in the Centers for Disease Control’s (CDC) Morbidity and Mortality Weekly Report (MMWR), researchers analyzed outbreaks of foodborne illness in the U.S. using data from 2014 to 2022, identifying trends in sources of contamination and contributing factors.

Their findings indicate bacterial outbreaks have increased over time while viral outbreaks have decreased, possibly due to safety measures taken to prevent the spread of coronavirus disease 2019 (COVID-19). Although contamination often occurred before the food was prepared, often due to environmental factors or from animals, contamination from sick food workers remained a significant cause of outbreaks, though it declined.

Reducing outbreaks requires retail food establishments to follow evidence-based guidelines for food safety, implement policies that prevent sick employees from handling food, and ensure that temperature and time controls are followed. Effective food safety plans such as Hazard Analysis and Critical Control Points (HACCP), as well as staff training, are essential in this effort.

Background

An estimated 800 outbreaks of foodborne illness occur in the U.S. every year, causing 15,000 illnesses that lead to 800 hospitalizations and 20 deaths. Outbreaks comprise a small proportion of foodborne illnesses, but investigating them can provide crucial insights into their occurrence and prevention.

State and local health departments collect outbreak data and report it to the Centers for Disease Control’s (CDC) Foodborne Disease Outbreak Surveillance System (FDOSS) through the National Outbreak Reporting System (NORS). This data helps track common sources of contamination and guides public health policies.

Foodborne illness outbreaks often result from contamination (when pathogens enter food), proliferation (when bacteria grow in food), or survival (when pathogens persist despite cooking or sanitization). Investigations use environmental assessments, epidemiologic data, and laboratory tests to determine these contributing factors.

Previous studies found that norovirus outbreaks are often linked to sick food workers handling food, while Salmonella outbreaks frequently stem from cross-contamination. However, cross-contamination, previously a leading cause of bacterial outbreaks, declined significantly in recent years, likely due to improved hygiene and sanitation during the COVID-19 pandemic.

Publishing updated outbreak data is essential, as cooking trends, policy changes, and the COVID-19 pandemic may have altered risk factors. For example, increasing consumption of unpasteurized milk and undercooked meats may contribute to more outbreaks.

About the Study

The research team analyzed FDOSS data to identify patterns in viral and bacterial outbreaks before and during the COVID-19 pandemic, helping health departments and restaurant owners implement better food safety measures. By identifying key risks, businesses and policymakers can develop strategies to reduce foodborne illnesses and improve public health.

The FDOSS dataset included food sources, exposure settings, laboratory findings, symptoms, case details, and identified contributing factors. Over half (57.2%) of outbreaks were excluded due to missing data on contributing factors. Outbreaks were analyzed over three periods: 2014–16, 2017–19, and 2020–22. Outbreak causes were categorized into five groups: bacterial, viral, parasitic, toxin/chemical, and unknown. The settings where contaminated food was prepared and consumed were also classified.

Findings

Between 2014 and 2022, 6,618 outbreaks of foodborne illness were reported. After excluding cases with missing or inconsistent data, 2,677 outbreaks were analyzed. These outbreaks were grouped into three periods: 2014–2016 (42.7%), 2017–2019 (42.2%), and 2020–2022 (15.1%). The sharp decline in reported outbreaks during 2020–2022 may reflect reduced health department capacity to investigate outbreaks during the pandemic rather than fewer actual cases. Additionally, the reduction may be partly due to restaurant closures and a shift toward home-prepared meals during COVID-19.

Over time, bacterial outbreaks rose from 41.9% in the first period to 48.4% in the third, while viral outbreaks declined from 33.3% to 23.2%. Outbreaks caused by contamination factors decreased from 85.6% to 81.0%, while those related to proliferation factors initially dropped from 40.3% to 35.0% but then stabilized at 35.1%.

Survival-related contributing factors (e.g., pathogens surviving inadequate cooking) declined in the second period (25.7% to 21.9%) but returned to the original proportion (25.7%) in the third period, indicating a resurgence that highlights the ongoing need for strict cooking and sanitation practices. The percentage of outbreaks linked to aquatic animal foods increased from 12.0% to 18.5% before slightly declining to 18.3%.

Outbreaks associated with restaurant-prepared food increased in 2017–2019 but then declined in the final period, rising from 57.6% to 63.2% and then falling to 58.8%. Institutional outbreaks, however, rose in the last time frame, from 6.4% to 5.3% to 9.6%. Many of these occurred in settings like schools, prisons, and hospitals, where meal services continued during the pandemic.

The most common contributing factor across all outbreaks was contamination from an environmental or animal source before final preparation, accounting for 26.0% of cases and increasing over time. While this type of contamination became more common, contamination from infectious food workers via barehand contact was the second most common factor, though it declined from 20.5% in the first period to 8.9% in the third. Viral outbreaks showed a notable shift from barehand contact to gloved-hand contact as a primary mode of transmission in the final period. However, barehand contact still accounted for nearly 29% of viral outbreaks. Improper temperature control while preparing food (13.1%) and displaying it (11.5%) were also major contributors.

Bacterial outbreaks were primarily linked to contamination from animals or the environment, inadequate cooking, and improper cooling. Improper cooling emerged as a top contributing factor to bacterial outbreaks during the final period, highlighting ongoing challenges with temperature control. Viral outbreaks, on the other hand, were caused mainly by direct contact with infectious food workers, shifting from barehand to gloved-hand transmission during the pandemic.

Conclusions

Outbreaks of foodborne illness reported from 2014 to 2022 were mostly caused by contamination from animals or the environment before food reached its final preparation stage. Cross-contamination, which had been a significant cause of bacterial outbreaks, was no longer among the top contributing factors in the final period, reflecting improved practices.

Inadequate food temperatures and improper cooling were major contributors to bacterial outbreaks. The pandemic influenced outbreak trends by reducing cross-contamination and likely underreporting due to strained public health resources while increasing institutional outbreaks. Unwell food workers remained a significant contamination source, but interventions like increased glove use helped minimize transmission.

Future research should improve food safety surveillance, training, and outbreak investigations. Practical measures such as HACCP plans, clear ill-worker policies, and pandemic preparedness strategies that cover all stages of the food supply chain—from production to retail—can help prevent future outbreaks.