Burden of Foodborne Illness in the United States: Questions and Answers

CDC estimated the average number of U.S. foodborne illnesses, hospitalizations, and deaths in 2019 caused by seven major pathogens: Campylobacter spp., Clostridium perfringens, invasive Listeria monocytogenes, norovirus, nontyphoidal Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Toxoplasma gondii.

In 2019,

• These pathogens minus Toxoplasma^* caused 9.9 million domestically acquired foodborne illnesses.
  
• Illnesses caused by all seven pathogens resulted in 53,300 hospitalizations and 931 deaths.
• Norovirus was the leading cause of domestically acquired foodborne illnesses (5.5 million) and hospitalizations from these illnesses (22,400); Salmonella was the leading cause of domestically acquired foodborne illnesses resulting in death (238).

_{*Laboratory-confirmed illness data are not available for Toxoplasma because it is not nationally notifiable, and illness is often mild or asymptomatic, so people do not seek medical care or get tested.}

We picked the four pathogens that the Interagency Food Safety Analytics Collaboration marks as top priorities, mainly because of how often and severely they cause illness. Those pathogens are Campylobacter, Listeria, Salmonella, and Shiga toxin-producing E. coli (STEC).

We also included norovirus, Clostridium perfringens, and Toxoplasma gondii so the assessment covers what were previously thought to be the top five pathogens leading to domestically acquired foodborne illnesses, hospitalizations, and deaths in the United States.

We did not estimate Toxoplasma illnesses because most infected people are asymptomatic and the proportion experiencing mild illness is unknown.

CDC estimated that the six major pathogens cause 37.6 million estimated episodes of illnesses in the United States in 2019.

• The six pathogens are Campylobacter spp., Clostridium perfringens, invasive Listeria monocytogenes, norovirus, Salmonella (nontyphoidal), and Shiga toxin-producing Escherichia coli.

Of those 37.6 million illnesses, CDC estimates that 9.9 million illnesses were acquired in the United States and spread through food.

We cannot meaningfully compare the estimates from 2019 with the ones published in 2011. During the past decade, substantial strides have been made to

• Understand, prevent, detect, and respond to foodborne illness;
• Improve the data systems that inform foodborne illness estimates; and
• Improve accurate testing through culture-independent diagnostic test (CIDT) usage.

Since 2011, CIDT use has

• Increased in most U.S. clinical laboratories to diagnose enteric pathogens,
• Improved the likelihood of identifying the pathogen causing illness,
• Increased observation of infections that may have otherwise gone undetected and unreported in U.S. surveillance systems,
• Increased diagnosis of infections from non-O157 STEC serogroups which cause three times as many U.S. foodborne illnesses as O157, and
• Increased overall pathogen number in the 2019 estimates.

CDC's 1999 and 2011 estimates were not wrong; they were the best estimates available when the data were collected.

New surveillance data, methods, and other factors have contributed to more accurate estimates for 2019. Every year, new findings emerge, new methods are developed for research and analysis, new data are added to surveillance systems, and new surveys are conducted.

The 2019 estimates are the result of innovations in methodology and improvements in the data made over the past decade, which we believe have resulted in more accurate estimates than their predecessors. The 2019 estimates give more descriptive information on the seven major pathogens instead of the broader 31 pathogens recorded previously in 2011. We hope future estimates will benefit from additional innovations and improvements.

The current estimates use better data sources and improved methods for adjusting for under-diagnosis.

These estimates are based on data reported for 2019, in part, because of the COVID-19 pandemic.

Officials implemented stay-at-home orders, restaurant closures, school and childcare closures, federal travel restrictions, and other public health interventions. These interventions and other health-care related factors likely influenced exposure to and detection of enteric infections.

• Exposure: Interventions to slow the spread of COVID-19 and other changes in hygiene and daily life likely influenced exposures to enteric pathogens.

• Detection: Other factors, such as changes in healthcare delivery, healthcare-seeking behaviors, and laboratory capacity, may have decreased the detection of enteric infections.

CDC did not want to postpone the development of new estimates until after reporting of surveillance data returned to pre-pandemic levels. Future estimates for the seven pathogens included in this report, as well as other pathogens, may include more recent years.

In addition to the influences of the COVID-19 pandemic, the foodborne illness burden estimates also rely on certain data sources that are not updated regularly. For example, the data that inform the under-reporting and under-diagnosis adjustments, in part, come from the FoodNet population survey. The most recent FoodNet population survey was conducted between December 2017 and July 2019. Without newer data sources and improved methods – including the population survey – we could not accurately and reliably analyze foodborne illness data from more recent years.

These estimates were the product of a multi-step process, with each step requiring careful and thorough analyses:

• Finding sources and studies to pull data from and compiling those data
• Developing methods and models to create the estimates and account for various factors, including under-reporting and under-diagnosis
• Generating the estimates and writing the report

For some of these steps, the necessary data were not immediately available. For instance, the structured expert judgments used to determine the proportion of illnesses transmitted by food were not published until 2021. Compiling, verifying, and publishing surveillance data for each pathogen also takes time, with finalized data usually not becoming available until well into the following year – and sometimes even later.

Our ability to conduct the analyses for these estimates was also limited by a lack of resources and bandwidth during the COVID-19 pandemic, which also affected the sources and studies that formed the foundation for the estimates.

Yes, CDC is working on estimates for another set of known pathogens.

Surveillance data can provide information on trends in foodborne illness. CDC publishes surveillance data annually, which can be found on CDC's pathogen sites, various CDC data dashboards, and the Interagency Food Safety Analytics Collaboration website.

These data inform the development of foodborne illness estimates, which are typically published when new or updated data sources or improved methods become available.