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FoodNet 2017 Preliminary Data

Documenting trends in foodborne illness – identifying which illnesses are decreasing and which are increasing –provides important information needed to help reduce foodborne illness. Each year, FoodNet reports on changes in the number of infections in the FoodNet surveillance area from pathogens transmitted commonly through food. Laboratory tests, including cultures for bacteria and culture-independent diagnostic tests (CIDTs), detected these pathogens.

This year’s report summarizes 2017 preliminary surveillance data and describes trends since 2006 for infections caused by the following pathogens monitored by FoodNet: Campylobacter, Cryptosporidium, Cyclospora, Listeria, Salmonella, Shiga toxin-producing Escherichia coli (STEC), Shigella, Vibrio, Yersinia. The report also summarizes cases of hemolytic uremic syndrome (HUS) for 2016, the most recent year for which those data are available.

Key Findings

  • In 2017, FoodNet received reports of 24,484 illnesses, 5,677 hospitalizations, and 122 deaths in its surveillance area, which includes 15% of the U.S. population.
  • The incidence of infections per 100,000 people was highest for Campylobacter and Salmonella, which is similar to previous years.
  • The number of infections diagnosed by CIDT, a newer type of test, is increasing. The overall number of Campylobacter, Listeria, Salmonella, Shigella, Vibrio, and Yersinia infections diagnosed by CIDT increased 96% in 2017 compared with the 2014–2016 average.
  • CIDTs are revealing many infections – such as those caused by Cyclospora, Yersinia, Vibrio, and STEC non-O157 – that would not have been diagnosed before because of limited testing.
  • The incidence of Salmonella infections overall did not change significantly, but there were significant changes among serotypes:
    • The incidence of infections caused by serotypes Typhimurium and Heidelberg has been decreasing since 2006–2008, with overall declines of more than 40% for both.
    • Infections cause by serotypes Javiana, Thompson, and Infantis have all increased by more than 50% since 2006–2008.
  • Infections caused by STEC O157 have decreased in the past 10 years. The increasing use of CIDTs makes interpretation of trends in STEC infections difficult because CIDTs do not indicate which STEC serogroup caused the infection. The incidence of HUS among children younger than 5 years decreased during 2016 compared with 2006–2008. Because most cases of HUS are caused by STEC O157, the decline in HUS provides evidence that supports the finding of the decline in STEC O157 cases.
  • CIDTs are fast and easy to use, and they detect some illnesses that would have otherwise been missed. However, CIDTs also challenge our ability to find outbreaks and monitor disease trends, because they do not provide certain information needed to characterize organisms that cause infections. For example, some information about the bacteria that cause infections, such as subtype and antimicrobial susceptibility, can be obtained only if a CIDT-positive specimen is cultured. FoodNet is gathering information to better understand the effect of CIDTs on surveillance.

Questions and Answers About the Report

What are culture-independent diagnostic tests (CIDTs)?

CIDTs detect the presence of a specific antigen or genetic sequence of an organism, such as Salmonella. These tests do not require laboratories to grow living organisms (a process known as culturing). Therefore, CIDTs can yield results far sooner than cultures can.

How do CIDTs affect our ability to detect illnesses and track trends?

Because CIDTs are fast and easy to use, they detect some illnesses that would have otherwise been missed. However, CIDTs do not provide certain information needed to characterize organisms. For example, CIDTs currently can detect Salmonella, but cannot identify the Salmonella serotype that caused the infection.

To counter these limitations, laboratories can culture specimens after a CIDT is positive (a practice called reflex culturing), and CIDT manufacturers can create new tests to provide information that is now available only from cultured specimens.

What is FoodNet doing to interpret trends?

Each year, FoodNet publishes a summary of surveillance data and trends information in CDC’s Morbidity and Mortality Weekly Report (MMWR). Recent changes in diagnostic practices challenge our ability to monitor disease trends. Because CIDTs are increasingly used for diagnosis, and positive results likely indicate true infections, FoodNet includes CIDT-positive results in counts of numbers of infections. However, more data are needed to understand how CIDTs have affected surveillance. For example, FoodNet is collecting data on the changes in number of people tested and on the accuracy of CIDTs to help assess the meaning of trends in the number of infections diagnosed.

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Interpreting Changes in Incidence of Infections

Why do Cyclospora infections appear to be increasing?

One reason Cyclospora infections appear to be increasing is that laboratories may be testing for Cyclospora more often, and therefore identifying more infections. More data are needed to determine if the number of infections is truly increasing.

Why do STEC infections appear to be increasing?

In 2017, the incidence of STEC infections increased 28% compared with 2014–2016. The increase was driven by an increase in the number of non-O157 STEC infections diagnosed, probably related to increased use of CIDTs.

STEC are classified by serogroup, and the most common is O157. Clinical laboratories have been able to test stool samples for O157 for decades. With the increasing use of CIDTs, clinical laboratories now commonly determine if a specimen contains a Shiga toxin, and then test the specimen for O157. If they do not find O157, they can assume that a non-O157 STEC is present. They can then send the specimen to a specialized laboratory to determine which non-O157 STEC serogroup is present.

Why do Yersinia infections appear to be increasing?

The increase in Yersinia infections might be due, in part, to more laboratories testing for Yersinia and more infections being diagnosed. Yersinia is difficult to culture. In the past, testing for this organism was limited, and infections may have been underdiagnosed. CIDTs are easier to use than traditional culture methods and are becoming widely used to diagnose Yersinia infections.

What are possible reasons that Salmonella Typhimurium and Heidelberg infections are decreasing?

The number of infections with Salmonella Typhimurium and Heidelberg has been decreasing in FoodNet sites for at least 10 years, long before CIDTs were used to diagnose foodborne infections. Laboratories perform reflex culture on most CIDT-positive Salmonella infections, so it is unlikely that this decrease is related to changes in diagnostic testing. People get Salmonella Typhimurium and Heidelberg infections from a wide variety of food sources. The decline in infections caused by particular serotypes of Salmonella may be due to a combination of efforts by regulatory agencies and industry to make food safer.

What can be done to reduce the incidence of Salmonella infections?

There is not one answer for addressing infections caused by all Salmonella serotypes.

Many sources can spread Salmonella to people. Most food animals and many wild animals carry Salmonella in their intestines without getting sick. Salmonella bacteria are most often spread to people through contaminated food, including meat, poultry, and eggs; raw produce contaminated with animal or human fecal matter (poop); and processed foods made with contaminated ingredients. People also can get infected with Salmonella by drinking contaminated water or having contact with animals or the places animals live.

Many different serotypes (types) of Salmonella bacteria exist, and their sources vary. The 10 most common serotypes reported to FoodNet in 2017 were Enteritidis, Typhimurium, Newport, Javiana, I, 4[5], 12:i:-, Muenchen, Infantis, Braenderup, Saintpaul, and Thompson. Some types of Salmonella are found mainly in one source (for example, Heidelberg in chicken) and some are found in many food and wild animals (for example, Typhimurium).

The particular interventions that work for each Salmonella serotype may vary. To reduce incidence, interventions need to target the types of Salmonella infections that are increasing or staying the same. For example, vaccines could be decreasing the incidence of some serotypes of Salmonella carried by poultry. Work to target additional Salmonella sources and to control a wider range of serotypes is ongoing. Whole genome sequencing can help in figuring out the sources of illnesses. FoodNet tracks illnesses to see if prevention measures are working.

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Questions and Answers About Food Safety

How does FoodNet contribute to food safety?

Since 1996, FoodNet has been counting cases and tracking trends for infections transmitted commonly through food. Information gathered on which illnesses are decreasing and which are increasing provides a foundation for food safety policy and prevention efforts. FoodNet’s surveillance data, such as those in this report, show where efforts are needed to reduce foodborne illnesses. Learn more about FoodNet >

What is CDC doing to control and prevent foodborne disease?

CDC uses the best scientific methods and information available to monitor, investigate, control, and prevent foodborne illness. Using the tools of epidemiology and laboratory science, CDC assesses public health threats. CDC works closely with state health departments to monitor the frequency of specific diseases and to conduct national surveillance for the diseases it monitors.

When food safety threats appear, CDC collaborates with public health partners, including state health departments, the U.S. Food and Drug Administration (FDA), and the U.S. Department of Agriculture (USDA), to conduct epidemiologic and laboratory investigations to determine the causes of these threats and how they can be controlled. Although CDC does not regulate the safety of food, CDC works with regulatory agencies to guide food safety policies and assess the effectiveness of current prevention efforts. CDC provides independent scientific assessment of what the problems are, how they can be controlled, and where the gaps exist in our knowledge. You can find more information on foodborne illness and CDC’s prevention activities on CDC’s Food Safety website.

What are FDA, USDA, and other regulatory agencies doing to make food safer?

Government regulation related to food safety is the responsibility of FDA, USDA’s Food Safety and Inspection Service (USDA-FSIS), the National Marine Fisheries Service, and other regulatory agencies.

Recent regulatory efforts include:

  • USDA-FSIS’s tighter standards for preventing Salmonella and Campylobacter contamination of ground chicken and turkey products, as well as raw chicken parts such as legs, wings, and breasts.
  • FDA’s ongoing implementation of the Food Safety Modernization Act, which requires larger facilities producing food for people and for animals meet preventive control and Good Manufacturing Practice requirements.

CDC works closely with regulatory agencies as they, along with industry, develop and implement measures to make our food safer.

Suggested citation: Centers for Disease Control and Prevention (CDC). Preliminary Incidence and Trends of Infections with Pathogens Transmitted Commonly Through Food — Foodborne Diseases Active Surveillance Network, 10 U.S. Sites, 2006–2017. MMWR Morb Mortal Wkly Rep. 2018 March 23.

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