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Questions and Answers about the 2013 FoodNet MMWR

Each year, FoodNet publishes a summary of surveillance data and trend information in CDC's Morbidity and Mortality Weekly Report (MMWR). The report contributes to our understanding of the human health impact of foodborne diseases. This information helps regulators, the food industry, and consumers understand the progress we are making in preventing them. Learn more about FoodNet.


Questions about this report

What is the MMWR article about? What are the main take-home points of the report?

This year’s  report summarizes 2013 preliminary surveillance data and describes trends since 2006 for nine infections caused by Campylobacter, Cryptosporidium, Cyclospora, Listeria, Salmonella, Shiga toxin-producing Escherichia coli (STEC) O157 and non-O157, Shigella, Vibrio, and Yersinia.

FoodNet provides accurate counts of the frequency of diagnosed infections.

  • In 2013, FoodNet collected information on 15% of the US population (about 48 million residents of 10 States) and identified:
    • 19,056 cases of culture-confirmed bacterial and laboratory-confirmed parasitic infection,
    • 4,200 hospitalizations, and
    • 80 deaths.
  • The most frequent infection was caused by Salmonella, accounting for 38% of reported infections, and the second by Campylobacter (35%). Vibrio accounted for 1.3% of reported infections.
  • The incidence of infection caused by nearly all pathogens tracked was highest among either children younger than five-years old or among adults aged 65 years or older.

FoodNet provides the best information we have on trends in specific foodborne illnesses.

  • Comparison of incidence rates in 2013 with rates in the preceding 3 years (2010–2012) shows some short-term changes:
    • Incidence of Salmonella infection was 9% lower (down to 15.2 cases reported per 100,000 population).
    • Incidence of Vibrio infection was 32% higher, but these infections are rare (0.51 cases per 100,000 population).
    • But most rates in 2013 did not change compared with rates in 2010–2012.
      • The incidence of culture-confirmed Campylobacter, Listeria, Shigella, STEC O157, STEC non-O157, and Yersinia, and laboratory-confirmed Cryptosporidium did not change significantly.
      • As a group, the incidence of infection with six key pathogens transmitted commonly through food (Campylobacter, Listeria, Salmonella, STEC O157, Vibrio, and Yersinia) was unchanged.

Compared with rates in 2006–2008, some longer-term changes were observed:

  • Incidence of infection with Campylobacter and Vibrio were both higher in 2013. Rates of the other pathogens did not change significantly.
  • In 2012, the incidence of HUS was 36% lower among children aged <5 years compared with rates from 2006–2008 (complete HUS case detection lags behind the rest of FoodNet surveillance).

Why has there been little change in the incidence of Salmonella infection?

Salmonella has many ways of spreading:

Salmonella infection is a complicated problem that is not likely to be controlled by any one single measure. Different types of Salmonella are carried in many food animals and wild animals, from where it can spread to humans in several ways.

Salmonella is spread through:

  • Contaminated food, including foods of animal origin like meat, poultry or eggs, raw produce contaminated with animal or human fecal matter
  • Processed foods with contaminated ingredients
  • Contact with animals that carry Salmonella in their intestines
  • Drinking contaminated water

Outbreaks and their effect on reported rates:

The incidence of Salmonella infection peaked back in 2010, and was lower in 2013 than in the preceding three years. It was back down to the rates seen in 2006–2008. The rate had gone up in between, partly because of a large egg-associated outbreak of the most common type of Salmonella, serotype Enteritidis, in 2010, and went down after that outbreak was stopped and new egg safety measures were mandated by FDA.

Prevention efforts by industry and consumers:

Foodborne Salmonella infections can be prevented by decreasing the frequency of Salmonella carriage in food animals, reducing contamination in slaughterhouses and factories, improving the safety of produce items consumed raw, monitoring the safety of ingredients of processed foods, and keeping foods refrigerated during transport. Consumers can also play their part by cooking meat and poultry thoroughly, and by keeping raw meat and poultry from contaminating other foods in the kitchen.

Regulatory efforts:

Regulatory agencies are developing and implementing measures to make food safer. USDA-FSIS has implemented tighter standards for Salmonella on poultry. In 2013, USDA-FSIS released the Salmonella Action Plan, which describes their plans to reduce foodborne illnesses from Salmonella; these plans include setting performance standards for cut-up poultry parts in 2014, such as:

  • Recently proposed regulations to make produce safer and to increase preventive controls in many food processing factories.
  • Training restaurant managers in food safety and educating consumers about preparing foods safely at home.
  • Investigating illnesses and outbreaks so that contaminated products are removed before more persons become ill, and to learn what measures are needed to prevent more illnesses.

What is happening with Vibrio infection?

Vibrio infections are rare but can be serious. Vibrio is a naturally occurring organism commonly found in marine and estuarine waters, including the Gulf of Mexico and the Atlantic and Pacific Oceans. It is estimated that 50–60% of Vibrio infections are acquired through food; many of these are due to eating raw oysters. Vibrio bacteria multiply when seawater where oysters grow is warm. As a result, these infections are most common during the warm months, when waters and oysters contain higher numbers of Vibrio organisms.

How is Vibrio spread?

The transmission of Vibrio infections varies considerably by species.

  • Vibrio parahaemolyticus causes the greatest number of infections; most are diarrheal illnesses caused by consumption of raw shellfish.
  • Vibrio alginolyticus infections are not foodborne; this species often causes ear or wound infections after contact with sea or estuarine water.
  • Vibrio vulnificus causes severe bloodstream and wound infections that are often fatal; approximately half of these infections are acquired through eating raw shellfish, and the other half through skin contact with sea or estuarine water. Immunocompromised persons, especially those with liver disease, are at greatest risk of acquiring Vibrio vulnificus infections.

Recent Vibrio data

During the summers of 2012 and 2013, infections with a particular strain of Vibrio parahaemolyticus serotype O4:K12 were associated with consumption of shellfish from several Atlantic coast harvest areas. This strain is characterized by its pulsed-field gel electrophoresis (PFGE) pattern, or DNA fingerprint.

Before 2012, infections of serotype O4:K12 had not been associated with shellfish outside the Pacific Northwest region of the United States. More information about this can be found in Notes from the Field: Increase in Vibrio parahaemolyticus Infections Associated with Consumption of Atlantic Coast Shellfish — 2013 and on the Vibrio parahaemolyticus investigation page.

Increasing rates

The incidence of Vibrio infection has increased in recent years. It is possible that global climate change has had a role. To learn more about Vibrio infections and how they can be prevented, visit the Vibrio web pages.

Prevention and control measures

Infections can be prevented by treating oysters at processing plants with heat, freezing, or high pressure. The risk of infection can be reduced by rapidly refrigerating oysters after harvest and by thorough cooking, especially when they come from warm growing waters.

  • Continued Vibrio infections highlight the lack of sufficient implementation of these control measures

What are culture-independent diagnostic tests (CIDTs) and why are they important?

Generally, diagnostic testing uses tissue culture to determine which pathogen is causing an illness. CIDTs are diagnostic methods that do not rely on culture to detect evidence of infection. Examples of CIDTs include antigen-based tests and nucleic acid-based assays. These tests do not grow the pathogens in culture, so we call them “culture-independent.” A familiar example is the rapid test for strep throat.

Changing diagnostic landscape

Clinical laboratories increasingly used CIDTs to diagnose gastrointestinal infections, because they are rapid, easy to use, and the accuracy of some tests is good. As the number of approved CIDTs increases, we anticipate that their use will increase rapidly, moving the diagnostic landscape away from culture-based methods. This will challenge the ability to identify cases, monitor trends, detect outbreaks, and characterize pathogens.

Limited information

CIDTs do not yield isolates that are needed to characterize the infecting strain’s antibiotic resistance, serotype, pulsed-field gel electrophoresis (PFGE) pattern for PulseNet, and whole genome sequence. Without isolates, we will not be able to monitor trends in the incidence of specific serotypes of Salmonella and of O157 and non-O157 STEC infections. Without PFGE patterns uploaded to PulseNet, our ability to detect outbreaks of Salmonella, STEC, and Listeria would be markedly reduced. Additionally, the performance characteristics (e.g., false positives) of CIDTs vary and differ from those of culture, so we do not yet know how many of the diagnoses identified by CIDT are correct.

What is FoodNet doing to examine the impact of CIDTs?

FoodNet began tracking infections diagnosed through culture-independent diagnostic tests (CIDTs) of Shiga toxin-producing Escherichia coli in 2008, Campylobacter in 2009, and all other FoodNet bacterial pathogens (Listeria, Salmonella, Shigella, Vibrio, and Yersinia) in 2011.

FoodNet has surveyed clinical laboratories about their diagnostic practices since 2010. For the first time, in this report, we describe the number of cases of bacterial enteric infections that were diagnosed using only CIDT (i.e., the organism was not isolated). Those suspected infections are not included in the confirmed cases reported by FoodNet. We are developing methods to assess incidence and trends using both culture-confirmed cases and those identified through CIDT.

What are possible solutions to the problems in surveillance created by CIDTs?

A positive culture-independent diagnostic test (CIDT) clinical specimen can be cultured to obtain an isolate if the specimen collection and transport process is compatible with culture.

For the short term, we plan to encourage laboratories to follow current recommendations for culturing of CIDT positives, or send specimens to their public health laboratory for culture. This practice may preserve our ability to detect outbreaks and track trends, but it is slow and expensive. 

For the longer term, we need to develop new tests for public health that are themselves culture-independent. These tests will need to specifically identify the organism AND its important characteristics directly in patient specimens. Once these new tests are developed, surveillance activities such as outbreak detection and trend analysis should be faster and more effective than they are currently. 

How does FoodNet help make food safer?

FoodNet provides a foundation for food safety policy and prevention efforts because FoodNet's surveillance data tell us where efforts are needed to reduce the burden of foodborne illnesses. FoodNet has been counting cases and tracking trends —which illnesses are decreasing and which are increasing— for infections transmitted commonly through food since 1996.

Where can I go for more information on foodborne illness?


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