Incidence of Disease Attributed to Some Major Foodborne Pathogens Appears to be on the Rise In-spite of Rigorous Efforts at the Pre-and Post-Harvest Levels to Improve Food Safety and Public Health

Over 200 infectious agents can be transmitted to humans through food, including three types of biological agents, including bacteria, viruses and parasites. Monitoring trends in foodborne illness in the US is performed by the Centers for Disease and Prevention (CDC) using two approaches, including continuous active surveillance and mathematical modeling of resultant active surveillance data. While the majority of foodborne illness cases have been predicted through mathematical modeling to be attributed to viruses (e.g., Norovirus and Hepatitis A; reference 3), active surveillance focuses on known bacterial pathogens and parasites for which robust and validated detection methodologies are widely available and implemented in clinical laboratories. Active surveillance of foodborne disease through the CDC’s FoodNet Network is based on laboratory confirmed cases caused by Campylobacter, Cyclospora, Listeria monocytogenes, Salmonella, Shiga Toxin producing Escherichia coli (STEC), Vibrio and Yersinia in 10 sites across the US, which represents approximately 15% of the US population (4).

The US bears a significant public health and economic burden attributed to disease caused by pathogens transmitted through food. Trends in the incidence of foodborne illness have been monitored through active surveillance of laboratory confirmed cases and mathematical models, which take into account estimations of under-reporting, method sensitivity etc., extrapolated from these surveillance data (3). The most recent active surveillance report published on preliminary 2018 FoodNet data showed an increased incidence of disease caused by multiple bacterial pathogens and the parasite Cyclospora as compared incidence rates from 2015-2017 surveillance data. In 2018, 25,606 illnesses, 5,893 hospitalizations and 120 fatalities were observed across the 10 FoodNet sites, where Campylobacter and Salmonella were responsible for approximately 73% of laboratory confirmed cases and 72% of hospitalizations. While Campylobacter and Salmonella infections were rarely associated with mortality (0.3 and 0.5% of cases resulted in death, respectively), L. monocytogenes infections were associated with a 96% hospitalization rate and 21% fatality rate (4).

The incidence of foodborne illness caused by Cyclospora, Vibrio, Yersinia, STEC, Campylobacter and Salmonella increased significantly in 2018 as compared to disease burden estimates based on surveillance data from 2015-1017. Specifically, the incidence of illness caused by Cyclospora, Vibrio, Yersinia, STEC, Campylobacter and Salmonella increased by 399%, 109%, 58%, 26%, 12%, and 9%, respectively. However, these trends should be interpreted with caution as the number of illnesses diagnosed by Culture Independent Diagnostic Techniques (CIDTs) was also markedly increased by 65% in 2018 as compared to those identified through CIDTs during 2015-2017. Reflex culture, or attempting to obtain an isolate of a bacterial pathogen identified in a clinical specimen following identification by CIDTs, was performed for 75% of CIDT positive samples. The percentage of CIDT positive samples that were confirmed by microbiological culture and from which an isolate was obtained ranged from 37% for Vibrio to 100% for L. monocytogenes. Additionally, 14%, 36%, 41%, 44% and 50% of specimens that were CIDT positive for Salmonella, STEC, Campylobacter, Shigella and Yersinia were culture negative for each respective pathogen, which represents a potential false positive result that may artificially inflate the true burden of disease caused by these infectious agents.

The observed increase in incidence of foodborne disease may be in part attributed to the increased use of CIDTs, where culture confirmation was not concordant with a CIDT result (potential false positive) along with large multi-state outbreaks of foodborne illness that occurred in 2018 (e.g., outbreaks linked to produce, poultry and shell eggs). Importantly, CIDTs represent a rapid screening technique to identify a causative agent in an illness or make a decision as to whether to accept or reject a food commodity; however, culture confirmation to obtain an isolate for subtyping is still important to confirm the presence of a viable pathogen in a sample. Obtaining a pathogen isolate from clinical specimens and other samples (i.e., food and food-associated environments) is needed to (i) facilitate outbreak detection and microbial source tracking investigations, (ii) monitor antimicrobial resistance patterns, and (iii) evaluate the effectiveness of mitigation strategies at the pre- and post-harvest levels to develop targeted prevention measures to further reduce the load of pathogens entering the human food chain in order to improve public health metrics.

Campylobacter has been the leading cause of gastroenteritis in the US since 2013 and the incidence of salmonellosis has essentially remained unchanged compared to the original 1996-1998 baseline data in-spite of diligent efforts to improve food safety and public health metrics in the US. Poultry represents a major food vehicle associated with both Campylobacter and Salmonella infections. The US Department of Agriculture Food Safety Inspection Services recently develop a new Campylobacter detection and isolation method and has plans to update Campylobacter performance standards. Salmonellosis in the US is most commonly associated with three serotypes, including Enteritidis, Newport and Typhimurium, where serotypes Enteritidis and Typhimurium are commonly linked to poultry and shell eggs. While incidence of salmonellosis cases caused by serotype Typhimurium has declined infections caused by Salmonella Enteritidis have not declined over the past decade warranting additional on farm interventions (i.e., vaccines and improved hygiene). In poultry, the prevalence of Salmonella is notably higher in chicken parts as compared to that observed for whole carcasses, supporting the need for additional interventions at the processing level. In beef mitigations that effectively reduce Salmonella contamination on the surface of carcasses may have limited efficacy to control Salmonella in internal tissues such as lymph nodes. Previous studies have demonstrated that Salmonella can be present in lymph nodes at high concentrations (2). Also, a few specific Salmonella serotypes tend to be predominant in beef trim and ground beef (e.g., Salmonella Montevideo and Dublin); further work is warranted to elucidate strain-specific characteristics that may explain the overrepresentation of these strains in beef trim and ground beef (1). Produce represents a major food vehicle associated with foodborne disease and in 2018 large multi-state outbreaks of illness attributed to Salmonella, STEC and Cyclospora may be responsible for the observed increase in incidence of disease caused by these biological agents. The CDC suggested implementing more targeted mitigation strategies at the pre-harvest level to prevent these pathogens from entering the human food chain in order to improve public health metrics. More microbiological testing, including isolation and further characterization of resultant isolates (i.e., Whole Genome Sequencing and Antimicrobial Resistance Profiling) is needed to further elucidate mechanisms that pathogens, and particularly specific strains within a given pathogen, employ to tolerate existing mitigations strategies.

  1. DOERSCHER, D. R., T. L. LUTZ, S. J. WHISENANT, K. R. SMITH, C. A. MORRIS, and C. M. SCHROEDER. 2015. Microbiological Testing Results of Boneless and Ground Beef Purchased for the National School Lunch Program, 2011 to 2014. J. Food Prot. 78:1656–1663.
  2. Gragg, S. E., G. H. Loneragan, M. M. Brashears, T. M. Arthur, J. M. Bosilevac, N. Kalchayanand, R. Wang, J. W. Schmidt, J. C. Brooks, S. D. Shackelford, T. L. Wheeler, T. R. Brown, T. S. Edrington, and D. M. Brichta-Harhay. 2013. Cross-sectional Study Examining Salmonella enterica Carriage in Subiliac Lymph Nodes of Cull and Feedlot Cattle at Harvest. Foodborne Pathog. Dis. 10:368–374.
  3. Scallan, E., R. M. Hoekstra, F. J. Angulo, R. V Tauxe, M.-A. Widdowson, S. L. Roy, J. L. Jones, and P. M. Griffin. 2011. Foodborne illness acquired in the United States–major pathogens. Emerg. Infect. Dis. 17:7–15.
  4. Tack, D. M., E. P. Marder, P. M. Griffin, P. R. Cieslak, J. Dunn, S. Hurd, E. Scallan, S. Lathrop, A. Muse, P. Ryan, K. Smith, M. Tobin-D’Angelo, D. J. Vugia, K. G. Holt, B. J. Wolpert, R. Tauxe, and A. L. Geissler. 2019. Preliminary Incidence and Trends of Infections with Pathogens Transmitted Commonly Through Food – Foodborne Diseases Active Surveillance Network, 10 U.S. Sites, 2015-2018. MMWR. Morb. Mortal. Wkly. Rep. 68:369–373.
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