Case Study 1
The Salmonella Heidelberg Infection Outbreak in 13 States
Brianna Dyson
Liberty University
Case Study 2
Introduction
Foodborne outbreaks refer to incidents where people experience illnesses after consuming
similar foods or beverages. The outbreaks can range from minimal to large-scale incidents
depending on the number of people affected. 1,3 In the past, several foodborne outbreaks have
been reported across different parts of the United States. Some of the popular incidents include
the outbreak of salmonella Chailey in the U.S. and Canada, the multistate, outbreak of
Salmonella Anatam, and the multistate outbreak of Listeriosis associated with Jensen Farms
cantaloupe 2,5. While the noted incidents were all significant based on how they impacted
people’s lives, this report will cover a much earlier outbreak that was reported in 2012. The
outbreak of Salmonella Heidelberg was reported by the Oregon Health Authority and
Washington State Department of Health in June 2012 3. The report was issued following an
increase in the incidents of the Salmonella enterica serotype, registering an identical pulsed-field
gel electrophoresis. An investigation conducted in August of 2012 led to the identification of 19
clinical isolates with matching patterns of infections 3, 4,8. This report encompassed the
pathophysiology and epidemiology related to the Salmonella Heidelberg agent, including the
origin of the reported outbreak in comparison to other similar outbreaks of the agent. The step-
by-step process incorporated in this investigation provides insights into approaches that the CDC
and other public health bodies take in addressing foodborne outbreaks.
Pathophysiology and Epidemiology of Salmonella Heidelberg
Pathophysiology
The investigation of the Salmonella Heidelberg strain commenced by analyzing the
pathophysiology of the agent. The impact of this agent started with the ingestion process as it
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accesses the body through contaminated foods or water. 2,3,5 Some of the contaminated foods
related to this agent include poultry products, eggs, and dairy. In the investigation process, the
cluster of 19 Salmonella Heidelberg isolates from people reported to have consumed one of these
products. A critical part of the agent’s pathophysiology is the mechanisms it uses to attach and
invade its host. Following the investigations carried out, the incident team noted that once the
agent gained access to the human body, it traveled down to the gastrointestinal tract and was
attached to the epithelial lining located within the intestines. 4,6,8 The Salmonella Heidelberg
agent has on its lower surface specialized structures that enable it to attach to the intestinal
mucosa within the gastrointestinal tract.
After attachment, the next step is the invasive process, marked by induction of the membrane
ruffing and injection of effector proteins into the epithelial cells.5 While inside these cells, a
notable mechanics of the invasion was the multiplication and colonization process. The
Salmonella Heidelberg agents multiply rapidly, and, in this process, they also invade the M cells,
comprising part of the gut-associated lymphoid tissue (GALT). 1,3,5 This mechanism is a critical
part of the Salmonella Heidelberg invasion as it enables them to evade the immune response
executed by the host, and, more so, penetrate much deeper into the intestinal tissue. A further
assessment of this process within the cluster of 19 isolates also helped determine that the
invasion of the M cells contributes to the systemic dissemination in severe infection cases.
The pathophysiology of the Salmonella Heidelberg agent is also marked essentially with
the inflammatory response noted across the isolates. Based on the observation made in the
evaluation, the increased invasion of the agent in the human body contributes to an inflammatory
response noted in the intestinal mucosa. 1,3 The laboratory reports reveal that the response
constitutes the release of the pro-inflammatory cytokines, including interleukin-1 (IL-1) and
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interleukin-6 (IL-6). 4,5 Another cytokine release noted in this process was the tumor necrosis
factor-alpha (TNF-α). A concurrent process that transpires along the mechanisms of the pro-
inflammatory cytokines is the recruitment of immune cells, including the neutrophils and
macrophages, which are delivered to the site of infection. 2,5,7 Overall, such interactions
contribute to salmonellosis, an outcome associated with such presentations as abdominal pain,
diarrhea, fever, and vomiting. Among the first isolates in the investigation, 12 presented with all
these symptoms, helping to make further deductions on the patterns. 3
Among the severe cases isolates, some distinguished aspects of systemic spread were
noted. These individuals comprise persons with significantly compromised immune systems. In
the cases, it was noted that the Salmonella Heidelberg agent could disseminate from the
intestines to other organs through the bloodstream. 3,4,6 Such incidents were marked as critical,
especially considering that the spread contributed to the development of bacteremia, sepsis, and
focal infections reported in several organs, including the liver, spleen, and lungs.1,3,4 Besides,
these developments are also associated with a Host Immune Response, a critical process
associated with controlling Salmonella Heidelberg agent infection. The mechanics of this
response are marked by the response of the innate immune cells, including the macrophages and
dendric cells. 2,4 After recognizing the presence of the bacterial agent within the host, the immune
system is triggered, initiating the inflammatory response, and simultaneously activating the
adaptive immune mechanisms. The critical elements associated with the process include the T
cells and B cells recruited to the infection site to initiate the production of specific antibodies,
also related to cell-mediated immunity. 2,5 While this process helps in controlling Salmonella
Heidelberg, the agent has evolved and adopted mechanics that allow it to evade and subvert the
host immune response.
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Epidemiology
When the CDC team was dispatched to investigate the spread of Salmonella Heidelberg
across 13 states, the epidemiological process included an investigation of the agent. The
investigation focused on prevalence, transmission, risks, and associated outbreaks. First, the
prevalence of Salmonella Heidelberg can be based on the historical data related to the agent. The
agent was initially discovered in 1933 in Heidelberg, Germany, and in 1954, isolated for the first
time in the U.S.1 Based on CDC reports, from 2001 to 2012, the Salmonella Heidelberg agent has
been isolated in 4% of 83,743 raw meat and samples collected for testing by the USDA-FSIS. By
2013, 134 individuals had been infected by the Salmonella Heidelberg strain. 3 The median age of
those infected was 22, and 55% of the patients were female. 3
Transmission
The transmission of Salmonella Heidelberg is a critical part of understanding the
preventative measures to adopt to avert the infection. Like other Serotypes of Salmonella
Heidelberg, the primary mode of transmission of the pathogen is through the ingestion of
contaminated food and water. In the contamination reported across the 13 states, there were
several contamination channels reported.3 The isolated individuals were infected through
ingesting contaminated food. Based on the interviews, they had ingested poultry products,
including chicken products, including chicken, turkey, eggs, and unpasteurized dairy products.3
Such foods become contaminated in different contexts, including production, processing,
transportation, and storage. The investigation also underpinned that some of the reported
infections developed because of improper hygienic practices when preparing the food.
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The historical reports encompassing Salmonella Heidelberg reveal that the agent can also
be transmitted through cross-contamination mechanisms. This infection mechanism is attributed
to processes where the pathogen is present in raw or undercooked foods and interacts with other
foods. In the 2013 case, four of the isolated presented with Salmonella Heidelberg infection
despite not having consumed the dairy product. 1,4,8 The inference of cross-pollution in these
cases was made following assessing the storage mechanisms in their respective households.
Therefore, because of the storage of dairy products with other foods, the pathogenic agents were
transmitted to the other foods, consequently leading to cross-contamination.3 Besides this vector,
another mode of transmission of Salmonella Heidelberg that can also infect individuals is
contaminated water. Although this case was not evident in the 2013 outbreak, historical reports
reveal that the bacteria can infect drinking water supplies and recreational water sources. 3,5 In
most of these cases, the incidents are marked with people interacting with fecal matter or runoff
water from such sources as agriculture and sewerage.
Risks
The comprehensive studies conducted on Salmonella Heidelberg reveal several risks associated
with the pathogen. One of the primary impacts reported is gastrointestinal illness. Based on
previous reports on people diagnosed with the illness, a common symptom they reported is
gastroenteritis, associated with such presentations as diarrhea, abdominal cramps, nausea, and
vomiting. 2,5,7 In the 2013 case, these symptoms ranged from mild to severe, warranting different
intervention mechanisms. The Salmonella Heidelberg infection is also associated with
bacteremia, attributed to bacterial presence in the bloodstream. In the previous cases reported,
the agent has been associated with other risks as developing severe illnesses. 4,5 The infection has
been reported to increase immune weaknesses in vulnerable populations, including young
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children and elderly. In populations diagnosed with such conditions as HIV/AIDS, cancer, and
diabetes, the treatment of Salmonella Heidelberg infection is more challenging.1 Lastly, the
pathogen is also associated with such risks as chronic sequelae, which comprise such conditions
as reactive arthritis and irritable bowel syndrome.
Origin of Reported Outbreak Compared to Other Outbreak Agents
Since the discovery of Salmonella Heidelberg, multiple reports have been made on the
outbreak of the bacterial infection. However, the 2013 incident had some notable peculiarities
not evident in the previous case. In determining the outbreak’s origin, the USSDA-FSIS
conducted a traceback investigation utilizing shopper card records from nine patients. Using
these sources, they managed to underpin that all these patients had purchased Foster Farms
chicken before the onset of their sickness presentations.3 They conducted further investigations
on the isolates, noting that four of the cases were unopened chicken samples that consequently
tested positive for the Salmonella Heidelberg strain. This finding helped the USSDA-FSIS team
trace the infection back to the Foster Farms slaughter establishments.3 Besides determining the
origin of the strain, the team also undertook a closer evaluation to underscore the nature of the
strain. In this investigation, they noted that three of the nine samples were susceptible to all the
microbials they tested. Based on these results, one of the samples was found resistant to
gentamicin, streptomycin, and sulfisoxazole.3 In response to this groundbreaking finding, the
incident investigation team commenced investigations in the Foster Farms establishment.
Nonetheless, following this step, the investigations were not finalized.3
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Compared to previous outbreaks, the 2013 Salmonella Heidelberg had some
distinguishable aspects. In the 1933 outbreak of the pathogen in Germany, the reports linked the
infection to the poultry production farms within Heidelberg, consequently giving the Salmonella
Heidelberg strain its name. 2,5,8 Another outbreak reported of the pathogen was in the United
Kingdom in 1960 (Nichols et al., 2022). In this case, the report noted the origin as the ingestion
of contaminated milk extracted from dairy cattle. In a more recent incident happening in
Wisconsin, the Wisconsin Veterinary Diagnostic Laboratory reported to the Wisconsin Division
of Public Health that an isolate of the pathogen had been traced in an ill dairy bull calf.4 In this
incident, they reported that the pathogen was associated with significantly high calf mortality.
Consequently, this discovery triggered further research that led to the discovery that
isolates drawn from people affected with the Salmonella Heidelberg infection matched that of the
index calf.4 The discovery of the stain in dairy cows initiated further research to underscore the
scope of infections related to the strain. The cases in the historical reports reveal a variation in
the origin of the Salmonella Heidelberg infection compared to the 2013 case, which was reported
and traced to the Foster Farms slaughter establishments. One of the significant differences in
these discoveries was that in the latter case, there were several incidents of cross-contamination
as several individuals diagnosed with the illness had not yet consumed the dairy products they
purchased from Foster Farms. Furthermore, unlike the 2013 case, attributing the infection to
chicken products, the previous incidents underpinned the infection vector to dairy products and
poultry. 3, 5 This discovery led the incident team investigators to deduce the need for education on
the best approach to use in storing raw food products to avert similar infections. Therefore,
compared to the other outbreak incidence in the 2013 case, the primary deduction made by the
investigation team was that it constituted multiple vectors of infections.
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Reported Steps in the Investigation and Lessons Learnt
Reported Steps
One of the most outstanding aspects of the Salmonella Heidelberg investigation includes
the reported steps undertaken in the investigation process. Based on the report, the first step
included conducting state-based interviews leading to the determination that chicken product was
commonly consumed, leading to the infections reported across the 13 states. 3,47The discovery
consequently triggered the traceback investigation using the shopper card records from
shortlisted nine patients. The information generated from these individuals, including where they
had purchased the chicken products, led the team to the Foster Farm establishments linked to the
origin of the outbreak. While further investigations were underway on the farm, the
investigations were never finalized in 2013. However, later reports noted that the pathogen
outbreak was linked to the farm.5
Lesson Learnt
In conducting the pathophysiology and epidemiology of the Salmonella Heidelberg led to
several lessons, including approaches to adopt in addressing such incidents. The evaluation led to
inferring that food storage lessons are critical in mitigating such infections. This conclusion was
made following the discovery of cross-contamination reported in several individuals diagnosed
with the infection. Furthermore, the incident led to the deduction that identifying the most critical
risk factors associated with foodborne infection is vital. Utilizing such sources as outbreak data
can help determine what high-risk foods comprise and, more essentially, the control points that
can help control such outbreaks. Furthermore, the incidents presented the need for a
comprehensive surveillance system that can help in developing relevant control measures and
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more importantly, develop preventive measures that can help minimize foodborne
contaminations across human populations. Considering that the infection origin was traced to a
food production establishment, the evaluation process determined the need for enhanced
regulatory frameworks to help ensure that all organizations involved in food production
implement all relevant measures to reduce infections. Lastly, the process revealed the need for
more research and innovation to help discover other risks associated with foodborne illnesses,
and, more so, approaches that can help treat the resulting conditions. In the Salmonella
Heidelberg case, several instances of drug-resistant strains can be challenging to treat.
Furthermore, the research revealed that some strains present significant challenges in vulnerable
populations. Consequently, such discoveries compel the need for further research to help
discover more effective treatment approaches.
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References
1. Kaldhone, P. R., Foley, S. L., & Ricke, S. C. Salmonella Heidelberg in layer hens and egg
production: Incidence and potential issues. InFProducing Safe EggsF2017; 235-256).
Academic Press.
2. Jones, T. F., & Yackley, J. Foodborne disease outbreaks in the United States: a historical
overview.FFoodborne Pathogens and Disease,F2018; 15(1), 11-15.
3. Outbreak of Salmonella Heidelberg infections linked to a single poultry producer -13
States. 2012-2013. Centers for Diseases Control and Prevention. 2013
https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6227a3.htm
4. Nichols, M., Gollarza, L., Sockett, D., Aulik, N., Patton, E., Francois Watkins, L. K., ... &
Klos, R. Outbreak of multidrug-resistant Salmonella Heidelberg infections linked to dairy
calf exposure, United States, 2015–2018.FFoodborne pathogens and disease,F2022; 19(3),
199-208.
5. Small, C., & Bidaisee, S. The Rise of an Invisible Enemy (Salmonella) in the United
States.FPublic Health,F2021; 4(1), 1061.
6. Federal Register: The Daily Journal of the United States Government. New performance
standards for Salmonella and campylobacter. 2024.
https://www.federalregister.gov/documents/2016/02/11/2016-02586/new-performance-
standards-for-salmonella-and-campylobacter-in-not-ready-to-eat-comminuted-chicken
7. Public Health LA County. Salmonellosis. 2023.
http://publichealth.lacounty.gov/acd/Diseases/Salmonellosis.htm
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8. USDA Food Safety and Inspection Services. Salmonella by the numbers. 2024.
https://www.fsis.usda.gov/inspection/inspection-programs/inspection-poultry-products/
reducing-salmonella-poultry/salmonella
