Investigation of an Enteritis Outbreak Caused by Salmonella enterica Serovar Aberdeen in China

Foodborne illnesses affect millions of people globally each year, with Salmonella being one of the most common and dangerous culprits. But a lesser-known serotype—Salmonella enterica serovar Aberdeen—recently caused a significant outbreak in China, shedding light on its ability to spread, make people sick, and resist antibiotics. A 2020 study by researchers from the Yantai and Fushan Centers for Disease Control and Prevention (CDC) investigated this outbreak to fill critical gaps in our understanding of this understudied pathogen.

The Outbreak: Who, When, and What Happened?

In June 2018, 75 employees in Fushan District, Yantai (Shandong Province) developed severe gastroenteritis. The group included 33 men and 42 women, with an average age of 34.2 years. An epidemiological investigation quickly linked the illness to a single source: a catered lunch the workers ate on June 9, 2018.

To confirm the cause, the CDC team collected 22 samples: 4 feces from patients, 5 surplus food items from the caterer, 8 cooking utensils, and 5 samples of chicken and eggs from the market where the caterer bought ingredients. Six samples tested positive for Salmonella Aberdeen—3 from patient feces (75% of fecal samples) and 3 from leftover food (60% of surplus food samples).

Proving the Source: Bacterial “Fingerprinting”

To confirm the outbreak was tied to the catered lunch, the team used pulsed-field gel electrophoresis (PFGE), a gold-standard technique for comparing bacterial strains. Following the PulseNet standardized protocol (used globally to track foodborne pathogens), they digested the bacteria’s DNA with two enzymes: XbaI (resulting in 96.8% similarity between strains) and BlnI (100% similarity). Using BioNumerics Version 6.0 software, they confirmed the strains from patients and food were nearly identical—proof the catered lunch was the source.

What Makes Salmonella Aberdeen Harmful? Virulence Genes

The researchers tested 9 key virulence genes—bits of DNA that make bacteria dangerous—using primers designed from GenBank data. They found 6 genes in all 6 isolates:

stn: An enterotoxin that triggers intestinal fluid secretion (a major cause of diarrhea).
fimA: A “sticky” gene that helps bacteria adhere to human cells, enabling colonization.
virK: Spreads toxins throughout the body.
invA: Lets bacteria invade intestinal M cells (a critical step in infection).
mgtC: Helps bacteria survive in immune cells and low-magnesium environments.
siiE: Aids adhesion to the gut lining, keeping the bacteria attached.

Three genes—sseL, sopB, and spvC—were not detected. These results explain why Aberdeen causes such severe gastroenteritis: it has the tools to stick to cells, invade the gut, and produce toxins.

Antibiotic Resistance: A Growing Concern

The team tested 15 antibiotics (including ampicillin, ceftazidime, and ciprofloxacin) using the Clinical and Laboratory Standards Institute’s broth microdilution method. All 6 isolates were sensitive to every drug except cefazolin (CFZ), a first-generation cephalosporin.

Next, they checked for 13 resistance genes using polymerase chain reaction (PCR) and capillary gel electrophoresis. Two CTX-M genes—group II (351 bp) and group IV (474 bp)—were present in all strains. CTX-M genes produce extended-spectrum β-lactamases (ESBLs), enzymes that break down cephalosporin antibiotics. This finding aligns with broader trends: a 2018 study found CTX-M is widespread in clinical Salmonella isolates across China, making it a major concern for antibiotic stewardship.

Clinical Symptoms: What Patients Experienced

The outbreak’s clinical pattern mirrored severe Salmonella gastroenteritis but with key markers:

Diarrhea (89.3%) and abdominal pain (89.3%): The most common symptoms.
Elevated C-reactive protein (CRP, 77.3%): A blood marker of inflammation, useful for diagnosis.
Nausea (65.3%), fever (52%), and vomiting (45.3%): Less universal but still prevalent.

The incubation period (time from eating to symptoms) ranged from 1 to 88 hours, with an average of 14 hours—similar to better-known serotypes like Salmonella Typhimurium or Enteritidis.

Treatment and Resolution

Severely ill patients received injections of cefotaxime (CTX) and levofloxacin, with noticeable improvement within 3–7 days. The CDC traced the outbreak to two critical food safety failures at the catering company:

Poor hygiene in the food processing environment.
No separation of raw and cooked foods, leading to cross-contamination.

Salmonella Aberdeen in China: A Brief History

While rare globally, Salmonella Aberdeen has a long history in China:

1964: First reported food poisoning outbreak caused by Aberdeen.
1976–1980: Detected in 0.39% of Salmonella isolates (105 out of 27,183).
2006–2007: Made up 3% of 807 Salmonella strains from 24,895 fecal samples (as China joined the WHO Global Salm-Surv surveillance program).

Key Takeaways for Public Health

This outbreak offers critical lessons for preventing future cases:

Food safety is non-negotiable: Separating raw and cooked foods and maintaining clean processing environments are essential to stop cross-contamination.
Virulence genes matter: Aberdeen’s ability to stick to cells and produce toxins explains its severity—monitoring these genes can help track risk.
Antibiotic resistance is a warning: The presence of CTX-M genes means Aberdeen can resist common cephalosporins. Healthcare providers should test for resistance before prescribing and prioritize stewardship.
CRP is a useful diagnostic tool: Elevated CRP levels were common in patients—routine testing can help identify severe cases early.

The study followed ethical guidelines, including the Declaration of Helsinki, and the authors reported no conflicts of interest.

The research was published in the Chinese Medical Journal in 2020 by Wen-Juan Liu, Hao-Jie Pan, Yan Song, Yan Li, Yan-Qing Wang, Xiao-Nan Zou, Dong-Nan Zhu, Yue-Hua Gong, and Ying-Chun Xu.

doi.org/10.1097/CM9.0000000000000938

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