Outbreak Reports: A Clostridium perfringens Related Foodborne Diarrhea Outbreak in an Elderly Care Center — Beijing Municipality, China, May 2024

Introduction: On May 21, 2024, an elderly care center in Beijing reported dozens of acute diarrhea cases. The local CDC immediately initiated an epidemiological investigation to identify the outbreak’s etiology and implement control measures.

Methods: Suspected cases were defined as individuals experiencing diarrhea ≥3 times or vomiting within 24 hours at the center during May 19–23, 2024. Cases were identified through review of the center’s medical records. Food exposure at breakfast and lunch on May 20 was compared between 24 cases and 70 controls. We interviewed kitchen staff regarding food preparation practices and collected samples for laboratory testing.

Results: Investigation identified 77 elderly residents and 21 staff cases, ranging in age from 22 to 99 years, all of whom presented with diarrhea. The epidemic curve exhibited characteristics consistent with a point-source outbreak. Breakfast and lunch served on May 20 were implicated as suspected exposure meals, with 88% of cases having consumed Chinese hamburgers compared with 51% of controls [odds ratio (OR)=6.6, 95% confidence interval (CI): 1.7, 37.1]. The preparation process for the pork filling in Chinese hamburgers involved portioning, blanching for 20 minutes, simmering in spiced broth for 40 minutes, followed by natural cooling. Laboratory analysis confirmed the presence of C. perfringens in 23 patient samples and one Chinese hamburger sample.

Conclusions: The outbreak was most likely caused by Chinese hamburgers contaminated with C. perfringens. We recommend comprehensive training for food handlers on proper cooling procedures when preparing Chinese hamburgers.

On May 21, 2024, an elderly care center in Beijing reported to the local Center for Disease Control and Prevention (LCDC) that dozens of elderly residents and staff members had experienced acute diarrhea within the previous 48 hours. The LCDC immediately initiated an epidemiological investigation to identify the outbreak’s etiology and implement control measures.

X Elderly Care Center (XECC), situated in southwestern Beijing, is a private residential facility providing comprehensive care services for elderly individuals with varying levels of independence, from self-sufficient to fully dependent. The facility encompasses 50,000 m² with 7 apartment buildings offering over 700 beds. At the time of the investigation, the facility housed 485 elderly residents and employed 204 staff members. A centralized kitchen and dining facility serves meals to both residents and staff.

The first case was identified as a 49-year-old female staff member who developed diarrhea at 16:00 on May 20. The patient’s symptoms resolved spontaneously without medical intervention or medication by May 21.

A suspected case was defined as any individual at XECC who experienced either vomiting ≥1 time/24 hours or diarrhea ≥3 times/24 hours between May 19 and 21, 2024. A probable case was diarrhea ≥3 times/24 hours and abdominal pain. A confirmed case was a suspected or probable case who had a positive bacterial isolation for C. perfringens.

By May 21, a total of 98 cases met the case definition (62 suspected, 16 probable, 20 confirmed), comprising 38 males and 60 females, with ages ranging from 22 to 99 years. Cases occurred between 16:00 on May 20 and 10:00 on May 21, with a median onset time of 23:00 on May 20 (Figure 1). Clinical manifestations are detailed in Table 1. No severe cases were reported. The overall attack rate was 14.22% (98/689), with elderly residents experiencing a higher attack rate of 15.88% (77/485) compared to staff members at 10.29% (21/204) [χ²=3.668, risk ratio (RR)=1.542, 95% confidence interval (CI): 0.980, 2.428]. Cases were distributed across all 7 apartment buildings, with attack rates varying from 8.16% to 22.09%.

Figure 1. 

Epidemic curve of diarrhea cases in an elderly care center, Beijing, China, May 2024 (N=98).

The epidemic curve demonstrated a point source exposure pattern. The absence of elevated diarrheal incidence in surrounding communities sharing the same water supply with XECC strongly indicated a foodborne outbreak. Based on the established incubation period for C. perfringens (2–36 hours) according to the Technical Guidelines for the Identification and Management of Foodborne Illnesses by The National Health Commission of the People’s Republic of China (2023), the exposure time was estimated between 22:00 on May 19 (calculated by subtracting the maximum incubation period from the last case onset) and 14:00 on May 20 (calculated by subtracting the minimum incubation period from the first case onset). This timeline implicated breakfast and lunch on May 20 as the suspected exposure meals.

A case-control study was conducted on May 23, comprising 24 confirmed/probable cases and 70 controls (asymptomatic individuals who lived or worked with the cases), maintaining a control-to-case ratio between 1∶2 and 1∶3. Face-to-face dietary history investigations focusing on breakfast and lunch consumption on May 20 were performed. Statistical analysis (Table 2) identified the Chinese Hamburger (Roujiamo) as the contaminated food item (χ²=9.743, OR=6.6111, 95% CI: 1.7089, 37.0583). The incubation period of this outbreak was 5–23 hours.

Chinese Hamburger (Roujiamo), a traditional Chinese dish consisting of a bun stuffed with chopped braised pork, was investigated for its preparation process. According to kitchen staff reports, raw pork was refrigerated upon purchase at 11:00 on May 19. Since C. perfringens is widely present in nature, pork might have been contaminated during production, transportation, or storage. The following day at 07:30, the pork was portioned, blanched for 20 minutes, and simmered in spiced broth for 40 minutes. However, due to the high thermal resistance of C. perfringens, combined with the possibility that large batches of meat may not have been thoroughly heated, the bacteria could form spores during this process, enabling survival. After cooking, the heat was discontinued, and the pork was left to cool naturally while submerged in the broth. This slow cooling process potentially allowed C. perfringens to proliferate significantly and produce toxins. At 11:00, the cooled pork was chopped and combined with cilantro and green peppers before being stuffed into buns.

On May 21, investigators collected 86 samples comprising 50 patient samples (42 anal swabs and 8 fecal samples), 13 anal swabs from healthy kitchen staff, 11 food samples, and 12 environmental swabs. Polymerase chain reaction (PCR) testing and bacterial isolation for C. perfringens were conducted on all samples. Among them, 23 patient samples and 1 food sample tested positive by PCR. By May 23, bacterial isolation confirmed C. perfringens presence in 23 patient samples (18 anal swabs and 5 fecal samples). The patients who tested positive by PCR were all positive in bacterial isolation. However, C. perfringens was not isolated from any food sample. Subsequent virulence gene detection revealed that 16 anal swabs were cpe(+)plc(+) and 2 were cpe(+), while all 5 fecal samples demonstrated cpe(+)plc(+) profiles.

The following public health interventions were promptly implemented: 1) Continuous monitoring of all patients’ health status; 2) immediate reassignment of kitchen staff with recent history of diarrhea to non-food handling positions; 3) implementation of targeted health education programs, with emphasis on kitchen staff hygiene practices. Following the implementation of these control measures, no new cases were reported.

Clostridium perfringens (also known as C. welchii) is a gram-positive, anaerobic, spore-forming bacillus ubiquitously distributed in environmental reservoirs, including soil and water. This organism commonly exists as part of the normal intestinal microbiota in humans and animals. Enterotoxin-producing strains that colonize the human gastrointestinal tract can precipitate diarrheal illness with an incubation period ranging from 2 to 36 hours. Outbreaks are characteristically associated with protein-rich foods, particularly meat dishes and their accompanying broths that have been prepared in large quantities, subjected to prolonged cooling at elevated temperatures, and served without adequate reheating.

Based on the epidemiological curve, clinical presentation, incubation period, case-control analysis, and laboratory findings, this outbreak was conclusively attributed to C. perfringens contamination of Chinese Hamburger (Roujiamo). The outbreak affected both elderly residents and young to middle-aged staff members. Overall, diarrhea remained the predominant symptom, consistent with previous reports (1–2). However, the symptoms of elderly cases appeared more diverse compared to those of staff cases, including nausea, vomiting, and fever, indicating a broader spectrum of clinical presentations compared to their younger counterparts.

Chinese Hamburger (Roujiamo) was identified as the contaminated food vehicle in this outbreak, with the braised meat filling being the likely source of contamination. The braising process, known as “lu” (卤) in Chinese cuisine, involves simmering over low heat followed by a cooling period to enhance flavor penetration. Although most C. perfringens spores are inactivated within minutes at 100 ℃, certain strains possess extreme heat resistance, enabling survival after 1–6 hours of boiling (3). The organism’s optimal growth temperature is approximately 45 ℃ (4), with rapid proliferation occurring during improper cooling between 37–50 ℃ (5). The cooling phase of the braised pork created ideal conditions for C. perfringens multiplication, combining protein-rich substrate, anaerobic conditions, and favorable temperatures. To mitigate this risk, rapid cooling through the critical temperature range of 15–55 ℃ may effectively prevent bacterial proliferation (6).

In China, the identification and management of C. perfringens outbreaks are governed by two national standards: “WS/T 7-1996 Diagnostic Criteria and Principles of Management for Food Poisoning of Clostridium perfringens” and “GB4789.13-2012 National Food Safety Standard Microbiological Examination of Food Hygiene — Examination of Clostridium perfringens”. These standards mandate bacterial isolation, culture, and confirmatory testing procedures. Furthermore, enterotoxin and enterotoxigenicity assessment requires animal experimentation, resulting in a complex and time-intensive diagnostic process. Given these limitations, the incorporation of multiplex PCR technology into national standards warrants consideration to enhance diagnostic efficiency.

This outbreak investigation was conducted by Fangshan CDC according to the national regulations of infectious control as part of the legally authorized mandate. Ethical approval and informed consent were not necessary as the study uses routinely collected and anonymous data.