First Local Cluster of Clonorchiasis Caused by Consumption of Raw Freshwater Fish — Zhengzhou City, Henan Province, China, 2026

Abstract

What is already known about this topic?

Clonorchiasis, a major foodborne parasitic disease in China, is transmitted by eating raw or undercooked freshwater fish and shrimp containing metacercariae. Although prevalent in southeastern and northeastern China, it remains low-endemic in Henan, where raw freshwater fish is not traditionally eaten; indigenous cases are rare and most infections are imported.

What is added by this report?

This report is China's first local clonorchiasis cluster in a low-endemic region. The Zhengzhou outbreak was linked to raw freshwater fish eaten at a dinner and drew online public concern. Three symptomatic and two asymptomatic patients were confirmed by the provincial CDC, and all recovered after anthelmintic treatment.

What are the implications for public health practice?

Rising popularity of raw freshwater fish has raised clonorchiasis risk in low-endemic regions. Authorities should strengthen public health education, train medical staff, and improve collaboration with market supervision agencies. Standardized surveillance of aquatic products, regular release of results, and coordinated responses to clusters and online public concern are essential.

ABSTRACT

Objective: To describe a clonorchiasis outbreak associated with consumption of raw freshwater fish in Zhengzhou, Henan Province, China, in 2026 and highlight the public health risks in low-endemic regions.

Methods: We conducted an epidemiological investigation and laboratory examinations, and analyzed findings in relation to health awareness, medical services, and food safety supervision.

Results: Five diners who consumed raw freshwater fish were diagnosed with clonorchiasis, corresponding to an attack rate of 100%. Patients presented with fever, abdominal pain, and other symptoms; two patients were asymptomatic. All patients had markedly elevated eosinophil counts, and four had impaired liver function. After praziquantel treatment, all patients fully recovered, and follow-up stool tests were negative. The outbreak was attributed to consumption of raw freshwater fish.

Conclusion: Consumption of raw freshwater fish caused this outbreak. Insufficient public awareness, limited clinical capacity, and regulatory gaps were major risk factors. Integrated prevention and control measures are needed to avert similar outbreaks.

On February 10, 2026, Henan CDC received a report of a suspected clonorchiasis case. The patient had consumed raw freshwater fish at a dinner party and presented with eosinophilia, recurrent fever, and abdominal pain. The provincial and municipal CDCs jointly investigated. Based on exposure history, clinical symptoms, and laboratory findings, the event was confirmed as a clonorchiasis outbreak caused by consumption of raw freshwater fish.

INVESTIGATION AND RESULTS

A patient with markedly elevated eosinophil counts presented to the Henan CDC. He had developed a fever and abdominal pain on January 24, 2026 and was treated for gastroenteritis. His condition worsened on February 1, when he developed a fever of 39.2 ℃, abdominal distension, chest tightness, and palpitations. Ten days of combination therapy did not relieve his symptoms. He reported consuming raw freshwater fish at a local Cantonese restaurant on December 31, 2025. Clonorchis sinensis (C. sinensis) eggs were detected in his stool using the modified Kato–Katz method (Figure 1), confirming infection. Anthelmintic treatment was arranged under Henan CDC guidance.

Figure 1.

Clonorchis sinensis egg in a Kato–Katz thick-smear preparation at 400× magnification.

Note: The specimen was prepared using the modified Kato–Katz thick-smear method. The main morphological features of the eggs are a sesame-seed shape, an operculum with shoulder-like elevations at the narrow end, and a small protuberance at the opposite end.

The Henan and Zhengzhou CDCs investigated the index patient's fellow diners. Of the seven attendees, five had consumed raw freshwater fish. One diner developed a fever on February 8 and took ibuprofen; he developed high fever again on February 11 and tested positive for C. sinensis eggs. The remaining diners were asymptomatic. Although informed of the risk of clonorchiasis and advised to undergo testing, they initially did not consider it necessary.

On March 11, 2026, a second fellow diner visited the Henan CDC. He had experienced abdominal pain without fever since mid-February but had not sought care promptly; stool testing later confirmed clonorchiasis. After the third case was confirmed, the four remaining diners underwent stool testing on March 12. Two were diagnosed with clonorchiasis. The two diners who had not consumed raw fish were asymptomatic, had normal laboratory findings, and were considered uninfected. All patients were treated with praziquantel at 75 mg/kg per day for 3 days. One month later, follow-up stool tests were negative. All patients recovered completely, with no severe outcomes or deaths.

Five premises were surveyed: the implicated restaurant and four other raw fish restaurants in Zhengzhou. Raw fish slices were mainly prepared from Ctenopharyngodon idella, Squaliobarbus curriculus, and Spinibarbus denticulatus, all purchased from two Zhengzhou aquatic product markets and sourced from Guangdong and Guangxi. Fish served at the implicated restaurant were sourced from Shunde District, Foshan; the origins of other batches were not recorded. The fish came from large-scale aquaculture farms, earthen ponds, or wild catches, and they were held in restaurants before processing. Slaughtering, filleting, and plating were performed in one area, whereas side dishes were prepared in another. Separate knives were allocated for raw and cooked foods; however, no disinfection was performed, and utensils were rinsed only with tap water.

This was not the first clonorchiasis cluster reported in Henan. In November 2024, an imported outbreak occurred in Anyang: all seven tourists who consumed raw freshwater fish in Foshan were tested, and five were positive, corresponding to an attack rate of 71.43%. Both outbreaks were associated with consumption of raw freshwater fish during group dinners. Patients presented with typical symptoms, mainly fever and gastrointestinal discomfort, accompanied by marked eosinophilia and liver injury. Early misdiagnosis was common; patients were initially treated for upper respiratory infection or gastroenteritis without symptom relief (Table 1).

Table 1. Comparison of epidemiological indicators, clinical characteristics, and laboratory findings among patients with clonorchiasis in two cluster outbreaks in Henan Province.

Features	Local outbreak				Imported outbreak
	n	Value range			n	Value range
Note: All cases identified in the imported outbreak were diagnosed on the same day. "–" means that the data are no applicable or no available for this group.
Gender
Male	2	–			3	–
Female	3	–			2	–
Age (y)	–	11–36			–	33–59
Location of exposure	–	Zhengzhou, Henan			–	Foshan, Guangdong
Food source	–	Guangdong			–	Guangdong
Attack rate	–	5/5			–	5/7
Incubation period (d)	3	24–46			4	19–25
Exposure-to-diagnosis interval (d)	5	41–70			5	49
Chief complaints
Fever	2	–			3	–
Nausea	1	–			3	–
Loss of appetite	1	–			4	–
Abdominal pain	2	–			4	–
No symptoms	2	–			1	–
Parameters
Eosinophil percentage (%)	5	10.1–61.1			5	23.2–76.5
Alanine aminotransferase (U/L)	4	60–122			2	137–323
Aspartate aminotransferase (U/L)	1	82			2	60–137
Alkaline phosphatase (U/L)	3	110–396			2	326–650
γ-glutamyl transferase (U/L)	1	166			2	313–534
Eggs per gram of stool	5	48–144			5	120–360

PUBLIC HEALTH RESPONSE

After the outbreak was identified, CDCs at the provincial, municipal, and district levels launched joint interventions. First, all seven dinner attendees were identified through the epidemiological investigation, and testing and follow-up were completed for all exposed persons. Second, health education and management of public concern were implemented. On February 14, 2026, the Henan CDC published a popular-science article on its WeChat account to disseminate clonorchiasis prevention information and reduce infection risk. On March 22, 2026, videos and media reports about patients with clonorchiasis seeking redress circulated online. The Henan CDC immediately notified the Zhengzhou CDC and guided multi-departmental consultations, patient management, risk assessment, and health education. A risk alert was issued to address public concern and mitigate online risk. Because the restaurant declined to release diner records, customers who had consumed raw fish during other service periods could not be screened. Two health advisories urged people who had consumed raw fish to undergo screening at the Henan CDC. Twenty people were tested; five newly-identified individuals had positive stool results. All five infections were associated with raw fish consumed in Guangdong, and no additional local infections have been identified to date. Sampling at the implicated restaurant could not be performed because raw fish dishes had been removed and no fish stock was available. Third, all confirmed patients remained under follow-up to ensure standardized treatment and re-examination until full recovery.

DISCUSSION

Henan remains a low-endemic region for clonorchiasis, with an infection rate of only 0.01% (5/42,759) reported in a 2015 survey (2). However, the two recent clusters indicate increasing risks of foodborne clonorchiasis in Henan. Together with imported exposure, the growing local popularity of raw freshwater fish sourced from high-endemic regions may make sporadic cases and clusters a long-term concern. These outbreaks highlight several important public health issues.

First, public health awareness remains insufficient. Local residents have limited knowledge of the transmission, hazards, and prevention of clonorchiasis and may underestimate the risk of consuming raw freshwater fish. This lack of awareness promotes risky dietary behavior and may facilitate local transmission. Most people with mild or asymptomatic infections do not seek screening or medical care, allowing infections to remain undetected in the community. Long-term infection can cause irreversible liver damage and markedly increase the risks of cholelithiasis, cirrhosis, and cholangiocarcinoma (3). Targeted health education is urgently needed to discourage raw fish consumption, interrupt community transmission (4), and encourage active screening and early treatment among high-risk populations.

Second, hospitals face major challenges in clonorchiasis management. Clinicians in low-endemic regions may have limited awareness of diagnostic clues, such as eosinophilia and a history of raw fish consumption, which can lead to delayed diagnosis or misdiagnosis as gastrointestinal or respiratory disease. Laboratory tests at primary-level medical institutions have limited sensitivity for detecting C. sinensis eggs and may miss mild infections. Most hospitals in Henan do not routinely stock antiparasitic drugs; therefore, patients with confirmed infection may need to obtain medication from CDCs or online pharmacies, delaying treatment and prolonging the transmission period. In addition, no official clinical guidelines are available, leaving clinicians uncertain about treatment regimens. Hospitals should establish prioritized specimen referral pathways, use provincial and municipal parasitology laboratory networks and CDC diagnostic capacity (5), develop rapid response mechanisms, and improve diagnosis and management of clonorchiasis. Although clonorchiasis is managed as an infectious disease requiring focused surveillance and statutory reporting under Category C requirements, formal designation as a Category C notifiable disease should be considered when appropriate.

Third, food safety supervision is inadequate. These surveys indicate major gaps in source control. Aquaculture quarantine and regular snail control in ponds should be strengthened in endemic regions, full traceability implemented in aquatic product markets, and certificate verification required of vendors, to reduce clonorchiasis transmission at the source. These processing practices create a high risk of cross-contamination during raw fish preparation. National Standard GB 2733–2015, Fresh and Frozen Aquatic Products (6), covers unprocessed aquatic raw materials but does not specify parasite limits for ready-to-eat products. In contrast, GB 10136–2015, Aquatic Animal Products (7), applies to edible raw fish and prohibits trematode metacercariae and nematode larvae. This outbreak reflects weak enforcement of GB 10136–2015 and a regulatory loophole that allows raw fish compliant with GB 2733–2015 to be sold for direct consumption. Market regulators should require any freshwater fish product intended for raw consumption to comply with GB 10136–2015, rather than permitting market entry based on GB 2733–2015 alone. Such products should be clearly labeled as "suitable for raw consumption" or "ready-to-eat."

Fourth, low-endemic regions may adopt mature, replicable full-chain management standards for raw freshwater fish developed in Hengxian and Shunde. Agricultural authorities should conduct seasonal snail control and parasite surveillance at fish farms that supply products intended for raw consumption. Market regulators should increase random sampling and routine testing of high-risk species, such as grass carp and barbel chub, to reduce source-related hazards. Restaurants serving raw fish should be subject to special registration and required to use separate utensils for raw and cooked foods and to follow standardized disinfection protocols. CDCs should provide regular technical training and health education to improve public awareness. When clonorchiasis outbreaks occur, a multi-departmental joint response should include source tracing, standardized treatment of patients, exposure investigation, screening of all diners, and management of public concern to interrupt transmission rapidly.

This study has several limitations. Although supply-chain tracing and on-site hygiene investigations suggested that contamination originated during the upstream aquaculture stage, no cross-provincial investigation was conducted; therefore, the supply sources, aquaculture environments, and contamination conditions could not be determined. Because the incubation period was prolonged, no residual fish samples from the implicated restaurant were available for testing. Because no formal request for regulatory intervention was submitted to market supervision authorities, parasitological examination was limited to diners in the outbreak cluster, and catering staff and other customers were not screened. Large-scale population surveys were not conducted locally; therefore, the infection prevalence among residents could not be accurately quantified.

This is the first local cluster of clonorchiasis caused by consumption of raw freshwater fish in Zhengzhou city of Henan province. It raised the importance of rising popularity of raw freshwater fish with clonorchiasis risk in low-endemic regions.

Conflicts of interest

No conflicts of interest.