Researchers have been unable to pinpoint the source of a Vibrio parahaemolyticus outbreak in China this past year.

In May 2021, an outbreak occurred caused by Vibrio parahaemolyticus O10:K4 infection.

Emergency departments of three hospitals in Huzhou City, Zhejiang Province, treated several patients with acute gastroenteritis. They attended the same wedding banquet.

Fifty-eight suspected cases, 28 probable, and 15 laboratory-confirmed cases were part of the outbreak among 500 banquet attendees, according to the study in Foodborne Pathogens and Disease.

In the first case, a female patient, aged 62, began to have abdominal pain and diarrhea on May 29, accompanied by nausea and vomiting. In all probable cases, the main symptoms were abdominal pain and diarrhea, with nausea and vomiting. Ten people had a fever. The final case occurred on May 30. Patients received emergency treatment in local hospitals, they were all cured without any severe cases or deaths.

Potential food sources
The suspicious meal was lunch on May 29. The incubation period ranged from six to 19 hours, and the average was about 13 hours. Among the 28 possible cases, the youngest patient was 7 years old and the oldest was 78. There were 13 males and 15 females.

A case-control study with 28 cases and 14 controls demonstrated that emperor crab with mashed garlic; goose liver geoduck; shrimp, and sea cucumber were the potential sources of food poisoning. Only seafood consumption was investigated during the investigation. 

Researchers said O10:K4, the new variant of O3:K6, poses a challenge for the prevention and control of Vibrio parahaemolyticus disease in the future.

Vibrio parahaemolyticus isolates from 18 laboratory-confirmed cases were all serotyped O10:K4, and of sequence type ST3. 

Four kitchen environmental samples were negative for Salmonella, Shigella, Vibrio parahaemolyticus, and E. coli. Three of these were coliform positive.

As all the remaining products had been removed after the wedding banquet, food samples could not be collected on-site.

Due to the lack of food samples and the negative Vibrio parahaemolyticus results from kitchen environmental samples, the source of the outbreak remains unclear, said, researchers. 

All 18 strains were susceptible to Azithromycin, Tetracycline, Gentamicin, Chloramphenicol, Ciprofloxacin, and Trimethoprim-sulfamethoxazole. Vibrio parahaemolyticus isolates showed resistance to Ampicillin, Cefoxitin, and Amoxicillin-clavulanate. Of these, the predominant antibiotic resistance type was to Ampicillin.

A closer look at Vibrio outbreaks
Another study, published in the journal Nature Microbiology, looked at Vibrio parahaemolyticus over a 17-year period in Shenzhen, China.

Contrary to the widely accepted notion that sporadic patients and independent point-source outbreaks dominated foodborne infections, they found most isolates from patients grouped into within-1-month clusters that were similar.

Analysis revealed the geographical heterogeneity of outbreaks and identified a coastal district as the potential hotspot of outbreaks and as the major source of cross-district events.

Researchers showed that despite the long time spans between clusters, many were closely related and could have come from a small number of common sources, which provides evidence that hidden persistent reservoirs generated most of the outbreaks rather than independent point sources. 

They said the findings present a different perspective on the major source of foodborne infections, which will inform future disease control strategies.

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