Scientists have found half of prawns sampled were contaminated with Vibrio in the United Kingdom but the strains of bacteria identified do not cause severe disease in humans.

Quadram Institute researchers studied Vibrio in prawns in the UK to understand the bacterium’s contribution to human disease and its resistance to antibiotics. Non-cholera vibrios are not a notifiable pathogen in the UK and surveillance programs do not actively test and analyze for it.

A total of 211 fresh and frozen prawn samples were collected at more than 200 shops between May 2018 and April 2019 and cultured for Vibrio species. Contamination was detected in 46  percent of samples, and multiple diverse Vibrio isolates were obtained from 34  percent of positive samples, according to the study published in the journal Microbial Genomics.

No major link between human Vibrio cases and prawns
Whole Genome Sequencing and further analysis showed differences between prawn-derived and cases of human-derived Vibrio genomes, suggesting prawns are not a major source of vibriosis cases in the United Kingdom. Genes associated with disease were not found in Vibrio parahaemolyticus in the study, indicating prawns did not carry pathogenic strains.

Researchers called for better future surveillance to protect the public against an increased risk of contamination driven by climate change, which could introduce more dangerous types of Vibrio into the UK as the vast majority of prawns are imported. Globally, Vibrio infections are rising and they follow a seasonal pattern attributed to an increase in sea temperatures.

Vibrio bacteria are common in estuary or marine environments. Most species are harmless but some can cause illness if eaten.

The high diversity of Vibrio within a single sample has implications for source attribution and outbreak investigation as if only a single isolate is tested per sample, it is possible that true transmission events will not be identified, according to the study.

Research was supported by the Food Standards Agency and Biotechnology and Biological Sciences Research Council (BBSRC).

Processing method impact
The proportion of samples testing positive for Vibrio was highest in raw full body prawns, followed by raw headless shell on and raw headless peeled. Of 62 cooked prawn samples, only five contained Vibrio.

Vibrio parahaemolyticus accounted for 83 of 130 of all Vibrio identified and was isolated from 66 of 211 prawn samples tested. Raw prawns were more likely to test positive for Vibrio than cooked ones and raw full body prawns were more likely to be positive than raw peeled prawns.

To combat viral and bacterial disease in production stocks, antimicrobials are commonly used in prawn farming for treatment and prevention.

Antimicrobial resistance genes were found in 77 percent of isolates, and 12  percent carried genes conferring resistance to three or more drug classes. Resistance genes were found mostly in Vibrio parahaemolyticus , though multiple resistance genes were also identified in Vibrio cholerae and Vibrio vulnificus.

“Through this survey, we found prawns purchased at retail in the UK carry a diversity of Vibrio species and antibiotic resistance genes. The survey has not only provided clarity into the current level Vibrio contamination in prawn products in the UK, it has created a framework to direct further efforts into protecting food safety as we move into the future,” said Dr. Nicol Janecko from the Quadram Institute.