Salmonella and antibiotic residues found in meat in Vietnam

Over two-thirds of meat samples from HCMC were found to contain Salmonella bacteria, according to a new study by the Oxford University Clinical Research Unit (OUCRU).

Researchers purchased 117 samples of chicken meat, beef and pork from retail sites including supermarkets and wet markets across the city between October 2016 and March 2017. Eighty of those samples (68.4%) were found to contain non-typhoidal Salmonella.

“That’s not too different from previous studies published in international journals about Vietnam,” said Nguyen Thi Nhung, OUCRU microbiologist and first author of the study. “But it’s extremely high compared with the European Union,” she added.

Ms. Nhung pointed out that, from the European Union’s 2014 surveillance data, the prevalence of Salmonella in 25g meat samples was 2.26%, 0.62% and 0.23% for chicken, pork and beef. Respective figures in the OUCRU study, with sampling from HCMC, were 71.8% (chicken), 70.7% (pork) and 62.2% (beef).

If it is not cooked well, meat contaminated with Salmonella can cause gastroenteritis in humans. While most non-typhoidal Salmonella infections are self-limiting, some can turn severe depending on the number of Salmonella bacteria present.

“The high counts of Salmonella in some types of meat, especially chicken meat sold in wet markets, is worrying. We found that not only does the meat contain Salmonella – it contains a lot of Salmonella,” Ms. Nhung added. She found that there were about 1,500 Salmonella bacteria in each gram of meat from wet markets, and noted that more research is needed to understand the reasons for this high level.

OUCRU researchers also examined how resistant the Salmonella that was present in the meat samples was to antibiotics. The researchers grew the Salmonella isolates in the lab, and then exposed them to 32 different types of antibiotics. Up to 52.2% of Salmonella isolates were multidrug resistant, meaning they showed resistance to at least three antibiotic classes. The most commonly identified type of Salmonella was multidrug resistant Salmonella Kentucky ST198, with high levels of resistance against β-lactams and quinolones. One isolate from pork, in particular, proved resistant to colistin. It was the first time that bacteria resistant against colistin, a last-resort antibiotic, had been identified in meat in Vietnam.

People can become infected with these antibiotic-resistant bacteria when they handle raw meat or undercooked meat, eggs and vegetables. To avoid infection people should wash hands and prepare food well, including careful washing of raw vegetables.

OUCRU also worked with the National Institute of Veterinary Research and the Dong Thap Sub-Department of Livestock Production and Animal Health to conduct antibiotic residue screening.

Antibiotic residues in animal-derived foods such as meat, dairy and eggs may cause adverse health effects in the people who eat them, including allergic/toxic reactions, and upsetting the balance of normal healthy bacteria in the gut. They can also cause the emergence of antibiotic-resistant bacteria.

The research team tested a total of 357 meat samples (chicken, pork and beef) from wet markets and supermarkets from HCMC, Hanoi and Dong Thap.

They found antibiotic residues in 7.3% of samples, with wet market meat showing a higher prevalence (9.6%) than meat from supermarkets (2.6%). The difference between these two retail channels was statistically significant (p value of 0.016), showing the difference was not by chance.

Such results appear low compared with similar studies previously done in Vietnam, but antibiotic residue in meat is always considered unacceptable since it reflects non-compliance with antibiotic withdrawal periods.

Detailed results by OUCRU researchers and collaborators were published in a scientific publication of the International Journal of Food Microbiology.

This study was conducted as an extension of ViParc, a farm-based trial in Dong Thap from 2016-2020 aiming to help farmers in the Mekong Delta raise healthy chickens while using less antibiotics.