Drug-resistant salmonella spreads

The Daily Mirror has reported that “a potentially deadly strain of salmonella that is resistant to drugs has been identified in Britain, [and] the strain – called S. Kentucky – has spread to these shores from poultry imported from Africa and the Middle East”.

This news story is based on research that used surveillance data from France, England and Wales, Denmark and the US to track the spread of a type of salmonella, referred to as S. Kentucky, and to estimate the number of drug-resistant cases seen in those countries. Salmonella is generally treated with ciprofloxacin. The researchers sought to estimate the number of cases of S. Kentucky infection which are resistant, or do not respond, to treatment with ciprofloxacin and similar drugs.

This research highlights the increasing number of drug-resistant S. Kentucky infections in several developed countries. However, the absolute numbers of infections resistant to this drug are still quite low: about 60 cases overall in England and Wales in 2008. Individuals should be aware of the symptoms of salmonella infection, and always be sure to wash fruit and vegetables before eating them. Read our Live Well articles on food safety to find the best way of preparing food.

Where did the story come from?

The study was carried out by researchers from the Pasteur Institute in Paris, the World Health Organization, the Technical University of Denmark, the French National Institute for Agricultural Research, the Health Protection Agency, the Staten Serum Institute in Denmark, the US Centres for Disease Control and Prevention, the Pasteur Institute of Morocco, the University of Ibadan in Nigeria, the French Agency for Food, Environmental and Occupational Health and Safety and the French Sanitary Surveillance Institute. The research was funded by the same organizations.

The study was published in the (peer-reviewed) Journal of Infectious Diseases .

Generally, the media reported the story accurately. However, reports by the Daily Mail emphasise that infections in Britain have trebled recently, which, while accurate, does not point out that absolute numbers of infections are still low.

What kind of research was this?

This was a cross-sectional analysis of national salmonella surveillance systems from France, England and Wales, Denmark and the US. These countries track salmonella infections, and collect information on those infected such as sex, age and international travel history. They also collect samples and isolate the specific salmonella strain for further examination.

The researchers focused on a specific salmonella type, S. Kentucky, and aimed to estimate how many infections with a drug-resistant strain of S. Kentucky had occurred between 2000 and 2008 in several developed countries, including England and Wales.

What did the research involve?

Researchers collected surveillance data from laboratories in France, England and Wales, Denmark and the US and analysed samples of the isolated salmonella strains. The samples were tested for antimicrobial resistance, and the type of salmonella was determined through DNA analysis.

The collected data was analysed using appropriate statistical methods.

What were the basic results?

All data are of salmonella cases reported between 2000 and 2008.

France

In France, 497 cases of S. Kentucky were reported, representing 0.5% of all salmonella cases reported in France during that time. There was an increase in the number of cases reported in recent years, with 139 cases reported in 2008 compared to 24 cases in 2000 and 54 cases in 2006. The first resistant French case was detected in 2002 in a tourist returning from Egypt. Cases resistant to the drug ciprofloxacin made up 40.2% of the reported cases of S. Kentucky. Individuals infected with the drug-resistant strains of S. Kentucky were more likely to have been hospitalised than those infected with the strains that respond to drugs.

England and Wales

In England and Wales, 698 cases of S. Kentucky were reported, representing 0.6% of all the salmonella cases reported during that time. Of these cases, 35% were resistant to the drug ciproflaxacin. The first resistant case in England and Wales was reported in 2004, and the resistant strain made up 50% of all reported S. Kentucky cases in 2008.

Denmark

In Denmark, 114 cases of S. Kentucky were reported, representing 0.2% of all salmonella cases reported during that time. The first drug-resistant Danish case was reported in 2002, and made up 56% of all S. Kentucky cases in 2008.

US

In the US, 679 cases of S. Kentucky were reported, representing 0.2% of all salmonella cases reported during that time. No drug-resistant cases were reported during that time.
Of the 307 identified cases of drug-resistant S. Kentucky for whom travel information was available, 89% of the individuals had travelled internationally in the two weeks before they became sick. Most reported travelling to north-eastern and eastern Africa, North Africa, West Africa and the Middle East.

How did the researchers interpret the results?

The researchers concluded that the increase in S. Kentucky cases found in France, England and Wales and Denmark were due mainly to increases in the number of cases resistant to the drug ciproflaxacin.

They say that the drug-resistant strain originated in Egypt, but is now found throughout Africa and the Middle East. Farms and food producers that are in these regions are likely to have reservoirs of drug-resistant S. Kentucky, and cases appearing in European and North American countries are likely to be due to the importing of contaminated food in addition to travel.

Finally, the researchers say that increased awareness by national governments and international organisations is needed to monitor and limit the spread of the drug-resistant S. Kentucky strain.

Conclusion

This was a well-conducted study of global salmonella surveillance data, which brings to light the increasing number of cases of a certain type of drug-resistant salmonella infections.

Concerns about the overuse of antibiotics have been growing in recent years. Overuse in food production and healthcare is thought to lead to increasing antibiotic resistance. The researchers say that the S. Kentucky strain of salmonella is closely linked to poultry, and that the observed increases may be due in part to the increased use of antibiotics in poultry production in Africa. The researchers say other sources of drug-resistant S. Kentucky strains may include imported fruits and vegetables, as well as shellfish, as water contamination has been reported. People should continue to wash fruits and vegetables before eating them.

It is important to remember that this study looked at just one type of salmonella, S. Kentucky, which only makes up a small proportion of reported salmonella cases. While the number of drug-resistant S. Kentucky cases in England and Wales does appear to be rising, the number of infections is still low.

Surveillance data is an important source of information for tracking diseases. However, comparing data between countries should be done cautiously, as each country collects and analyses samples differently. The researchers say that differences in testing for drug resistance may lead to an underestimation of drug-resistant cases. For instance, the US tested approximately 5% of their salmonella cases for drug resistance, compared to approximately 75% in Denmark, 99% in England and Wales, and 100% of certain subtypes (including S. Kentucky) in France.