Image caption Most Salmonella infections are mild

A strain of Salmonella resistant to the most powerful antibiotics has been found in the UK, France and Denmark.

The outbreak emerged in Africa then spread to Europe, picking up antibiotic resistance along the way, says a team of international researchers.

They are calling on health officials to step up monitoring to stop the "superbug" spreading globally.

Cases have grown from a handful in 2002 to 500 worldwide in 2008, they report in The Journal of Infectious Diseases.

Co-researcher Dr Simon Le Hello of Institut Pasteur in Paris, France, said: "We hope that this publication might stir awareness among national and international health, food, and agricultural authorities so that they take the necessary measures to control and stop the dissemination of this strain before it spreads globally, as did another multidrug-resistant strain of Salmonella, Typhimurium DT104, starting in the 1990s."

Most of the millions of Salmonella infections a year are not serious, causing only mild stomach upsets. Occasionally, however, particularly in the elderly or in people with weakened immune systems, they can be life-threatening and may need treatment with antibiotics.

We recommend people follow some basic food safety rules: wash hands properly and keep them clean, cook food thoroughly, chill foods properly and avoid cross-contamination Spokesperson, Food Standards Agency

The strain, known as S. Kentucky, has developed resistance to the antibiotic Ciprofloxacin, often used for treating severe Salmonella cases.

French researchers started monitoring the strain after noticing a handful of cases in travellers returning from Egypt, Kenya and Tanzania.

Data from other countries suggests S. Kentucky arose in poultry in Egypt in the 1990s, and then spread to farm animals in various parts of Africa and the Middle East.

Although the first infections outside Africa seem to be in international travellers, more recent cases seem to have been acquired in Europe, perhaps through contaminated food, say the researchers. Cases have been seen in England, Wales, Denmark and France.

Hygiene care

The Health Protection Agency said there had been 698 cases of S. Kentucky from 2000 to 2008, 0.6% of all Salmonella cases reported in England and Wales .

Just 244 of the cases had resistance to ciprofloxacin.

Altogether, there are around 13,000 cases of Salmonella each year. Infections are only treated with antibiotics when they become invasive, and there are a number of drugs which can be used.

A spokesman said: "The number of cases we have been seeing in England and Wales has shown some increase but remained fairly stable over the latter part of the study.

"Care should be taken with hand and food hygiene particularly while travelling as over 50% of S.Kentucky cases reported in England and Wales are known to be travel-related."

A spokesperson from the UK's Food Standard's Agency (FSA) said human Salmonella infections are rarely treated with antibiotics, and cooking food thoroughly will destroy any bacteria irrespective of whether the organism is resistant to antibiotics or not.

He added: "As part of the FSA strategy to reduce foodborne illness we recommend people follow some basic food safety rules: wash hands properly and keep them clean, cook food thoroughly, chill foods properly and avoid cross-contamination.

"These principles, which are designed to reduce the risk from pathogens, such as Salmonella, are equally applicable whether these pathogens are resistant to antimicrobials or not."