The E. Coli bacteria behind the outbreak in Europe that has left 17 people dead and sickened at least 1,500 others, including a third with kidney failure, is a new strain not seen before, the World Health Organization told the media earlier today, Thursday. Experts suggest the new strain is a combination of two aggressive strains, that we may never find the source, and the real shape of the outbreak is yet to reveal itself.

The outbreak has been associated with cucumbers and other vegetables in Germany, although the authorities are still trying to trace the source.

Reports coming in today suggest that more than 1,500 people have been infected, with 17 deaths reported, one in Germany and one in Sweden.

Preliminary genetic analysis shows the strain is likely to be a mutation of two distinct strains of E. coli: enteroaggregative and enterohemorrhagic E. coli (EAEC and EHEC).

A WHO food safety expert told the Associated Press that the new strain has never been isolated from patients before and has several features that make it more toxic and virulent than the many strains of E. coli we normally carry around in our guts.

It is not unusual for bacteria to swap genes, and even to cross over from one species to another.

It appears in this case that one particularly aggressive strain has acquired a toxin from another aggressive strain, and the result is an even more aggressive bacterium, producing two nasty toxins.

Dr Paul Wigley, a senior lecturer in infection biology at Liverpool University’s School of Veterinary Science in the UK says that while most strains of E.coli don’t cause disease, some, like the new one, produce toxins that damage the gut, causing bloody diarrhoea, and can also damage other parts of the body, including the the kidneys.

“The most serious consequence of infection is Haemolytic Uraemic Syndrome (HUS) that frequently leads to kidney failure resulting in the need for kidney dialysis or sometimes death,” says Wigley.

So far, Germany has reported 470 cases of HUS, including 16 deaths. Sweden has also reported 15 cases and one death, Denmark 7 cases, The Netherlands 3 cases, the UK 3 cases, and Spain 1 case, according to the BBC.

Speculating about the possible source of the bacteria, Wigley says that animals, and cattle in particular, can carry the more dangerous forms of E. coli in their guts without becoming ill themselves, and shed it in their faeces.

His guess is that the source is manure used as a fertilizer on farms that produce organic salad vegetables, including cucumbers.

The Food Standards Agency in the UK says so far there is no evidence that produce from possible sources identified so far has been distributed in the UK.

The four new cases in the UK, three with HUS and one with bloody diarrhoea, are all people who probably got infected in Germany.

The HPA is reminding people in the UK intending to travel to Germany to avoid eating tomatoes, cucumbers and leafy salad, including lettuce, especially if they are going to northern Germany, until further notice.

They also urge anyone coming back from Germany who feels ill, especially if they have bloody diarrhoea, to see a doctor and tell them where they travelled recently.

A mysterious feature of this outbreak is that the number of cases of HUS is predominantly young women. Normally HUS outbreaks affect children and older adults.

Dr Dilys Morgan, head of the gastrointestinal, emerging and zoonotic infections department at the HPA, told the BBC this outbreak is “alarming”, and appears to be caused by a very “rare organism”.

Health officials still don’t know why this outbreak is affecting adults, and women in particular. One reason could be that there are lots of much milder cases that aren’t being reported, and another could be that the new strain has particular properties that affect this population more.

Dr Paul Hunter, a professor of health protection at the University of East Anglia in the UK told the Associated Press that because this is a new strain, there may be a delay factor in the reporting anyway. He suggests we may see more cases in the next few days, as the incubation period is expected to be between 3 to 8 days and most people likely to recover within 10 days.

It is possible that the source of the infection will never be found, especially since the produce affected has a short shelf life, narrowing the window of opportunity for the search.

However, Hunter reckons there could be a secondary wave, caused by humans infecting each other.

In the meantime, experts advise people to be scrupulous about kitchen hygiene and thoroughly wash all salads, raw vegetables and fruit before eating, even peeling or cooking them to be on the safe side, and wash hands well with soap before handling food, and regularly at other times to prevent person-to-person spread.

Sources: Associated Press (London), BBC, HPA (UK), University of Liverpool, WHO.

Written by: Catharine Paddock, PhD