Aussie expert explains E.coli outbreak
Australian scientist Dr Robert Hall explains how health experts are in a race against time to find the source of the deadly German E.Coli outbreak.
The outbreak has already killed at least 17 people in Germany and Sweden and sickened 1614 in 10 European countries.
But while suspicion has fallen on raw tomatoes, cucumbers and lettuce as the source of the germ, researchers have been unable to pinpoint the food responsible for the illness.
“The German outbreak appears to be due to a Shiga toxin-producing Escherichia coli infection (E.coli O:104), which also apparently has some antibiotic resistance,'' Dr Hall said.
"From an immediate public health (as opposed to a clinical) point of view the antibiotic resistance is not of great significance. The damage caused by the infection is actually due to the Shiga toxin. It causes haemolytic-uraemic syndrome, which can be fatal and can require kidney dialysis.
"The DNA that codes for the toxin is spread by a virus, so the E.coli first needs to be infected by the virus for it to produce the Shiga toxin.
Dr Robert Hall is Senior Research Fellow in Epidemiology and Preventive Medicine and he was Director of Communicable Disease Control in South Australia. He was also Director of Public Health in Victoria.
"The immediate public health problem is the identification of the source of infection so that it can be controlled. Finding the source is done through a combination of epidemiological, microbiological and environmental investigations.
"These are all highly skilled tasks that need to be done rapidly and are nearly always done in a glare of publicity.
"An epidemiological investigation is essentially a two-stage process. A hypothesis on the cause is built, usually by interviewing people with the infection (cases). The interviews would cover all foods eaten by cases in the two weeks before onset of illness. There will be a large number of foods that are common to many of the cases.
"The second and more important stage is to test this hypothesis by looking for differences in the foods eaten between cases and people without disease in a case-control epidemiological study. This is a complex statistical process, and is the heart of disease detective work.
"Once a food or short-list of foods is available from a case-control study, then microbiological testing of the implicated foods can be done to further test the hypothesis.
"Also environmental investigation and trace-back of the food can be done to identify any risks, for instance likely contamination or cross-contamination. The trace-back should be comprehensive, from paddock to plate.
"It is common for the evidence found to be inconclusive or even contradictory. The intent is to collect evidence for rational and effective control measures. The process requires a high degree of specific scientific skills.
"In addition the public want to know what the source is and the process requires very good public communication.
"Training people in these skills is very important for the maintenance of public health. Investigations such as these are commonly needed, and greatly assisted by specialised public health agencies such as the German Robert Koch Institut, the European Centre for Disease Control, or the US Centers for Disease Control and Prevention.
"The situation in Japan brings up exactly the same issue as in Germany, and just shows how common such problems are, and how complex it can be with the global distribution of food. In Japan the E.coli strain seems to be O157, a much commoner strain than O104, but it is still producing exactly the same Shiga toxin as in Germany, which causes the same lethal disease.
"The reason for this is that the genetic controls for strain type and for toxin production are unrelated. Strain type depends on the coating of the cell, toxin production depends on machinery within the cell. And just to make it more interesting the toxin production depends on the E. coli itself being infected with a specific virus! This explains why the O157, O26, O111, O104 strains only sometimes produce the toxin.
"The problems in Germany and Japan, from a public health perspective, are the same for two reasons: the outbreak is of unknown origin in both countries and the Shiga toxin is the same in both outbreaks, despite the different strains.
"The investigative process is also the same. The outbreaks illustrate that both meat and vegetables are risks for this disease, and show just how important food safety is.
"There is a weakness in one of the proposed solutions I have seen, that of testing meat for contamination. The number of samples of meat you need to test, to be confident of zero contamination, is much higher than most people realise, and this is due to a simple law of statistics that cannot be worked around.”