Vaccinating cattle against E. coli O157 could cut the number of human cases by 85%, according to scientists.
The study used veterinary, human and molecular data to examine the risks of E. coli O157 transmission from cattle to humans, and to estimate the impact of vaccinating cattle.
Risk of E. coli O157 infection is significant when the cattle are 'super-shedding' – excreting high numbers of bacteria in their faeces for a limited period of time.
The stx2 variant of the Shiga toxin gene is likely to be a critical factor in the shedding phenotype and appearance in human clinical isolates and supershedding of the pathogen by cattle seems to heavily influence risk to humans.
Cattle are the main reservoir for E. coli O157, and vaccines for cattle now exist. One in Canada where it is fully licensed but with little uptake by farmers, and two in the US where restricted licenses permit only limited use.
However, adoption is being delayed by conflicting responsibilities of veterinary and public health agencies, economic drivers, and because clinical trials cannot easily test interventions across species boundaries and a lack of information on the public health benefits, said the researchers.
The findings confirm that a vaccine based on-farm prevention program would be a logical addition in the fight against the pathogen, said the Canadian Food Safety Alliance (CFSA).
“This latest study is further evidence that vaccinating cattle could drastically reduce E. coli O157 human infections and the related healthcare costs in Canada,” said Bliss Baker, CFSA spokesperson.
“This is the only country in the world with a fully licensed vaccine for the immunization of cattle that reduces the shedding of the dangerous E. coli O157 pathogen at the source.”
Cattle vaccines reduce the frequency of shedding by around 50% but how this translates into the reduction in human cases requires investigation of the cross-species transmission step to identify the factors promoting transmission across the species barrier.
Vaccination and shedding
Matthews et al simulated the effect of vaccination on shedding frequency and concentration in cattle and predicted the reduction in human cases based on the frequency of shedding above the threshold of 1,300 cfu/g feces(the threshold of human risk).
In their simulations, eliminating just the 12% highest shedding densities produces a 50% drop in the frequency of shedding in cattle but an 83% drop in human cases.
Vaccines exist that can reduce super-shedding and the researchers predict that using them could reduce human cases by nearly 85%, higher than the 50% predicted by studies looking at the efficacy of current vaccines in cattle.
Lead author, Dr Louise Matthews, senior research fellow in the Institute of Biodiversity, Animal Health and Comparative Medicine said E. coli O157 is a serious gastrointestinal illness.
“The economic impact is also serious – for instance studies in the US suggest that healthcare, lost productivity and food product recalls due to E. coli O157 can cost hundreds of millions of dollars each year.
"Treating cattle in order to reduce the number of human cases certainly makes sense from a human health perspective and, while more work is needed to calculate the cost of a vaccination programme, the public health justification must be taken seriously."
Controlling the problem
In Scotland, an average of 235 culture positive cases of E. coli O157 infection per year (people who had the organism in their stools) were notified to Health Protection Scotland from 2008 to 2012.
Reducing occurrence in cattle is one route to control, but a lack of effective interventions before slaughter means that control currently relies heavily on good hygiene practices by food producers and individuals in the domestic kitchen.
Several preslaughter interventions have been tried, including altered diets, adding probiotics to feed, spraying cattle with bacteriophage, and vaccination, with the latter proving most effective.
Source: Proceedings of the National Academy of Sciences of the USA
Online ahead of print, doi: 10.1073/pnas.1304978110
“Predicting the public health benefit of vaccinating cattle against Escherichia coli O157”
Authors: Louise Matthews, Richard Reeve, David L. Gally, J. Chris Low, Mark E. J. Woolhouse, Sean P. McAteer, Mary E. Locking, Margo E. Chase-Topping, Daniel T. Haydon, Lesley J. Allison, Mary F. Hanson, George J. Gunn, and Stuart W. J. Reid