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To Develop a Cost Effective and Practical Method to Reduce E. Coli O157 Infection in Cattle Prior to Slaughter


Escherichia coli bacteria that produce the poison (verocytotoxin) are called verocytotoxigenic E. coli (VTEC) and cause a variety of disease symptoms, and potentially death, in humans. The very young and old are especially vulnerable. Food-producing animals, mainly cattle and sheep, are carriers of these bacteria and many ways have been identified for humans to acquire infection from carrier animals. Contaminated food, water, or direct contact with infected animals or their environment have all led to outbreaks or isolated cases of human infection. The most important type is E. coli O157:H7 which is harmless in animals. Of the cattle positive for E. coli O157:H7, a minority excrete large numbers of the bacteria in their faeces and these are considered to be essential for the transmission and maintenance of infection. Considerable research on this bacterium has been carried out since its discovery in 1983, and most methods to minimise the risk of human infection have focused on preventing food-borne transmission from animals to humans. However, it is accepted that control within animals would offer a better approach to prevent human infection, by eliminating both food-borne and direct infections. However, there is currently no method being applied to control in animals. <P>

Previous work carried out by SAC has identified that the major site where E. coli O157:H7 colonises cattle is the surface of the rectum. The organism can be entirely restricted to this site or the levels sufficiently high, relative to the rest of the gastrointestinal tract, that removal of bacteria here will result in a massive reduction in numbers passed in the faeces. This has been demonstrated in Defra project OZ0712 using the administration of an antiseptic to kill the bacteria in the rectum. The potential impact of any intervention that eliminates high-level faecal shedding in groups of cattle has been measured by mathematical modelling. It is thought that successful prevention of the maintenance of E. coli O157:H7 in the cattle population can be achieved by treating the colonised and high-shedding animals.

In this work it is proposed to study commercial farms where cattle are brought from different sources. From our knowledge of prevalence rates of E. coli O157:H7 these groups should include carrier animals. The likelihood of the organism continuing to circulate within a group is likely to be linked with the number of positive animals and the levels of bacteria being passed in their faeces at the start of a housing period. Therefore, when a group of animals is brought-in, treatment of all animals will be used to reduce or eliminate faecal shedding so as to reduce the likelihood that the organism will colonise new animals. In this study, finishing beef cattle on commercial farms will be screened for E. coli O157:H7 and the impact measured of this antiseptic treatment of the rectum to reduce the prevalence rate and shedding levels of E. coli O157:H7 in each of three consecutive winter housing periods. The ability of a rapid method for identifying high-level shedders in the field will also be assessed in the trial. If successful, this may be used in subsequent trial periods as part of an enhanced intervention strategy to target high-shedders.

It is also important to estimate the relative impact of the ways the organism may pass between animals so as to improve the field trial and to optimise the treatment for use in other cattle farming systems. It is anticipated that the two main routes of transmission of the bacteria in a group of cattle will be either by direct contact (via external body surfaces) or via environmental surfaces contaminated with faeces containing E. coli O157:H7. The intervention treatment will reduce or eliminate faecal shedding prior to animals entering their winter housing environment, but bacteria on external body surfaces will not be affected. It is therefore proposed to test if the direct contact transmission route is insignificant relative to the environmental transmission route. Therefore, the proposal includes two experimental designs, based upon our established experiments, which will determine the relative contribution of these transmission routes. One of these experiments will be conducted both indoors and outdoors. Electronic proximity loggers will be worn by the animals to quantify the rates of between animal, and animal-contaminated pasture contacts, and relate these to rates of transmission.

In summary, the field trial will use a simple on-farm intervention treatment to reduce the prevalence of a major human pathogen in its most important animal host. The design of supporting experiments will also allow an assessment of some of the likely means to apply the intervention strategy in different husbandry systems. This work has the potential to be of considerable benefit to public health by providing a means of reducing the >1,000 human cases of E. coli O157:H7 infection seen in people each year in UK.

Scottish Agricultural College
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