Routes by Which Pathogens Associated with Livestock Slurries and Manures may be Transferred from Farm to the Wider Environment

Progress: Livestock manures are a valuable source of crop available nutrients, but can also contain pathogenic micro-organisms which may reach the water and air environments. In order to manage these risks, it is necessary to assess pathogen sources and potential loss pathways, so that practical advice on reducing the risks of transfer can be developed.
An initial desk-study reviewed data on the movement of pathogens from animal housing, manure storage systems and following landspreading and grazing, to the water and air environments. Experimental work then sought to examine the routes and processes involved in the transmission of pathogens during and following landspreading, and to quantify losses of viable pathogens to the water and air environments. A non-pathogenic strain of E. coli was used as a marker to track both movement to water and via aerial dispersion. <P>
Transport to water: Work on pathogen transport to water was carried out using hydrologically isolated plots, where the aim was to study ‘worst-case’ risk situations following the landspreading of manures and during cattle grazing i.e. on heavy-textured underdrained soils, where ‘by-pass’ flow was likely to provide a rapid and important loss route. Also, small plots and soil monoliths were used to examine in more detail the effects of soil type on pathogen transfer routes to surface and groundwaters. <P>
The behaviour in slurry of the E. coli marker organism was similar to that of generic E. coli, however, the marker was less robust than generic E. coli when introduced into the soil environment. The marker organism was detected in drainage and surface waters where rainfall events generated drainflow/runoff a few days after manure application, but not where drainage occurred after a longer time period (i.e. weeks). This indicated that the marker was not able to survive for long periods in the soil and was only suitable for use as a short-term marker of pathogen movement (most of the field experiments used the K12 marker organism).
Drainage waters from fields which were being grazed by cattle or where manures had been recently applied contained E. coli concentrations which exceeded the EC limit for bathing water quality (2000 colony forming units-cfu/100ml). Also, E. coli was detected in surface runoff samples collected during grazing and following recent manure application.
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The soil monolith experiments indicated that there was little vertical movement of slurry-borne pathogens in free draining soils, where rainfall volumes immediately after slurry application were low.
Transport to air: The transmission of pathogens via aerosol dispersion downwind of a landspreading source was studied by collecting aerosol particles on agar plates, using specialist portable air sampling units. Some
short-range dispersion modelling was also undertaken. The measurements showed that aerosol-borne pathogens generated during the landspreading of cattle slurry and dirty water using a splash-plate applicator, could travel at least 200m at wind speeds of 2 m/s and over 400m at wind speeds of 4.2 m/s. Projections from these data indicated that some pathogens could be transported at least 1500m during slurry spreading.
Risks of pathogen transfer: Information from the desk-study and experimental work was used to estimate the relative risks of pathogen transfer from manures/excreta to the water and air environments. The highest risks to water from the landspreading of manure were assessed to be direct application into watercourses and where drainflow or surface runoff occurred from ‘wet’ soils within 7 days of landspreading. Similarly, the highest risks to water during grazing were assessed to be the direct deposition of excreta into watercourses and where drainflow or surface runoff occurred during or soon after the end of grazing. The most important point-sources identified were uncontained runoff from farmstead hardstanding areas and woodchip corrals, and runoff from 'wet' farmstead middens. The highest risks from aerosol transmission were assessed to be from slurry/dirtywater spreading using rainguns and vacuum tankers with splash-plates, particularly where wind speeds were high and fresh slurry was being spread. It was not possible to make absolute risk comparisons between the different loss routes, but it appears likely that the risks of transfer are considerably higher via water (particularly from farmstead hardstandings, following manure spreading and during livestock grazing) than air, and that the risks via other vectors (e.g. wild animals and flies) are relatively low. <P>
Practical recommendations: Potential pathogen control measures to minimise the risks of pathogen transfer from livestock manure management systems to the water and air environments were reviewed. These included the manipulation of diet and dietary additions, minimum storage periods (typically 90 days) for slurry and solid manures, avoidance of recontamination of stored manure, slurry treatment, solids composting, landspreading methods and timing, management of grazing livestock in the field, management of farmstead runoff and general good practice. The effectiveness and reliability of the suggested measures to reduce microbial pathogen loads was assessed, both in terms of maximum potential reduction (from one to six log10 reduction) and reliability (i.e. is the process likely to be easily controllable under typical farm conditions). For handled manures, measures associated with extended storage periods or the treatment of solid manures and slurries generally gave the best reductions in pathogen loads, ranging from two to six log10 reductions, and represented the most practical and cost-effective options to reduce the risks of pathogen transfer during manure spreading to the water and air environments. In terms of field management, the avoidance of spreading manures directly into surface water systems, fencing of streams/watercourses and provision of bridges to prevent direct livestock access, and avoiding manure spreading and livestock grazing when soils are wet and drainage/runoff is likely, were assessed to be the most effective measures. In the case of point-source loss routes to surface water systems (e.g. from hardstandings, woodchip corrals etc.), investment in runoff minimisation (e.g. roofing of hardstandings) and collection/storage systems were likely to be the most cost-effective approaches. <P>
Recommendations for future research: The highest priority for future research was identified as a need to improve our understanding of the processes and pathways through which pathogens are lost from farming systems to the water environment, particularly surface water systems, to provide robust data to inform agricultural mitigation measures to ensure compliance with the EU Bathing Water Directive. In parallel with this work, there is a need to synthesise our knowledge on pathogen loss routes from agricultural systems within a modelling framework, and in particular microbial transport in soils via vertical mechanisms and by-pass flow. There is also a need for further work on pathogen transport in aerosols (e.g. the effects of manure type, spreading technique and climatic conditions), as both the literature review and preliminary results from this project show that there are potential human infection risks from aerosols generated during the landspreading of manures.
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