B17002: Assessment of the risks to food safety associated with spreading of animal manure and abattoir wastes on agricultural land

Study Duration: May 2001 to August 2002

Contractor: WRC-NSF Ltd

Traditionally, animal manure has been used as a fertiliser, along with certain wastes from abattoirs, such as 'gut contents' and blood. Whilst the fertiliser and soil conditioning value of these products is undeniable, the impact that the pathogen content of these products may have on food safety is now being questioned.

Farm animals are known to be the source of many of the pathogens that are of concern to food safety and the health of livestock. In the past salmonella was the main concern, particularly in poultry waste. More recently, other enteric bacterial pathogens such as campylobacter and E. coli O157 have increased concern about the potential routes of transfer from animals to food. The use of manure and abattoir wastes as fertilisers may represent one of the routes of infection.

Pathogens such as E. coli O157 and campylobacter are known to be present in high numbers in the faecal waste of farm animals along with protozoan pathogens such as cryptosporidium, which have recently become of concern in sources of drinking water derived from surface waters.

The practice of using animal wastes as a fertiliser may therefore pose risks to food safety by direct contamination of crops and livestock or more indirect routes such as the contamination of water sources. This may be of particular relevance to E. coli O157 where infected animals may show no symptoms of disease but still be infected and so excrete large numbers of the pathogen.

Investigation of all the potential routes of transfer of pathogens from these wastes to food crops and livestock is impractical, because of the wide spectrum of micro-organisms which could be present and the analytical difficulties of quantifying them in the various environments involved. Environmental factors such as temperature, rainfall and soil type will also influence the survival characteristics of differing types of pathogens making the derivation of representative survival rates almost impossible.

Microbiological risk assessment (MRA) offers a cost-effective option. MRA can be envisaged in three stages, referred to as load, pathway and receptor. In the context of this project 'load' is the number of each of the pathogens likely to be present. 'Pathway' is consideration of the routes that can bring about transfer to the 'receptors', which in this project are food crops and livestock. Pathways may serve to increase or decrease the load of pathogens, depending on whether treatment processes or storage or environmental disposal are involved. MRA assesses these on the basis of existing knowledge of the reaction of different types of pathogens to differing environments.

Good information on the load of pathogens in manure and abattoir waste is vital to the completion of MRA and the use of this data and other information gathered elsewhere would be fed into this project.

Successful completion of this project will provide a tool for assessing risks to food safety posed by the existing waste disposal strategies used on farms. It should also be possible to predict the effect of changes in disposal strategies.

The objective of this study is to develop a risk assessment of the risks to food safety from pathogen contamination through spreading of animal manures and abattoir wastes on agricultural land. The approach used will be to construct a conceptual model defining the use of these wastes and the pathways through which humans and livestock could be exposed to specific selected pathogens. The risk assessment framework will be defined as event trees. Loadings of pathogens in the manures derived from a variety of farm sources will be modelled. Information on the reductions or increases in pathogen numbers which can occur during collection and storage of the wastes will be obtained from published data. In addition, data on the decay/survival of the selected pathogens after application of the wastes to agricultural land will be used to model pathogen exposures on the crops at the point of harvest. Finally, the feasibility of quantifying the impact of the use of manures on the health of farm animals will be assessed.

Objective 01
The approach taken to achieve the overall objective will be basically similar to that used to carry out a microbiological risk assessment (for the UK Water Industry Research) of the risks to food crops at the point of harvest resulting from the application of sewage sludge to agricultural land. In this respect, the following pathogens will be studied: verotoxigenic E. coli, salmonellae, campylobacter, listeria, giardia, and cryptosporidium. Also, the risks to seven crop categories will be studied. These are:

• Fruit - includes top fruits, stone fruits, vines, hops and nuts
• Salad crops - ready to eat crops including lettuce, carrots and garlic
• Vegetables - includes potatoes, rhubarb and artichokes
• Horticulture - includes soil-based glasshouse and polythene tunnel production
• Combinable and animal feed crops - wheat, barley, oats and rye
• Silage - cut grass and cut maize
• Grazing land - includes grass, swedes, fodder and turf production

Objective 02
Formulate the conceptual model, which identifies the key sources and pathways of pathogen exposure. Define framework in the form of an event tree for each pathway.

Objective 03
Obtain the best possible data on pathogen loadings and pathway reductions required by the model to simulate exposure to crops and livestock.

Objective 04
Predict the added risk of specific pathogens being present due to manure and abattoir waste disposal strategies.

Objective 05
Determine the feasibility of estimating the health risks to farm animals of the land application of animal manures and abattoir.

Event tree models for predicting the pathogen loading on crops at harvest have been developed for slurries and farm yard manure (FYM). So far these models have been used to predict the fate of salmonella, cryptosporidium and E. coli O157. Where data is available (see below), this work will be extended to include slurry and FYM from pigs, sheep and poultry.

However, the work done so far has identified several gaps and shortcomings in the data that are influencing the validity of the risk assessments.

The data for the pathogen loadings in the faecal material from different animals is either incomplete or none have so far been identified. Of particular concern are data for the pathogen loadings of pig faeces. Similarly, there is a general lack/absence of data on the prevalence and concentration of listeria in the faeces of all the animals being considered in this study. This may be because listeria may only be present in animals fed on silage during the winter months.

The risk assessment modelling carried out so far has shown the importance of decay rates in determining the efficiency of the farm disposal route. More detailed information on the decay rates of the pathogens in stored manures and after disposal to land is required. Ideally this data should be comprehensive enough to allow decay rates to be determined. As stated previously, data of this type is difficult to obtain for manures applied to land because of the range of environmental factors that could influence the decay rate. However, further information on the effects of storage on farm (slurry tanks and FYM heaps) is also required and studies of this type are easy to control and carry out.

Of particular concern is the lack of data on the decay rates of pathogens in manure from layers and broilers both during 'on-farm' storage and after disposal to land.

The final report is available from the Agency's Information Centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk)

Contact: For any enquiries concerning this research project, please contact the relevant Programme contact or email science@foodstandards.gsi.gov.uk