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Pathogens in organic wastes: their levels and survival both during storage & following application to agricultural land

Project Code: B05003

31/03/2004

Direct Laboratories Ltd
Walters, L ; Hutchison, M; Moore, T;
SAC Veterinary Sciences Division
Synge, B;
Direct Laboratories Ltd
Avery, S

Background and Objectives

Strategies which prevent contamination of food during its production are likely to be effective in lowering the cases of food-borne gastroenteritis. This study addresses an obvious source of contamination of the human foodchain. On UK farms, animal manures are spread to land as a way to recycle the small amounts of nutrients contained in the waste material. Although some manures can provide a supply of nutrients for crops on some soil types, any potential fertiliser benefits are offset by the fact that UK livestock manures can contain pathogens such as E. coli O157, Salmonella and Campylobacter. This study aims to assess the risks to food safety of manure spreading by determining the incidence and levels of pathogens present in GB livestock manures, determining how long pathogens take to completely die off in manures during batch storage, and how long pathogens can survive after contaminated manures are spread to agricultural land for different seasons.

Approaches

A GB-wide study to determine the incidence and levels of pathogens present in animal manures.

Between 2000 and 2002, a total of 1549 manure samples were collected from cattle, sheep, poultry and pig farms in England, Scotland and Wales. The samples were examined using microbiological analyses methods which were custom-designed for this study. Results showed that British animal manures do contain measurable amounts of pathogenic microorganisms. Pathogens were isolated at incidences of 5.7% for E. coli O157, 6% for Salmonella, 11.9% for Listeria, 6% for Campylobacter, 3% for Cryptosporidium and 1.4 % for Giardia. There is a 1 in 3 chance that a manure sample collected from a British farm will contain at least one of the above pathogens. Livestock and manure management factors were found to influence some pathogen levels and prevalences. When straw bedding was a component of the waste lowered Campylobacter incidence and prevalence was measured. Young stock in livestock groups produced wastes with increased levels and prevalences of E. coli O157. Stock which consumed grass as a principal feed had significantly lower E. coli O157 and Salmonella levels in their fresh wastes; although it was not clear whether having access to pasture or consumption of grass caused the effect.

Die off of pathogens in manures that are stored

Pathogens that are present in manures will die off if the material is stored without adding any fresh waste. Tanks of slurry and heaps of solid manures were spiked with pathogens grown in the lab and these wastes monitored over time to determine how long it takes for the pathogens to die off completely. Pathogens died very rapidly in solid manure heaps. The temperature in some manure heaps rose to greater than 60°C within a few days and consequently less than one month of storage was required for total kill of all pathogens. Die off in slurries took longer. Although most pathogens had completely disappeared within four months, Cryptosporidium was able to survive for at least six months in liquid manures.

Die off in manures that are spread to land

Manures which had been spiked with pathogens were spread to a loam soil and also a grass pasture to determine if pathogens in contaminated manures survived long enough to contaminate crops or grass subsequently grown on the land. These experiments were repeated during winter and summer to determine if season influences pathogen survival. Generally, pathogens survived for longer during winter when temperatures were cooler, but the effect was not statistically significant. However, pathogens declined significantly faster when manures were left unincorporated on the surface of the land rather than being ploughed under. Pathogens were no longer isolated from soil four months after manure spreading.

Direct deposition of pathogens to land

Roughly 60% of the manures generated by British animals are from housed stock. The remaining 40% (~60 million tonnes of waste annually) are deposited directly onto land by grazing and foraging livestock. Experiments were carried out to determine how long E. coli - which is naturally present in faeces, could survive after being deposited directly onto land. The type of E. coli that was present was not pathogenic and occurred at higher levels than pathogens are typically found in wastes. Thus the results of these experiments gave a conservative indication of how long pathogens could survive after direct deposition to land. E. coli numbers returned to their background levels six months after livestock were moved out of the fields being monitored.

What it means and why it’s important

This study has shown than a significant proportion of British livestock manures contain pathogenic microorganisms. Often without storage, this waste is subsequently spread to land, which is used to grow crops or graze livestock destined for human consumption. Thus the common agricultural practice of manure spreading may be contributing to the increasing incidence of foodborne disease in the UK. The results of the manure storage and spreading experiments have shown that pathogens die off over time. The studies determined safe batch storage time intervals for fresh solid manures as three months to ensure complete die off for any pathogenic bacteria that are present. Slurries should be stored for at least four months if the pathogens are bacterial, or eight months if the wastes contain Cryptosporidium. At least six months should be left between the spreading of fresh or directly deposited manures and harvest of crops as long as the manure is left unincorporated. The results of these studies are currently being written into an easy-to-follow FSA guidance document that farmers can use to ensure that food produced on their farms is free from pathogen contamination

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