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The levels of pathogens in abbattoir wastes
Project Code: B05008
Division of Farm Animal Science, Department of Clinical Veterinary Science, University of Bristol,
The scope of this study was small, being essentially limited to establishing which types of wastes from abattoirs are being applied to agricultural land, and what levels of the main foodborne pathogens they contain. Therefore, the study design was based on surveys rather than determining any mechanisms for the prevalence of foodborne pathogens.
The results of the study relate to 28 commercial abattoirs, selected so to be as representative as possible of the UK meat industry as a whole. Nevertheless, since the total number of abattoirs in the UK is above 400, one should be aware that the surveyed abattoirs represent only 6 to 7% of that number. Approximately 75% of the included abattoirs slaughter one or more red meat animal species (30% are single-species, 15% two-species and 30% three-species), and remaining 25% slaughter poultry only. The former group contains one abattoir slaughtering both sheep and poultry.
The abattoir wastes being applied on agricultural land comprise two main groups: “effluentbased wastes” and “animal-based wastes”.
The effluent-based wastes include three main types: separated solids, sludge and water. Approximately 68% of surveyed abattoirs apply one or more types of effluent-based wastes to land. Among these abattoirs, sludge-type wastes are the most common, and in one half of cases they are previously treated (dissolved air flotation, DAF) or untreated, while in another half they are mixed with blood, before disposal on land. Mixing sludge and blood is a regular practice at poultry-only abattoirs. Some of the above abattoirs also dispose separated effluent solids on land, either alone or as mixed with other types of wastes. Also, some of the above abattoirs dispose the water phase of the effluents to land, either after treatment in aerated lagoons or as untreated.
“Animal-based wastes” include two main sub-groups: digestive tract content-based and blood-based. All surveyed red meat abattoirs apply some of these wastes to land, and 37 such wastes were counted. Approximately 70% of these wastes are digestive tract-based including: a) lairage waste mixed with lorry waste, and/or with stomach content, and/or with some other wastes types; b) lairage-only wastes; and c) stomach content wastes. The remaining 30% of “animal-based wastes” comprise blood, either alone or together with some other component - such as sludge. All surveyed poultry-only abattoirs dispose some wastes to land; most commonly it is a mixture of blood and sludge (70%) or blood alone and sludge alone (15% each).
With respect to storage conditions and handling of the wastes at the premises, relatively clear differences between red meat abattoirs and poultry abattoirs were observed. At red meat abattoirs, lairage/lorry waste types are commonly stored as a manure/muck heap, in quantities ranging from a few tonnes to tens of tonnes, often for periods of only 0 to 2 days, or less commonly for much longer periods (from few months to two years). Blood based wastes at red meat abattoirs, where handled separately, are usually stored in a tank, in quantities of 5 to 10 tonnes, and for periods of 0 to 2 days. However, where blood is mixed with sludge and/or water, the quantities may be much larger (26 to 300 tonne), but commonly is not stored for longer than 3 days. If sludge is handled separately, it is usually treated and stored for much longer periods of time than other abattoir wastes. In poultry abattoirs, almost invariably, handling of the wastes is more simple and standardised than in red meat abattoirs. Regularly, blood or blood/sludge mixtures are stored in tankers (often mobile) in quantities of 10 to 60 tonnes for 0 to 3 days, and are entirely handled by contractors.
A microbiological survey of the abattoir wastes was conducted using methods specifically developed for these types of substrates. Generally, the methods performed well during their routine use, but could be considered as quite laborious, which may limit the total number of samples to be handled daily in the laboratory and they are relatively expensive. However, a large number of different types of wastes and their various mixtures are produced at abattoirs, and these variations may have caused variations in the performance (e.g. sensitivity) of the microbiological methods. In that respect, in this study, the methods for bacterial pathogens (recovering only viable forms) may have been affected more than the protozoan pathogens methods (viability not assessed).
Bacterial and protozoan foodborne pathogens were surveyed only in abattoir wastes being applied on agricultural land. The average incidence (from all wastes tested) of the most commonly isolated viable bacterial pathogen, Campylobacter, was 5.7% - but was higher in positive types of wastes: effluent from poultry abattoirs and lairage and blood from red meat abattoirs. Listeria monocytogenes was found only in 1.1% of all waste samples (4.2% in lairage waste), and not in any sample from poultry abattoirs. Salmonella and E. coli O157 were not isolated from any of the abattoir waste samples. A number of possible explanations for these relatively low levels of the bacterial pathogens in abattoir wastes exists:
A) The bacterial pathogens may have been shed only by a small proportion (and/or in low numbers) of slaughtered animals, and subsequently become “diluted” by mixing of these wastes with wastes from non-shedding animals, and/or blood, and/or water. This “diluting” could reduce the overall pathogens’ level below limit of detection.
B) Much of the microbiological sampling was conducted before the official end of the Foot & Mouth situation, during which time significantly increased amounts of both water and disinfectants were used daily at abattoirs. This practice would have both, “diluted” and eliminated a proportion of pathogens in abattoir wastes at the time.
C) A proportion of pathogens could have died off in stored wastes before sampling, which could decrease their counts to below the limits of detection.
The low incidence of bacterial pathogens in abattoir wastes did not allow analysis of between-abattoir, between-wastes, or between-season differences for these pathogens.
Since the two protozoan pathogens are not expected in poultry, they were not examined in samples collected from poultry-only abattoirs. The overall incidence of total Giardia and Cryptosporidium (viability not assessed) in red meat abattoir wastes was relatively high – around 50% and 40%, respectively – and the incidences were used to consider various waste type- and abattoir-type-related trends. The waste type most frequently contaminated with protozoan pathogens was lairage waste, followed by effluent. In lairage wastes from single-species abattoirs, the incidences of Giardia and Cryptosporidium were higher at sheep and pig abattoirs than at cattle abattoirs. Also, the incidences of both protozoan pathogens in lairage wastes at three three-species abattoirs were higher as the throughput was higher, and vice versa. On the other hand, the sampling season did not show any significant effect on either overall incidences of Giardia or Cryptosporidium or on their average total counts/g in abattoir wastes.
Due to the highly variable nature of abattoir wastes, and because numbers of samples tested (per type of waste and/or per abattoir) were relatively limited, a direct extrapolation of the above microbiological results to all abattoirs would have been difficult.
Globally, in simple terms, from the abattoir hygiene perspective, wastes should be not be stored on the abattoir premises, while from the agricultural land contamination perspective they should be stored at abattoirs as long as possible. To balance these opposing interests, more information is required on pathogens’ time/survival rates during abattoir storage and through treatment processes. If waste is to be stored at abattoirs, further research would be needed to address the issue of optimising and standardising the storage conditions so to minimise risks for both the meat and the environment, as well as to develop related control and monitoring mechanisms.
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