Risk assessment concerning health risk from the spreading of slurry
Report no: 2009: 29
VKM has carried out a risk assessment concerning a health risk from the spreading of slurry under Norwegian conditions, with special focus on potential transfer of Salmonella, Cryptosporidium, Campylobacter, Escherichia coli and Yersinia enterocolitica to humans, either directly or indirectly via ingestion of water, plants or animals.
Microorganisms present naturally in slurry originate mainly from farm animal’s intestinal flora, hence are intestinal pathogens rather than respiratory pathogens. Of these potential pathogens,
Salmonella seldom occurs in Norwegian farm animals. Campylobacter and verotoxin producing E. coli (VTEC) are generally found routinely in farm animals but the occurrence of EHEC (the humanpathogenic group of VTEC) commonly found in human infections (e.g. O26, O103, O111, O145 and O157) is very low.
However, the presence of EHEC in animal faeces cannot be excluded. Pigs are the main reservoir for Yersinia enterocolitica and the parasites Cryptosporidium and Giardia are also found routinely in farm animals. It can therefore be assumed that slurry from cattle and pigs can potentially spread EHEC, Campylobacter, Yersinia and both parasites while the transfer of Salmonella is less likely.
Exposure to slurry is depending on the following factors that risk reducing measures must to take into account:
Method of spreading
Generally there are two methods used to spread slurry onto pasture;
- Trailing hose - the spreader boom has hoses connected to it which distribute slurry close to the ground in strips or bands or Injector - slurry is injected into the soil. In both cases exposure to the air and consequently the risk of spreading the contamination very low
- Broadcast spreader - slurry is forced under pressure through a nozzle, often onto an inclined plate (splash plate) to increase the lateral spread. Exposure to the air is great and spreading of slurry less controlled.
Meteorological conditions e.g. relative humidity, temperature, and air velocity and turbulence. Potential for aerosol formation is lowest in conditions of high humidity and low temperature and highest in dry, warm weather. At low wind speed aerosols would not be spread widely but can contaminate the immediate surrounding area to almost the same level as in the source of contamination.
Strong winds and turbulence would increase dispersion but also the evaporation rate of droplets that are reduced to small residual nuclei that can be spread over longer distances.
Timing of spreading
The risk for indirect infection via the contaminated produce is increasing if spreading occurs close to harvest. Infection due to spreading of slurry can occur:
- Directly by (i) large aerosol particles falling in the source area affecting humans nearby (primarily the farmer spreading slurry) or (ii) by small residual nuclei of aerosols spread widely and subsequently inhaled or ingested
- Indirectly by contaminating water or produce close to source area.
There are no known outbreaks of infections caused by slurry spreading in Norway, and significant sized outbreaks appear unlikely. However, the role of slurry in the epidemiology of sporadic infections is unknown and an association cannot be excluded.