Information describing the risks associated with the drifting of human pathogens contained inside aerosols during manure spreading is sparse. There are a small number of papers that have researched the issue. A seminal study by Sorber et al. (1984) found that more aerosols were generated using a rain gun-type spreader when compared with that of a vehicle-mounted sprayer. The authors believed that the height from which the waste was released was a key consideration. Irrespective of waste release height however, the Sorber study concluded that it was difficult to recover microorganisms from the air. Although it used a thicker sludge, a later study by Pillai et al. (1996) confirmed an apparent low risk associated with its disposal and only sporadic detection of aerosolized bacterial and viral faecal indicators. A likely explanation for the finding was reported previously by Teltsch et al. (1980), who found that the numbers of aerosolized Escherichia coli were lowered by 1 log after between 10 seconds and 2 minutes depending on environmental conditions.
Hutchison and colleagues (2008) similarly report that the risks are low- even when using a rain gun to spread freshly inoculated pig slurry. This group's experiment introduced antibiotic resistant auxotrophic E.coli (which can’t survive for very long without food supplements rarely found in the environment) into the slurry before spreading it onto a cereal fodder crop using a raingun spreader (a commonly-used waste disposal method in the UK; Figure 1). The workers found that they could recover the marker bacteria using air samplers at 150 metres from the rain-gun, but did not recover any marker at 250 m. Thus the authors conclude that if manures are being spread more than 250 m from fresh produce, the risks of cross contamination appear to be quite low.
Figure 1 Dilute pig slurry being spread using a raingun spreader
Most recently, Murayama and colleagues (2010) assessed the airborne microorganisms present during the spreading of bovine cattle slurry. Using a slit-style air sampler, 38 bacterial isolates, representing 16 bacterial genera (types) were identified which included both Gram-positive and Gram-negative genera. No gastro-intestinal pathogens capable of causing human illness were detected, although it was not clear whether there were zoonotic agents present in the manure being spread. However, the study demonstrated the wide range of micro-organisms that are typically present in livestock wastes, although only a few of the isolates have previously been shown to be of importance as human pathogens. The Murayama study also assessed the risks of aerosolised drift as low, although it should be stressed these are only for the particular batch of material being spread. Hutchison and colleagues (2004) report that there is a one in three change of UK livestock wastes containing a commonly-encountered zoonotic agent which could compromise food safety.
Hutchison, M. L., Avery, S. M. and Monaghan J. M. 2008. The air-borne distribution of zoonotic agents from livestock waste spreading and microbiological risk to fresh produce from contaminated irrigation sources. J. Appl. Microbiol. 105,848-857.
Murayama, M., Kakinuma, Y., Maeda, Y., Rao, J., Matsuda, M., Xu, J., Moore, P.A., Millar, B. C., Rooney, P. J., Goldsmith, C. E et al. 2010 Molecular identification of airborne bacteria associated with aerial spraying of bovine slurry waste employing 16S rRNA gene PCR and gene sequencing techniques. Ecotoxicology and Environmental Safety 73, 443-447.
Pillai, S.D., Widmer, K.W., Dowd, S.E. and Ricke, S.C. (1996) Occurrence of airborne bacteria and pathogen indicators during land application of sewage sludge. Appl Environ Microbiol 62, 296–299.
Sorber, C.A., Moore, B.E., Johnson, D.E., Harding, H.J. and Thomas, R.E. (1984) Microbiological aerosols from the application of liquid sludge to land. J Wat Poll Cont Fed 56, 830–836. (reference is too old to be available electronically).
Teltsch, B., Shuval, H.I. and Tadmore, J. (1980) Die-away kinetics of aerosolised bacteria from sprinkler application of wastewater. Appl Environ Microbiol 39, 1191– 1197.