Salmonella uptake in sheep exposed to pastures after biosolids application to agricultural land

Abstract

Young adult sheep grazing pasture on land treated with dewatered biosolids 1–3 weeks before pasture sowing were used to assess the risk of transfer of Salmonella spp. from biosolids to the food chain. Monthly determinations of Salmonella spp. concentrations in the biosolids after land application showed survival for at least 7 months, with concentrations fluctuating between 104/g and below detection limits at various time points. Low concentrations were measured 4–5 months after application, followed by a return to 102–103/g on subsequent samplings. Sheep introduced at 3 or 6 months after biosolids applications of 0, 10, or 15 dry t/ha were assessed monthly for faecal shedding of salmonellae, and at slaughter, 8 months after biosolids application, for intestinal carriage of Salmonella spp. None of 80 sheep grazing the biosolids-treated land were found to excrete salmonellae. A single sheep among those grazing treated land showed Salmonella carriage in slaughter tissues, but the serovar isolated from this animal was different from the 6 serovars of Salmonella detected in the applied biosolids. Use of dewatered biosolids on land later sown for pasture and grazed by sheep thus posed no risk to animal uptake or faecal shedding of Salmonella spp. This in part relates to a low risk of direct pathogen ingestion by grazing sheep and the period of several months necessary for pasture establishment before grazing stock can be introduced.