BackgroundThe prevalence of Salmonella in food producing animals is very low in Sweden due to rigorous control programmes. However, no active surveillance is in place in sheep. The authorities decided to perform a prevalence study in sheep herds because findings at slaughter indicated that sheep associated S. diarizonae (S. enterica subspecies diarizonae serovar 61:(k):1, 5, (7)) might be common in sheep. Sampling was stratified by herd size in two groups, small herds with ≤ 30 animals and large herds with > 30 animals. In each stratum, 237 herds were selected at random. Faecal samples received from 244 out of the 474 randomly selected herds were analysed.ResultsA total of 40 of 100 (40%) of large herds and 17 of 144 (12%) of small herds were positive. The overall adjusted prevalence was 17.6% (95% CI, 12.9-22.2). Sheep associated S. diarizonae was detected in all counties (n = 21). Scientific opinions and an evaluation of on-farm control measures performed concluded that the impact of sheep associated S. diarizonae on human health is very low, and that risk management measures applied in response to findings of sheep associated S. diarizonae in sheep or sheep meat can be expected to have very little impact on reducing risks to human health. As a result, Swedish authorities decided to make an exemption for sheep associated Salmonella diarizonae in sheep and sheep meat in the current Salmonella control measures.ConclusionsSheep associated S. diarizonae is endemic in Swedish sheep herds. It is more common in large herds and not limited to certain parts of the country. The responsible authorities concluded that current risk management actions regarding sheep associated S. diarizonae in sheep and sheep meat are not proportional to the risk. This is the first time in the history of the Swedish Salmonella control programme that an exemption from the legislation has been made for a specific serovar. If there is any future indication of an increasing risk, due to e.g. change in the pathogenicity or development of antimicrobial resistance, the risk assessment will be re-evaluated and control measures reinforced if needed.

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While agricultural activities, such as the application of manure on arable land and animal grazing on pastures, provide economic and environmental benefits, they may also pose microbial risks to water sources. The aim of this paper was to study the microbial fate and transport in an agricultural catchment and recipient water source through further development of the hydrological model HYPE. Hydrological modelling was combined with hydrodynamic modelling to simulate the fate and transport of Salmonella spp., verotoxin-producing Escherichia coli O157:H7 (VTEC) and Cryptosporidium parvum in an agricultural catchment of a drinking water source, Lake Vombsjön, in Sweden. This approach was useful to study the influence of different processes on the pathogen fate and transport, and to interpret the relative changes in the simulated concentrations. Sensitivity analysis indicated that the largest uncertainties in the model were associated with the estimation of pathogen loads, parameterisation of the pathogen processes, and simulation of partitioning between surface runoff and infiltration. The proposed modelling approach is valuable for assessing the relative effect of different risk-reducing interventions.

Waterborne outbreaks of gastrointestinal diseases are of great concern to drinking water producers and can give rise to substantial costs to the society. The World Health Organisation promotes an approach where the emphasis is on mitigating risks close to the contamination source. In order to handle microbial risks efficiently, there is a need for systematic risk management. In this paper we present a framework for microbial risk management of drinking water systems. The framework incorporates cost-benefit analysis as a decision support method. The hydrological Soil and Water Assessment Tool (SWAT) model, which was set up for the Stäket catchment area in Sweden, was used to simulate the effects of four different mitigation measures on microbial concentrations. The modelling results showed that the two mitigation measures that resulted in a significant (p<0.05)reduction of Cryptosporidium spp. oocyst and Escherichia coli concentrations were a vegetative filter strip linked to cropland and improved treatment (by one Log10 unit) at the wastewater treatment plants. The mitigation measure with a vegetative filter strip linked to grazing areas did not achieve any significant reduction in E.coli (p>0.05) but for Cryptosporidium spp. (p<0.05). The mitigation measure enhancing the removal efficiency of all on-site wastewater treatment systems (total removal of 2 Log10 units), did not achieve any significant reduction in E.coli or Cryptosporidium spp. concentrations (p>0.05). The SWAT model was useful when characterising the effect of different mitigation measures on microbial concentrations. Hydrological modelling implemented within an appropriate risk management framework is a key decision support element as it identifies the most efficient alternative for microbial risk reduction.