In 1996 the VLA expanded its programme of experimental BSE challenges, such that it was no longer possible to accommodate sufficient numbers of experimental animals on the existing VLA estate, and subsequently, new cattle buildings were erected at Drayton. In 1998 SEAC confirmed that after 28 days following oral inoculation, excreta produced from TSE infected animals could be composted for a year and then safely used to fertilise agricultural land at Drayton. SEAC's recommendations were implemented and furthered, with excreta produced within the first 28 days for any experimentally inoculated animals being incinerated, together with any materials or excretions associated with lambing and calving. The remaining excreta is composted in clamps for at least 12 months.
In 2001 landspreading of the composted manure and wastewater began at Drayton, arising from experimental and control animals at that premise and also surplus composted excreta from VLA Weybridge, where the same protocols are in operation. Such arrangements have been in place since 2001 with short rotation willow coppice being grown on fertilised land and sold for incineration. However, TSE experiments at Drayton are due to be completed by March 2010, with resulting composted manure applications finishing by March 2011. A risk assessment has been requested to investigate the residual TSE risk associated with the fertilised land at Drayton and whether any restrictions on land use at Drayton may be necessary. A parallel risk assessment is currently being completed (project end date 31/03/2009) on the residual TSE risk associated with composted manure arising from VLA, Weybridge.
To assess this risk, the OIE framework for risk assessment of release, exposure and consequence assessment, will be used. Following this framework, firstly, the release of TSEs (BSE and scrapie) in excreta from experimental animals from 1998 to 2010 will be considered. This will include an estimate of the amount of potential TSE agent being released in urine, faeces, saliva and residue milk over time. Secondly, the amount of TSE to which livestock (cattle or sheep) could be exposed to from the fertilised agricultural land will be estimated over time. Lastly, the number of any new TSE infections resulting from exposure to TSEs from the agricultural land will be quantified taking into account the dose-response relationship of ingestion, and the time interval between applications and exposure. The end result of the risk assessment, therefore, will be a quantitative estimate of risk (the number of possible new BSE infections in cattle or sheep, or scrapie infections in sheep) per time interval post 2011.
A sensitivity analysis will also be undertaken to ascertain the impact that any uncertain parameters have on the final output of risk. In doing so, those parameters which have a large impact on the results can be identified and further research undertaken to assist in reducing the uncertainty in the model inputs. In summary, the risk assessment model proposed will follow a similar approach to that produced by Cummins & Adkin (2007).
The risk assessment model will be parameterised with the most recently available data from the scientific literature and VLA studies. It is likely that there may be gaps in knowledge for some model parameters in which case expert opinion will be sought. The resulting model framework, data, model assumptions, results and sensitivity analysis will be written within a peer-reviewed scientific paper and subject to review by Defra and other scientific bodies (e.g. SEAC).