Investigation into the Persistence of Scrapie Infection on Pasture Land

<p>In DEFRA project SE1845 2 day-old TSE free lambs were introduced to pasture on an endemically infected farm (VLA Rectory) to follow the flock, but with no direct contact with the infected sheep. These SE 1845 lambs became infected with scrapie and reached a clinical end stage between 726-868 days post exposure.</p>

<p>This proposal will investigate the persistence of this scrapie infection on pasture, using grazing sheep as a bioassay tool. The experimental design is based on a designated field on the same farm (at the VLA Rectory farm, the same farm that was used in SE1845). This field will be grazed by the infected flock at Rectory prior to being divided up into 4 equal paddocks which will then be restocked with TSE free animals. Each paddock will be double fenced, leaving a minimum of 2 metres between the fences, to prevent any nose to nose contact with sheep from other paddocks. Any area in the field whereby the infection may be of a higher concentration i.e. around water troughs, will be omitted from any paddock area to try and maintain an equal distribution of infectivity throughout the 4 paddocks. Each paddock will be treated independently with protective clothing and equipment being dedicated to each paddock.</p>

<p>In the 1st year a pilot of 4 groups of 6 sheep classical scrapie free 2 day old lambs, of the VRQ/VRQ PrP genotype, will be sourced from the classical scrapie free flock at Arthur Rickwood (SE 1931) and along with their dams will be introduced to the paddocks (6 lambs in each paddock). This 2 day old introduction follows the design of SE1845. The dams will be removed at lamb weaning and the lambs will be RAMALT tested at 6 and 9 months of age until positive, then they will be removed 10 months after introduction assuming the majority are positive. It is assumed that this will happen within the first 10 months but this study has been included to provide a proof of principal stage before the main experiment which will be as follows.</p>

<p>Groups of 25 classical scrapie free 2 day old lambs, of the VRQ/VRQ PrP genotype, will be sourced from SE1931 as above and along with their dams will be introduced to one of the paddocks at the start of 2nd, 3rd, and 4th years of the project and a subsequent year to be agreed with FFG (not costed). This 2 day old introduction follows the design of SE1845. The dams will be removed at lamb weaning and the lambs will stay in the paddocks and will be RAMALT tested quarterly from 6 months of age until positive or for up to a maximum of 36 months. The removal once RAMALT positive is to reduce the potential for cross contamination in other members of the group. Once removed from the pasture the sheep will be placed in the Rectory flock (this differs for the project design of SE 1845 in which they were placed in scrapie free accommodation).</p>

<p>The use of 25 sheep per group in four paddocks looking at a fallow period of approximately 2, 14 and 26 months plus a longer period in the paddock without an introduction date. Animals will be treated for routine husbandry procedures in their individual paddocks and equipment for this will be designated to each individual group. The empty paddocks will be mowed when necessary to maintain the grassland following normal farm practice when fields are left fallow.</p>

<p>All equipment and facilities used in this procedure will be dedicated to an individual group of animals, where this is impractical, it will be decontaminated between groups to minimise the risk of any cross contamination.</p>

<p>Samples of soil will be taken over the timecourse of the project (not costed in this proposal) and frozen for potential analysis for the presence of PrPres in soil from the experimental paddocks. In addition, areas outside the paddocks will provide a control to be used to study PrPres decline in the abscence of grazing sheep. If this analysis becomes available the study will provide data to assess the link between levels of soil-derived PrPres and its bioavailability measured in the attack rate and incubation period of the sheep.</p>

<p>The study will enhance the knowledge of the persistence of environmental scrapie infectivity and the implications for farms that have had scrapie cases.</p>