Michigan reports more CWD TSE Prion aka mad deer disease 111 cases to date
Rapid recontamination of a farm building occurs after attempted prion removal
Kevin Christopher Gough, BSc (Hons), PhD1, Claire Alison Baker, BSc (Hons)2, Steve Hawkins, MIBiol3, Hugh Simmons, BVSc, MRCVS, MBA, MA3, Timm Konold, DrMedVet, PhD, MRCVS3 and Ben Charles Maddison, BSc (Hons), PhD2
Author affiliations
School of Veterinary Medicine and Science, The University of Nottingham, Loughborough, UK ADAS, School of Veterinary Medicine and Science, The University of Nottingham, Loughborough, UK Animal Sciences Unit, Pathology Department, Animal & Plant Health Agency Weybridge, New Haw, Addlestone, Surrey, UK E-mail for correspondence; ben.maddison@adas.co.uk
Abstract
The transmissible spongiform encephalopathy scrapie of sheep/goats and chronic wasting disease of cervids are associated with environmental reservoirs of infectivity.
Preventing environmental prions acting as a source of infectivity to healthy animals is of major concern to farms that have had outbreaks of scrapie and also to the health management of wild and farmed cervids.
Here, an efficient scrapie decontamination protocol was applied to a farm with high levels of environmental contamination with the scrapie agent.
Post-decontamination, no prion material was detected within samples taken from the farm buildings as determined using a sensitive in vitro replication assay (sPMCA).
A bioassay consisting of 25 newborn lambs of highly susceptible prion protein genotype VRQ/VRQ introduced into this decontaminated barn was carried out in addition to sampling and analysis of dust samples that were collected during the bioassay.
Twenty-four of the animals examined by immunohistochemical analysis of lymphatic tissues were scrapie-positive during the bioassay, samples of dust collected within the barn were positive by month 3.
The data illustrates the difficulty in decontaminating farm buildings from scrapie, and demonstrates the likely contribution of farm dust to the recontamination of these environments to levels that are capable of causing disease.
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PrPC is ubiquitous in its distribution in vivo2 and with both scrapie and CWD the in vivo dissemination of infectivity is also widespread with PrPSc usually accumulating within peripheral lymphatic tissues before the CNS.3 4 With scrapie, PrPSc can be secreted/ excreted via a multiplicity of routes including saliva,5 6 milk,7 faeces,8 skin9 and urine.10 The accumulation of this material within the environment (particularly the built farm environment),11 12 creates levels of infectivity that can be transmitted to naïve animals. These reservoirs of infectivity can remain infectious for prolonged periods of time, in one such recorded incident at least 16 years.13 The advent of high sensitivity prion replication assays such as protein misfolding cyclic amplification (PMCA) with application to sheep/goat scrapie14 15 has allowed the monitoring of prions within environments.11
Attempts to decontaminate pens on a scrapie-affected farm and measuring efficacy using a sheep bioassay were previously reported.12 It was concluded that the failure of effective decontamination within that study was likely to have been due to the incomplete farm decontamination and the presence of dust containing infectious prions that recontaminated the pen surfaces. The serial protein misfolding cyclic amplification (sPMCA) technique was recently used to confirm the presence of prions within extracts prepared from dust samples that had settled on sterile surfaces.16 Given the presence of mobile infectious prions within dust, it was proposed that for effective scrapie decontamination emphasis should be given to the removal of all sources of dust within the decontamination strategy for a farm. More recently, the sPMCA technique has been used by the authors' laboratory to look at effective methods of decontaminating prions bound to concrete surfaces within the laboratory setting.17 This study demonstrated that current methodology based on a one-hour exposure to 20000 ppm free chlorine was likely to be ineffective at removing surface-bound scrapie prion. However, there was an enhanced effectiveness of this chemical decontamination when using multiple applications over four hours. Here, a study is described where a scrapie-affected farm was decontaminated using four applications of 20000 ppm free chlorine to livestock barns and concreted areas. The decontamination also included a high-level clean of the buildings that had housed sheep to remove all traces of dust as far as practicable before the chemical decontamination procedure. Following these treatments the surfaces within the barn were demonstrably free from prion using a sensitive sPMCA assay. The presence of any residual infectivity was then evaluated by sheep bioassay and dust samples collected during the bioassay were assayed for prion seeding activity by sPMCA.
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Discussion
The authors' previous work on this farm indicated that dust harbours low levels of mobile scrapie prions that can accumulate on surfaces16 and this is likely to perpetuate transmission of scrapie within such a farm environment.12 In addition, previous in vitro modelling of scrapie prions bound to a concrete ‘fomite’ demonstrated that prion seeding activity could be inactivated by four applications of 20,000 ppm free chlorine as measured by a sPMCA assay. This previous modelling demonstrated that residual contamination of the swab extract with hypochlorite at levels which would inhibit the sPMCA are unlikely, and the authors consider these results as reduction in seeding titre.17 Here, this same decontamination methodology was tested within a farm-scale study which also included steps to remove dust within the barns. This study demonstrated that this thorough decontamination method applied to a farm with a high incidence of naturally acquired scrapie was sufficient to remove scrapie prions on surfaces to levels that were undetectable by sPMCA, one of the most sensitive biochemical assays for prions. The authors' sPMCA assay has an limit of detection of around 1–10pg scrapie-infected sheep brain per sPMCA reaction. The authors assume that the samples negative by sPMCA had less than this amount (of brain equivalent) within the extracts that were prepared. This treatment together with measures designed to minimise the amount of dust retained within the buildings (vacuuming all surfaces, pressure washing and then hypochlorite treatment) was expected to have removed all infectivity from the buildings and the concrete areas surrounding them, and it was anticipated that the sheep bioassay would confirm absence of infective prion.
However, the introduction into this decontaminated barn of 25 VRQ/VRQ sheep (a genotype highly susceptible to classical scrapie) demonstrated that all animals, with the exception of 1 lamb that died at 122 dpe, had detectable PrPSc in lymphoid tissue, indicating infection with the scrapie agent. This included 14 animals (54 per cent) that were PrPSc-positive on the first RAMALT analysis at 372 dpe or 419 dpe. Although infected sheep were removed based on a positive RAMALT result, it is possible that lateral transmission or subsequent contamination of the environment from infected sheep had contributed to the rapid spread of scrapie in nearly all sheep. It has been shown previously that objects in contact with scrapie-infected sheep, such as water troughs and fence posts, can act as a reservoir for infection.23 As in the authors' previous study,12 the decontamination of this sheep barn was not effective at removing scrapie infectivity, and despite the extra measures brought into this study (more effective chemical treatment and removal of sources of dust) the overall rates of disease transmission mirror previous results on this farm. With such apparently effective decontamination (assuming that at least some sPMCA seeding ability is coincident with infectivity), how was infectivity able to persist within the environment and where does infectivity reside? Dust samples were collected in both the bioassay barn and also a barn subject to the same decontamination regime within the same farm (but remaining unoccupied). Within both of these barns dust had accumulated for three months that was able to seed sPMCA, indicating the accumulation of scrapie-containing material that was independent of the presence of sheep that may have been incubating and possibly shedding low amounts of infectivity.
This study clearly demonstrates the difficulty in removing scrapie infectivity from the farm environment. Practical and effective prion decontamination methods are still urgently required for decontamination of scrapie infectivity from farms that have had cases of scrapie and this is particularly relevant for scrapiepositive goatherds, which currently have limited genetic resistance to scrapie within commercial breeds.24 This is very likely to have parallels with control efforts for CWD in cervids.
Acknowledgements The authors thank the APHA farm staff, Tony Duarte, Olly Roberts and Margaret Newlands for preparation of the sheep pens and animal husbandry during the study. The authors also thank the APHA pathology team for RAMALT and postmortem examination.
Funding This study was funded by DEFRA within project SE1865.
Competing interests None declared.
***> see more of study here;
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