Detection of Sub-Clinical CWD Infection in Conventional Test-Negative Deer Long after Oral Exposure to Urine and Feces from CWD+
Posted Dec 06 2009 3:04pm
Detection of Sub-Clinical CWD Infection in Conventional Test-Negative Deer Long after Oral Exposure to Urine and Feces from CWD+ Deer
Nicholas J. Haley1, Candace K. Mathiason1, Mark D. Zabel1, Glenn C. Telling2, Edward A. Hoover1*
1 Department of Microbiology, Immunology, and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America, 2 Department of Molecular Biology and Genetics, University of Kentucky, Lexington, Kentucky, United States of America
Abstract Top Background Chronic wasting disease (CWD) of cervids is a prion disease distinguished by high levels of transmissibility, wherein bodily fluids and excretions are thought to play an important role. Using cervid bioassay and established CWD detection methods, we have previously identified infectious prions in saliva and blood but not urine or feces of CWD+ donors. More recently, we identified very low concentrations of CWD prions in urine of deer by cervid PrP transgenic (Tg[CerPrP]) mouse bioassay and serial protein misfolding cyclic amplification (sPMCA). This finding led us to examine further our initial cervid bioassay experiments using sPMCA.
Objectives We sought to investigate whether conventional test-negative deer, previously exposed orally to urine and feces from CWD+ sources, may be harboring low level CWD infection not evident in the 19 month observation period. We further attempted to determine the peripheral PrPCWD distribution in these animals.
Methods Various neural and lymphoid tissues from conventional test-negative deer were reanalyzed for CWD prions by sPMCA and cervid transgenic mouse bioassay in parallel with appropriate tissue-matched positive and negative controls.
Results PrPCWD was detected in the tissues of orally exposed deer by both sPMCA and Tg[CerPrP] mouse bioassay; each assay revealed very low levels of CWD prions previously undetectable by western blot, ELISA, or IHC. Serial PMCA analysis of individual tissues identified that obex alone was positive in 4 of 5 urine/feces exposed deer. PrPCWD was amplified from both lymphoid and neural tissues of positive control deer but not from identical tissues of negative control deer.
Discussion Detection of subclinical infection in deer orally exposed to urine and feces (1) suggests that a prolonged subclinical state can exist, necessitating observation periods in excess of two years to detect CWD infection, and (2) illustrates the sensitive and specific application of sPMCA in the diagnosis of low-level prion infection. Based on these results, it is possible that low doses of prions, e.g. following oral exposure to urine and saliva of CWD-infected deer, bypass significant amplification in the LRS, perhaps utilizing a neural conduit between the alimentary tract and CNS, as has been demonstrated in some other prion diseases.
Citation: Haley NJ, Mathiason CK, Zabel MD, Telling GC, Hoover EA (2009) Detection of Sub-Clinical CWD Infection in Conventional Test-Negative Deer Long after Oral Exposure to Urine and Feces from CWD+ Deer. PLoS ONE 4(11): e7990. doi:10.1371/journal.pone.0007990
Editor: Jiyan Ma, Ohio State University, United States of America
Received: September 29, 2009; Accepted: October 29, 2009; Published: November 24, 2009
Funding: This work was supported by NIH/NCRR Ruth L. Kirschstein Institutional T32 R07072-03 and NIH/NIAID NO1-AI-25491-02 (EAH, GCT). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: Edward.Hoover@colostate.edu
In summary, we provide evidence for the presence of infectious prions in the brains of conventional prion-assay-negative deer orally exposed 19 months earlier to urine and feces from CWD-infected donor deer. This apparent low level of prion infection was amplified by sPMCA, confirmed by Tg[CerPrP] mouse bioassay, and detected only in the obex region of the brain. These results demonstrate the potential for CWD prion transmission via urine and/or feces, and highlight the application of more sensitive assays such as sPMCA in identification of CWD infection, pathogenesis, and prevalence.
Michael Jones, Darren Wight, Rona Barron, Martin Jeffrey, Jean Manson, Christopher Prowse, James W. Ironside, and Mark W. Head
To assess interspecies barriers to transmission of transmissible spongiform encephalopathies (TSEs), we investigated the ability of disease-associated prion proteins (PrPd) to initiate conversion of the human normal cellular form of prion protein of the 3 major PRNP polymorphic variants in vitro. Protein misfolding cyclic amplification showed that conformation of PrPd partly determines host susceptibility.
Our results are best appreciated in terms of the molecular interaction between seed PrPd and substrate PrPC, specifi cally the species-specific amino acid sequence and PRNP polymorphic status of PrPC and PrPd and the PrPd conformers involved (Table). Regardless of the seed PrP amino acid sequence, the PrPd conformers associated with bovine BSE, ovine BSE, and human vCJD were amplified in the humanized mouse substrate and displayed similar PRNP-129 genotype preferences (PRNP-129MM >PRNP- 129MV >PRNP-129VV). In contrast, the PrPd conformer associated with the ovine scrapie strain, although sharing the same PrP amino acid sequence as the PrPd in ovine BSE, could not be amplified in any of the PRNP humanized mouse substrates but could be amplifi ed in a sheep brain substrate. These observations are consistent with conformation of a TSE agent's PrPd (rather than solely its amino acid sequence) having a role in determining the susceptibility of a host's PrPC to conversion. They similarly suggest that these molecular factors could in turn have a powerful influence on disease susceptibility and incubation time.
To date, all clinical cases of vCJD have occurred in persons with the PRNP-129MM genotype, as might be predicted from the effi ciency of amplification of BSE-related PrPd shown here. Extrapolating from these results, one would predict that the next genotypic group most likely to show susceptibility to the BSE agent would be heterozygous (MV) at codon 129 of the PRNP gene, as previously suggested from the corresponding in vivo transmission studies (14).
In the wake of BSE epidemics in the United Kingdom and elsewhere, enhanced surveillance has identified apparently new TSEs (15), raising concerns regarding animal and human health. PMCA with suitable substrate sources could provide a rapid way to estimate the molecular component of transmission barriers for particular TSE agents between species, including humans. These estimates could thus indicate whether, like classical scrapie, the agents rep- resent little risk for human health or whether, like classical BSE, they represent cause for concern.