Protein misfolding cyclic amplification corroborates the absence of PrP accumulation in placenta from foetuses with the ARR/ARQ genotype in natural scrapie.

Ovine scrapie is a worldwide spread prion disease that is transmitted horizontally under field conditions. Placenta from scrapie-infected ewes is an important source of infection, since this tissue can accumulate high amounts of PrP depending on the foetal genotype. Therefore, placentas carrying susceptible foetuses can accumulate PrP but there is not PrP accumulation in presence of foetuses with at least one ARR haplotype. In scrapie eradication programs, ARR/ARR males are used for breeding to increase the resistant progeny and reduce the horizontal transmission of the disease through the placenta. The development of highly sensitive techniques, that allow the detection of minimal amounts of PrP, has caused many secretions/excretions and tissues that had previously been deemed negative to be relabeled as positive for PrP. This has raised concerns about the possible presence of minimal amounts of PrP in placentas from ARR foetuses that conventional techniques had indicated were negative. In the present study we examined 30 placentas from a total of 23 gestations; 15 gestations resulted from naturally ARQ/ARQ scrapie-infected ewes mated with ARR/ARR rams. The absence of PrP in placentas carrying the foetal ARR haplotype (n=19) was determined by IDEXX HerdChek scrapie/BSE Antigen EIA Test, Prionics-Check WESTERN and corroborated by the highly sensitive Protein Misfolding Cyclic Amplification technique (PMCA). By immunohistochemistry, several unspecific stainings that might mislead a diagnosis were observed. The results of the present study support that using ARR/ARR males in scrapie eradication programs efficiently decreases the spreading of the agent in the environment via shed placentas.