Much uncertainty surrounds transmission of transmissible spongiform encephalopathies (TSEs) through blood and blood derived products. A first expert elicitation with 14 experts was conducted in March 2008, and a second re-elicitation involving 11 experts was held a year later in March 2009. Both expert groups were calibrated using a series of seed questions for which values are known, and then were asked to provide their individual judgements on a set of seven target questions for which values are not known or have not been determined through conventional research. Questions dealing with uncertainty of TSE prevalence, genotype effects, susceptibility, and infectivity were answered by the experts. Elicitation can be used to obtain quantitative values for parameters affecting prion uncertainty gaps. We show that the method is amenable to re-elicitation over time allowing refinement of expert opinion as new knowledge becomes available for improved risk assessments where data gaps continue to exist.