Assays are provided for rapid detection, with high specificity of the pathogenic form of prion protein responsible for neurodegenerative diseases affecting humans and animals, such as transmissible spongiform encephalopathy in bovine, sheep, and cats. Also provided are assays for testing animal feedstock, such as animal feed, for the presence or concentration of pathogenic prion protein. Results are available in from about 0.5 to about 20 minutes and preferably within from about 5 to about 10 minutes. The assays employ proteinase-K to remove normal prion protein from a biological sample, so that the sample may be analyzed by immunochromatography to determine the presence and concentration of pathogenic prion protein. Because the proteinase-K is immobilized on a solid support for in situ removal of interfering components, the present invention obviates the need for subsequent extraction of the desired analyte. All aspects of the present invention are suitable for quantifying the minimal detectable amount of pathogenic prion protein in a biological sample. Moreover, the simplicity of sample preparation makes the present invention suitable for use in the field.