Transmissible spongiform encephalopathies (TSEs), or prion diseases, affecting human and animal species can be transmitted from TSE-infected individuals to naïve susceptible hosts during the long asymptomatic (years to decades) and symptomatic disease stages. The presence of infectious hematogenous prions in asymptomatic TSE-infected hosts demonstrates the highly infectious nature of blood-borne prions in hosts lacking overt clinical symptoms. It is currently unknown when and how infectious prions first enter the blood following initial exposure. We have previously shown that the whole-blood real-time quaking-induced conversion assay (wbRTQuIC) possesses 100% specificity and >92% sensitivity, making it an ideal tool to address this question. Here, we use wbRT-QuIC to analyze whole blood collected from oral, extranasal or aerosol TSE-exposed hosts for blood-borne prions. Our results demonstrate that conversion competent prions in the inoculum are capable of crossing mucosal surfaces and entering the circulatory system within 30 min—no matter the route of exposure.