Food-borne transmission of prions can lead to infection of the gastrointestinal tract and neuroinvasion via the splanchnic and vagus nerves. Here we report that the transmission of transmissible mink encephalopathy (TME) is 100,000-fold more efficient by inoculation of prions into the tongues of hamsters than by oral ingestion. The incubation period following TME agent (hereinafter referred to as TME) inoculation into the lingual muscles was the shortest among the five nonneuronal routes of inoculation, including another intramuscular route. Deposition of the abnormal isoform of the prion protein, PrPSc, was first detected in the tongue and submandibular lymph node at 1 to 2 weeks following inoculation of the tongue with TME. PrPSc deposits in the tongue were associated with individual axons, and the initial appearance of TME in the brain stem was found in the hypoglossal nucleus at 2 weeks postinfection. At later time points, PrPSc was localized to brain cell groups that directly project to the hypoglossal nucleus, indicating the transneuronal spread of TME. TME PrPSc entry into the brain stem preceded PrPSc detection in the rostral cervical spinal cord. These results demonstrate that TME can replicate in both the tongue and regional lymph nodes but indicate that the faster route of brain invasion is via retrograde axonal transport within the hypoglossal nerve to the hypoglossal nucleus. Topical application of TME to a superficial wound on the surface of the tongue resulted in a higher incidence of disease and a shorter incubation period than with oral TME ingestion. Therefore, abrasions of the tongue in livestock and humans may predispose a host to oral prion infection of the tongue-associated cranial nerves. In a related study, PrPSc was detected in tongues following the intracerebral inoculation of six hamster-adapted prion strains, which demonstrates that prions can also travel from the brain to the tongue in the anterograde direction along the tongue-associated cranial nerves. These findings suggest that food products containing ruminant or cervid tongue may be a potential source of prion infection for humans.
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