Checking for Neurotropic Prions
The next step for researchers was to see if neurotropic prions, which have a special affinity for the central nervous system, also replicate more easily in spleen tissue. Ovinized mice were infected with disease-causing prions from hamsters; here too, substantially more spleens than brains harbored replicating disease-causing hamster prions. Importantly, after 500 days, hamster prions were detected in every mouse spleen yet none of the mice showed any signs of illness.
In the fourth experiment, “humanized” transgenic mice were challenged with BSE prions – the public health official’s big fear. Only 3 of the 44 inoculated mice acquired the disease-causing proteins in their brains, and none acquired them before 600 days post-inoculation. Of the 41 spleens analyzed, however, 26 were positive for BSE prions; a transmission barrier far leakier than brain tissue, according to Béringue. Even more worrying, there were no signs of clinical illness in any of the mice.
Experts Call for Expanded Prion Surveillance
Hubert Laude DVM, PhD, the other principal INRA scientist on this study, noted to this writer that because spleen was markedly more permissive than nervous tissue in all the mice tested, despite an intracerebral infection route, the human species barrier to prions is probably “dramatically lower than suggested by brain testing alone,” and called for routine inclusion of lymphoid tissue surveillance in everyone exposed to CWD or BSE. He also noted in the Science article that these study results “reinforce the legitimacy of current investigations aimed at evaluating the proportion of people harboring silent PrPres,” referring to the UK’s ongoing vCJD survey of surgically removed appendices, which has so far revealed 4 prion-positive samples out of 14,000 tested, meaning that as many as 1 in every 3,500 people could be carrying and spreading vCJD.