Allergic encephalitis, rabies antibodies, and the blood/brain barrier.

Similarity of the syndromes of rabies and of encephalomyelitis resulting from rabies vaccination poses a problem in differential diagnosis that has had tragic consequences. That difficulty may also be largely responsible for the traditional belief that rabies is inexorably fatal, in that recovery from paralysis is interpreted as evidence that the disease was postvaccinal encephalomyelitis rather than rabies. Diagnosis is additionally complicated by presence of rabies antibodies in serum in both conditions. However, we have found that rabies antibodies elicited by vaccination do not pass the blood/brain barrier to enter the fluids of the CNS in EAE, the experimental counterpart of postvaccinal encephalomyelitis; whereas antibodies are present in high titer as a result of production in situ after recovery from rabies. Therefore a ratio of antibody concentrations of greater than 1:10 in CSF and serum indicates chronic rabies or recovery from that disease.