Alphaherpesvirus saimiri infection in rabbits. 2. Morphometric studies of cutaneous spinal nerves.

To provide a better insight into the ultrastructural pathology of herpetic neuropathy, quantitative studies were made on cutaneous spinal nerves of normal rabbits and rabbits intradermally infected with alphaherpesvirus saimiri (alpha HVS) isolate KM 322. Marked reductions in the numbers and densities of myelinated and unmyelinated axons were found in the nerves of the rabbits killed 17 and 45 days after the infection. Abnormalities in the size distribution of unmyelinated axons were seen at 45 days post-inoculation where axonal sprouting caused a noticeable shift in the fiber population. Two years after virus inoculation reduction in unmyelinated axons and abnormalities in the fiber size distributions characterized by smaller diameters of both myelinated and unmyelinated axons were detected. In these nerves conspicuous fibrosis caused a significant increase in the endoneurial area. At this stage of the infection regenerative changes involving myelinated fibers were found. Since attempts to detect spontaneous reactivation of alpha HVS infection in rabbits have been unsuccessful, the finding of regeneration 2 years after exposure seems in agreement with the view that regenerated myelinated fibers never attain their original size. In the present study although both types of fibers were damaged, morphometric data suggest that unmyelinated axons were more severely affected. Whether this seemingly selective involvement was due to spreading of the virus between axons sharing the same Schwann cell subunit remains to be proved.