Pattern and extent of demyelination in the optic nerves of mice infected with Semliki Forest virus and the possibility of axonal sprouting.

The optic nerves of all mice infected with Semliki Forest virus (SFV) showed patchy demyelination, although the extent of demyelination varied between animals. There was a significant increase in small diameter unmyelinated fibres in the optic nerves of SFV infected mice, paralleled by a loss of small myelinated fibres, suggesting that the smaller myelinated fibres may be more susceptible to virus-induced demyelination. The small unmyelinated fibres were more numerous in the peripheral region of the optic nerve trunk, where blood vessels are found in greater numbers. In addition, a particularly large increase in the proportion of unmyelinated fibres which had diameters of less than 0.2 microns suggested the possibility of axonal sprouting.