The neural spread of pseudorabies virus in calves is described. Twenty-four calves were infected subcutaneously with the virus and killed from 48 h after inoculation until the terminal stages of the disease at 120 h post-inoculation. Virus tissue isolations and histological, fluorescent and electron microscopic techniques were employed.

Virus multiplied in the fasciae at the inoculation site for 60 h and subsequently appeared almost simultaneously in the entire 75 cm length of peripheral nerve, and related spinal ganglion and spinal cord segment. As the disease progressed virus spread cranially and caudally along the spinal cord so that by death virus was present throughout the central nervous system. The findings were indicative of a centripetal spread of virus along the peripheral nerve.

The studies established that a cellular progression of virus to the central nervous system was not possible. The pathway of the virus must have involved a fluid medium since it travelled over 75 cm in less than 72 h. The media available were the perineurial fluid, endoneurial fluid and axoplasm. No evidence of virus transport in the perineurial fluid was observed, and although some movement probably occurred in the endoneurial fluid, extracellular virus was not seen in either the nerve or ganglion. Both naked and enveloped virus particles were seen in the axoplasm of nerve fibres throughout the peripheral nerve and ganglion following replication in ganglionic neurons but virus particles were not identified during the incubation period. The passage of one or two infecting particles during the latter period, however, would be very difficult to detect by all the techniques employed. Evidence is presented for the axoplasm as the pathway of the virus to the spinal ganglion and central nervous system.


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