After intramuscular, intradermal and subcutaneous inoculation, the pseudorabies virus reaches the central nervous system by way of the peripheral nerves, although it is circulating in the blood. Centrifugal spread from the infected nervous tissues by the neural route also occurs. After intracerebral inoculation the virus passes in the reverse direction, down the nervous axis. The Aujeszky strain invades the blood stream more readily than does the Iowa strain; but possibly with repeated passage the latter is approximating in this respect more closely the classical Aujeszky strain. After intravenous inoculation, effective with even small doses, virus is rapidly removed from the blood, and multiple infective foci are established in various organs; thence ascent of the virus by the peripheral nerves leads to infection of the central nervous system, the symptomatology differing according to whether the spinal cord or the medulla is first reached. The lack of evidence that the virus can penetrate directly the hemato-encephalic barrier deserves emphasis. When subcutaneous inoculation is practised in an area deprived of its nerve supply, the ability of the virus to invade the blood stream permits it to establish infective foci in the various viscera, and, after a predictable delay, the course of infection resembles that following intravenous injection. The pseudorabies virus is pantropic; i.e., it readily attacks cells derived from any embryonic layer. Lesions in the adrenal gland following intravenous inoculation are very like those due to herpes virus similarly introduced, this being one point of similarity in the pathogenic action of the two organisms. The relation of the pseudorabies virus to other viruses affecting the central nervous system is discussed.

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