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presumably by the blood-brain barrier Moreover, most antibodies and immunocompetent cells are excluded as well, so that the same mechanism that limits the entry of viruses also deters their removal Viral particles are cleared from the blood by the reticuloendothelial system; but if the viremia is massive or other conditions are favorable, they will invade the CNS probably via the cerebral capillaries and the choroid plexuses Another pathway of infection is along peripheral nerves; centripetal movement of virus is accomplished by the retrograde axoplasmic transport system Herpes simplex, VZV, and rabies viruses utilize this peripheral nerve pathway, which explains why the initial symptoms of rabies (and the similar but rare B-virus infection of monkeys) occur locally, at a segmental level corresponding to the animal bite In animals inoculated subcutaneously with VZV, sensitive DNA ampli cation techniques identify fragments of viral genome only in the dorsal root ganglion or ganglia corresponding to the dermatome(s) containing the site of inoculation It has been shown experimentally that HSV may spread to the CNS by involving olfactory neurons in the nasal mucosa; the central processes of these cells pass through openings in the cribriform plate and synapse with cells in the olfactory bulb (CNS) Another potential pathway is the trigeminal nerve and gasserian ganglion However, the role of these pathways in human infection is not certain Of these different routes of infection, the hematogenous one is by far the most important for the majority of viruses The steps in the hematogenous spread of infection include crossing the blood-brain barrier through small vessels of the brain and the choroids plexus Viruses cross both within migrating lymphocytes and directly through areas of glial and vascular regions that are permeable to the organisms

Viruses, once they invade the nervous system, have diverse clinical and pathologic effects One reason for this diversity is that different cell populations within the CNS vary in their susceptibility to infection with different viruses To be susceptible to a viral infection, the host cell must have on its cytoplasmic membrane speci c receptor sites to which the virus attaches Thus, some infections are con ned to meningeal cells, enteroviruses being the most common, in which case the clinical manifestations are those of aseptic meningitis Other viruses will involve particular classes of neurons of the brain or spinal cord, giving rise to the more serious disorders such as encephalitis and poliomyelitis, respectively In some viral infections, the susceptibility of particular cell groups is even more speci c In poliomyelitis, for example, motor neurons of cranial and spinal nerves are particularly vulnerable; in rabies, there is an af nity for neurons of the trigeminal ganglia, cerebellum, and lim631

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Viruses gain entrance to the body by one of several pathways Mumps, measles, and VZV enter via the respiratory passages Polioviruses and other enteroviruses enter by the oral-intestinal route, and HSV, mainly via the oral or genital mucosal route Other viruses are acquired by inoculation, as a result of the bites of animals (eg, rabies) or mosquitoes (arthropod-borne or arbovirus infections) The fetus may be infected transplacentally by rubella virus, CMV, and HIV Following entry into the body, the virus multiplies locally and in secondary sites and usually gives rise to a viremia Most viruses are prevented from entering central nervous system (CNS) tissues,

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bic lobes The virus or its nucleocapsid must be capable of penetrating the cell, mainly by the process of endocytosis, and of releasing its protective nucleoprotein coating For viral reproduction to occur, the cell must have the metabolic capacity to transcribe and translate virus-coated proteins, to replicate viral nucleic acid, and, under the direction of the virus s genome, to assemble virions Also, the pathologic effects of viruses on susceptible cells vary greatly In acute encephalitis, susceptible neurons are invaded directly by virus and the cells undergo lysis There is an appropriate glial and in ammatory reaction Neuronophagia (phagocytosis of affected neurons and their degenerative products by microglia) is a mark of this phenomenon In progressive multifocal leukoencephalopathy (PML), there is a selective lysis of oligodendrocytes, resulting in foci of demyelination With other viral infections, zones of tissue necrosis of gray and white matter occur, and these changes may localize in particular regions, eg, in the inferomedial parts of the temporal lobes in cases of HSV encephalitis VZV and HSV may remain latent in neurons of the sensory ganglia for long periods, until some factor triggers reactivation and retrograde spread of virus to cutaneous dermatomes or mucocutaneous epithelium Only then will the in ammatory reaction to viral replication create symptoms (pain, weakness, sensory loss) In certain congenital infections, eg, rubella and CMV, the virus persists in nervous tissue for months or years Differentiating cells of the fetal brain have particular vulnerabilities, and viral incorporation may give rise to malformations and to hydrocephalus (eg, mumps virus with ependymal destruction and aqueductal stenosis) In experimental animals, cerebral neoplasms can be induced when the viral genome is incorporated into the DNA of the host cell There is suggestive evidence of such a mechanism in humans (relating to EBV in B-cell lymphoma of the brain) In still other circumstances, a viral infection may exist in the nervous system for a long period before exciting an in ammatory reaction (eg, progressive multifocal encephalopathy; subacute sclerosing panencephalitis); in these cases it may be so indolent as to simulate a degenerative disease.