Infections: Viruses

Viruses can cause meningitis, encephalitis, myelitis, arteritis when affecting the nervous system. Viruses are classified as DNA viruses and RNA viruses. Imaging of viral encephalitis is often nonspecific showing focal or diffuse edema (acute infection) or focal atrophy (chronic infection). Location of the lesion depends on causative agent, thus, HSV encephalitis displays an almost pathognomonic involvement of the limbic system. The various steps in the pathogenesis include entry into the body, delivery of the virus to the target tissue, interaction of virus with target tissue, cytopathogenesis, host responses, immunopathology, virus production in a tissue with release of the virus to other people (contagion), and transmission of viruses. Treatment consists of administration of antiviral drugs which aim at altering virion disruption, attachment, penetration and uncoating, RNA synthesis, genome replication, virion assembly and release. Clinical outcome depends on age of infection, nature of disease, severity of disease, and progression of viral disease. Human immunodeficiency virus (HIV)-1 infection is a serious health problem worldwide as 33 million adults and 2 million children are infected with HIV-1. The brain is often involved which leads to the HIV-associated neurocognitive disorders (HAND) of which asymptomatic neurocognitive impairment (ANI), mild neurocognitive disorder (MND), and HIV-associated dementia (HAD) represent various stages. The neuropathologic changes in HIV-1 induced lesions, i.e. HIV-1 encephalitis (characterized by multiple disseminated foci composed of microglia, macrophages, and multinucleated giant cells (MGCs) predominantly located in the cortex, deep gray matter, and the white matter), HIV-1 leukoencephalopathy (diffuse damage to the white matter), lymphocytic meningitis (LM), perivascular lymphocytic infiltration (PLI), vacuolar myelopathy (VM), vacuolar leukoencephalopathy (VL) are described. Therapy might lead to the immune restituiton inflammatory syndrome (IRIS). The sequelae of HIV-1 infection of the nervous system include changes in neuronal number, neuronal size, synapses, dendrites, nerve fibers, astroglia, oligodendroglia, microglia/macrophages, vessels, vascular endothelial cells, and capillaries. Pathogenetic mechanisms deal with the mode of entrance of HIV-1 into the brain, target cells of HIV-1, mechanisms of brain lesions, and interactions between the blood–brain-barrier (BBB) and HIV. Cytomegalovirus infection (CMV), progressive multifocal leukoencephalopathy (PML), Herpes Simplex Virus (HSV) encephalitis, and Tick-borne encephalitis are further described.