Human Immunodeficiency Virus Infection

The human immunodeficiency virus-1 (HIV-1), a RNA-retrovirus, infects the immune system and the neural tissue, leading to a slowly progressive, multifaceted disease with a fatal outcome. Since the isolation of the virus in 1983, HIV infections have become epidemic worldwide; an estimated 60 million individuals have been infected. The incidence is still rising, particularly in Africa and Asia. HIV-associated diseases are a leading cause of death among men aged 25 to 45 years.

FIGURE 6.1 1

Cytomegalovirus encephalitis. A. Periventricular mononuclear cell infiltration. B. Intranuclear inclusion in a cytomegalic cell (HE).

HIV infection is transmitted by several routes: (a) sexual contact, among both homosexual men and heterosexual men and women; (b) blood transfusion; (c) intravenous illicit substance abuse; (d) transmission from mother to infant via placenta, during delivery, or via breast milk; and (e) via infected blood or needlestick injury among health care workers (this risk is considered minimal).

The virus infects the CD4+ T lymphocytes and the macrophages. Infection of CD4+ T cells results in a slowly progressive decline and ultimate loss of these cells, along with a decline of all cellular components of the immune system. The infection of the macrophages is significant, because they carry the viruses to the nervous system. When the viruses reach the neural tissue, they infect the cells of macrophage/microglial lineage and usually remain clinically asymptomatic for months to years. Occasionally, 3 to 6 weeks after infection, the viruses may produce a transient encephalopa-thy, myelopathy, or aseptic meningitis, all attributed to HIV-induced immunologic processes. The infection becomes clinically symptomatic when immunosuppres-sion is severe; that is, when the CD4+ cell count drops below 200 microliters (guidelines of the Center for Disease Control). This final stage of the disease, known as acquired immunodeficiency syndrome (AIDS) is associated with a variety of neurologic complications, some directly related to the virus and some related to the immunosuppressed state of the host.

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