Human Immunodeficiency Virus

HIV Background

Human immunodeficiency virus (HIV) is an enveloped, single-stranded virus and member of the genus Lentivirus and Retroviridae family. Two types of HIV are recognised: HIV-type 1, responsible for most HIV infections globally and HIV-type 2, which is restricted to some regions of Africa.

HIV-1 viruses may be further divided into groups M, N, O and P. The HIV-1 group M viruses predominate and are responsible for the AIDS pandemic. Within the M group of HIV-1 there are a number of genetically distinct sub-types (known as clades). Different subtypes can also combine genetic material to form a hybrid virus or ‘circulating recombinant form’ (CRF). Subtype B is the most common in the Americas and Western Europe, whilst subtype C is the predominant form in Africa and India. Most research has been carried out on subtype B, although it accounts for only 12% of infections worldwide.

Human Immunodeficiency Virus is transmitted from person-to-person through contact with contaminated blood, seminal fluid, vaginal secretions, rectal fluids and breast milk. HIV particles can be detected in the blood 10-12 days after infection. Characteristics of HIV infection vary and may include flu-like symptoms, fever, arthralgia, lymphadenopathy, pharyngitis, weight loss and general malaise. An asymptomatic phase follows the acute phase of HIV infection. During this phase, the effects of HIV infection continue resulting in a decrease of CD4+ T-cell populations and impairment of the immune system. Seroconversion is estimated to occur between 3-5 weeks after infection. (Fanales-Belasio, E. et al).

HIV targets cells of the host’s immune system, including circulating T-cells, precursor T-cells in the bone marrow and thymus, dendritic cells, macrophages and monocytes, eosinophils and microglia cells of the central nervous system. The HIV gp120 envelope glycoprotein enables the virus to enter host target cells by interacting with CD4 – a cell surface glycoprotein. Chemokine receptors have also been identified, primarily CXCR4 and CCR5, which act as co-receptors for viral entry (Wilen, C.B. et al).

In the absence of treatment, HIV infection can progress further, resulting in a decrease in T-cell numbers and the development of Acquired Immunodeficiency Syndrome (AIDS). AIDS renders an individual susceptible to a plethora of opportunistic infections, including Microcystis carinii, Candida albicans, Cytomegalovirus and Herpes zoster, which are life-threatening to those infected. (Fanales-Belasio, E. et al). Effective antiretroviral therapy (ART) is widely available for the treatment of HIV infection. In 2016, approximately 19.5 million people worldwide were reported to be receiving ART, resulting in significant reductions in mortality, in both children and adults (UNAIDS factsheet).