Human immunodeficiency viruses (HIV-1 and HIV-2)

There are two types of HIV virus; HIV-1 and HIV-2. [Move next text to line below]
HIV-1 is responsible for the majority of global HIV infections and cases of acquired immune deficiency syndrome (AIDS), while the relatively less common HIV-2 is mainly restricted to West Africa.

HIV-1 and HIV-2 are very similar in almost every respect, although growing evidence indicates that progression of disease is slower in HIV-2 infection. Unless specifically highlighted, the properties of these viruses are presented under the generic term 'HIV'.

Pathogenesis of HIV infection

HIV infects certain types of white blood cell; specifically helper T-lymphocytes, monocytes and some other cells that are key elements of the human immune system. This usually results in the death of these cells. The hallmark of HIV infection is the gradual loss of helper T-lymphocytes from an infected person. This ultimately leads to a state of generalised immunodeficiency and AIDS. In some cases, infection of the central nervous system occurs, often leading to progressive brain damage (encephalopathy).

Several different conditions may occur as a result of HIV infection that precedes the development of AIDS. Most infected individuals generate antibodies to HIV within a few weeks after infection and, during this period, may develop a self-limiting illness resembling glandular fever (infectious mononucleosis). After a longer period, some develop a long-lasting generalised enlargement of the lymph glands. Other non-specific symptoms (including fever, night sweats and swollen lymph glands) are associated with progressive immune dysfunction. When AIDS develops fully, which often takes several years, it is characterised by the appearance of secondary opportunistic infections and tumours.

Transmission of HIV infection

Infectious virus is present at all stages of HIV. However, the viral load (the amount of HIV in the blood, measured by viral RNA in blood plasma) is proportional to the chances of the infected person transmitting the virus to a recipient. Viral loads are higher in the initial acute infection and towards the end of disease in an untreated person. It is usual for a person receiving anti-HIV therapy to have low - or even undetectable - viral loads, and to be less likely to transmit virus. The relationships between viral proteins, antibody generation and the progression of this disease are complex.

Despite considerable genetic variation in HIV, there has been no discernible change in its routes of transmission. Available evidence indicates that, by far, the most important vehicles of infection are blood, semen and female genital tract secretions. Thus, worldwide, most infections have been transmitted sexually or by blood - the latter being principally via blood transfusion or from contaminated injecting equipment. Infection of babies from infected mothers has been attributed to trans-placental infection (the passage of infection through the placenta), exposure during delivery or breast-feeding. As a greater understanding of transmission has developed, steps have been taken to mitigate HIV transmission in the UK. These include screening of blood donations, needle exchange programmes and antenatal screening coupled with antiretroviral therapy, obstetric management and avoidance of breastfeeding to prevent mother-to-child transmission.

Prevalence of HIV and AIDS in the UK

Since the first reports in the 1980s from North America of an immunodeficiency syndrome affecting men who have sex with men (MSM), HIV infection has spread to become a global pandemic. Initially, MSM were the most affected group, but more recently, the UK HIV epidemic has shown a rapid increase in the number of diagnoses among heterosexuals and a steady increase in the number of diagnoses in MSM. There continues to be a constant small number of new diagnoses among intravenous drug users, children born to HIV-infected women and blood/blood product recipients. The increase in diagnoses of heterosexually acquired infections has been greater among women than men. A key factor to detecting this is routine antenatal screening.

At the end of 2007, it was estimated that 77 400 people were living in the UK with HIV. This equates to around 0.13% of the population. An estimated 28% of carriers were unaware of their infection. In the UK, London remains the focus of the epidemic, with higher infection rates relative to the rest of the UK. The number of people living with diagnosed HIV is rising each year, due to increased numbers of new diagnoses and improved survival rates due to anti-retroviral therapies.

Further reading on HIV markers and progression:

• Public Health England
• Centers for Disease Control and Prevention HIV/AIDS web pages