Human immunodeficiency virus HIVl and HIV2

The classical descriptions and the vast majority of the literature on AIDS refer to HIV-1; a second retrovirus capable of causing AIDS, HIV-2, mainly occurs in West Africa.

Human immunodeficiency virus (HIV) can be transmitted both in cellular and plasma components. Most of the patients infected by the transfusion of blood components were transfused before the introduction of the screening of blood donations for HIV antibodies. The majority of recipients of blood products who were infected in the past were transfused before 1985 with unheated, non-pasteurized pooled plasma products, mainly factor VIII and factor IX. Thus, HIV infection became an important sequel to transfusion of factor VIII concentrates to haemophiliacs in the early 1980s, before the introduction of heat treatment and other viral inactivation methods. HIV is more heat labile than HBV, especially when in solution. Prolonged heat treatment and other viral inactivation methods of plasma products are effective means of protecting haemophiliacs from infection, although too late for those who were receiving regular factor VIII therapy in the late 1970s and early 1980s.

Albumin solutions are pasteurized, and carry no risk of HIV transmission. Similarly, i.m. immunoglobulin preparations are rendered safe from HIV infectivity by their manufacturing processes.

Blood and all fresh blood components (platelets, white cells, single donor plasma, fresh-frozen plasma and cryoprecipitate) are capable of transmitting HIV. Certain behaviour patterns place individuals at greater risk of HIV infection. Accordingly, male homosexuals and bisexuals, intravenous drug users and prostitutes are permanently deferred from blood donation. The sexual partners of such individuals and of haemophiliacs treated with blood products are also excluded. In addition, large areas of sub-Saharan Africa and South-East Asia have a high incidence of HIV seropositivity in the general population. Inhabitants of these areas and their partners are also considered to be at greater risk of HIV infection than heterosexual non-drug users in other areas of the world. Donor education and encouragement of those whose behaviour may have exposed them to HIV to exclude themselves from blood donation are highly cost-effective methods for the prevention of transmission of HIV infection by blood transfusion. Systems for donor deferral should continue to be in place, even in the presence of sensitive assays for the detection of infectious donors.

In most HIV-infected subjects, antibody develops within 34 weeks and it coexists with the virus thereafter. Hence, HIV seropositivity is an indicator of infectivity. The tests that lend themselves most readily to the rapid mass screening required in blood transfusion are all ELISA based. As many of the commercially available kits have some false-positive reactions, due mainly to cross-reactivity, confirmatory tests using alternative methodologies are carried out before a positive result is reported.

Routine screening, combined with the well-established donor education and self-deferral schemes, has reduced even further the already small risk of the transfusion of a contaminated donation. There have been only three documented cases of HIV transmission by transfusion in the UK since screening began. The small residual number of HIV transmissions through screened blood will arise through donations given in the 'window period' of infectivity, i.e. soon after the donor has been infected but before anti-HIV has become detectable (as in the three UK documented cases referred to), or through system errors. With current combined antigen-antibody screening techniques, the window period has been estimated to be less than 2 weeks on average. The prevalence of HIV antibodies in repeat UK blood donors is less than

I in 100 000, and is 1 in 25 000 in new donors; there is no doubt that energetic donor education has contributed to this very low rate by excluding high-risk individuals. In England and Wales, it is estimated that the residual risk of HIV infection by transfusion at present, per donation, is approximately 1 in 8 million. Although small, the risk of HIV transmission by transfusion is greater in the USA (approximately 1 in 500 000-1 million donations) than in the UK. In order to reduce the length of the window period even more, i.e. to about 2 weeks, the USA and other countries (such as Thailand) introduced screening for HIV antigen at a considerable cost. In the USA, this has been replaced by nucleic acid testing (NAT) for HIV RNA, which reduces the window period to approximately 1 week. However, in areas of low HIV incidence, the number of extra donors detected by such screening will be extremely small. In the UK, combined antigen-antibody serological testing has proved extremely effective, although NAT for HIV has been introduced in some parts of the country.

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