Primary HIV infection is often asymptomatic but may present as an acute illness with fever, sweating, myalgia and arthralgia, sore throat, lymphadenopathy, nausea and vomiting, diarrhoea, headache and other neurological symptoms and a rash lasting between 2 and 4 weeks, but symptoms such as fatigue may persist for many weeks or months. Most patients then become asymptomatic, usually for years. The incubation period is 2-4 weeks after infection, with a range of 5-90 days or longer. Following primary infection, there may be a slow and progressive decrease in the number of CD4+ cells and an increase in CD8 + cells.
AIDS is the late manifestation of infection with HIV, characterised by a marked depletion of CD4+ cells, resulting in a reversal of CD4+ :CD8+ cell ratio. The progressive immunodeficiency is accompanied by a wide range of opportunistic infections, neoplasms, and may present with AIDS encephalopathy (AIDS dementia complex) and other neurological complications that may occur in the absence of opportunistic infections. The Centers for Disease Control (Atlanta, USA) definition of AIDS, adopted in the USA in 1992, is helpful. The definition is based on a positive test for HIV and the following:
1. A CD4+ T cell number of less than 200 mm (the normal count is 600-1000 mm ~3) of whole blood, or a CD4 + T cell/total lymphocytes percentage of less than 14%, or
2. A CD4 + T cell number of 200 mm or over and any of the following conditions: fungal diseases, including candidiasis, coccidioidomycosis, cryptococcosis, histop-lasmosis, isosporiasis; Pneumocystis carinii pneumonia; cryptosporidosis, or toxoplasmosis of the brain, bacterial diseases including pulmonary tuberculosis and other Mycobacterium species, recurrent Salmonella septicaemia; viral diseases, including cytomegalovirus infection, HIV-related encephalopathy, HIV wasting syndrome, chronic ulcer or bronchitis due to herpes simplex, or progressive multifocal leucoencephalopathy; malignant diseases such as invasive cervical carcinoma, Kaposi sarcoma, Burkitt lymphoma, primary lymphoma of the brain, or im-munoblastic lymphoma; recurrent pneumonia due to any age.
Laboratory screening tests for HIV-1 and HIV-2 are based mostly on a variety of ELISAs based on antigens consisting of viral lysates or recombinant proteins corresponding to the immunodominant epitopes of HIV-1 (including the group 0 variants) and HIV-2. Rapid and simple laboratory tests are also used in developing countries, based on filtering serum through a membrane coated with recombinant HIV-1 and HIV-2 antigens. Confirmatory assays are generally based on Western blot techniques, but other immunoblot methods are also available. Strain serotyping methods and subtyping techniques are available. Virus isolation is carried out in high security laboratories. PCR and nested-PCR is used for the detection of proviral HIV DNA. The amount of virus in peripheral blood (viral load) is assessed by measurement of plasma RNA. p24 antigenaemia is measured by ELISA. Genotypic drug resistance assays are important for treatment and monitoring antiviral therapy.
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