AIDS
The advent of AIDS (acquired immunity deficiency syndrome) in early 1981 surprised the scientific community, as many researchers at that time viewed the world to be on the brink of eliminating infectious disease. AIDS, an infectious disease syndrome that suppresses the immune system, is caused by the Human Immune Deficiency Virus (HIV), part of a group of viruses known as retroviruses. The name AIDS was coined in 1982. Victims of AIDS most often die from opportunistic infections that take hold of the body because the immune system is severely impaired.
Following the discovery of AIDS, scientists attempted to identify the virus that causes the disease. In 1983 and 1984 two scientists and their teams reported isolating HIV, the virus that causes AIDS. One was French immunologist Luc Montagnier (1932– ), working at the Pasteur Institute in Paris, and the other was American immunologist Robert Gallo (1937– ) at the National Cancer Institute in Bethesda, Maryland. Both identified HIV as the cause of AIDS and showed the pathogen to be a retrovirus, meaning that its genetic material is RNA instead of DNA. Following the discovery,
a dispute ensued over who made the initial discovery, but today Gallo and Montagnier are credited as co-discoverers.
Inside its host cell, the HIV retrovirus uses an enzyme called reverse transcriptase to make a DNA copy of its genetic material. The single strand of DNA then replicates and, in double stranded form, integrates into the chromosome of the host cell where it directs synthesis of more viral RNA. The viral RNA in turn directs the synthesis protein capsids and both are assembled into HIV viruses. A large number of viruses emerge from the host cell before it dies. HIV destroys the immune system by invading lymphocytes and macrophages, replicating within them, killing them, and spreading to others.
Scientists believe that HIV originated in the region of sub-Saharan Africa and subsequently spread to Europe and the United States by way of the Caribbean. Because viruses exist that suppress the immune system in monkeys, scientists hypothesize that these viruses mutated to HIV in the bodies of humans who ate the meat of monkeys, and subsequently caused AIDS. A fifteen-year-old male with skin lesions who died in 1969 is the first documented case of AIDS. Unable to determine the cause of death at the time, doctors froze some of his tissues, and upon recent examination, the tissue was found to be infected with HIV. During the 1960s, doctors often listed leukemia as the cause of death in many AIDS patients. After several decades however, the incidence of AIDS was sufficiently widespread to recognize it as a specific disease. Epidemiologists, scientists who study the incidence, cause, and distribution of diseases, turned their attention to AIDS. American scientist James Curran, working with the Centers for Disease Control and Prevention (CDC), sparked an effort to track the occurrence of HIV. First spread in the United States through the homosexual community by male-to-male contact, HIV rapidly expanded through all populations. Presently new HIV infections are increasing more rapidly among heterosexuals, with women accounting for approximately twenty percent of the AIDS cases. The worldwide AIDS epidemic is estimated to have killed more than 6.5 million people, and infected another 29 million. A new infection occurs about every fifteen seconds. HIV is not distributed equally throughout the world; most afflicted people live in developing countries. Africa has the largest number of cases, but the fastest rate of new infections is occurring in Southeast Asia and the Indian subcontinent. In the United States, though the disease was concentrated in large cities, it has spread to towns and rural areas. Once the leading cause of death among people between the ages of 25 and 44 in the Unites States, AIDS is now second to accidents.
HIV is transmitted in bodily fluids. Its main means of transmission from an infected person is through sexual contact, specifically vaginal and anal intercourse, and oral to genital contact. Intravenous drug users who share needles are at high risk of contracting AIDS. An infected mother has a 15 to 25% chance of passing HIV to her unborn child before and during birth, and an increased risk of transmitting HIV through breast-feeding. Although rare in countries such as the United States where blood is screened for HIV, the virus can be transmitted by transfusions of infected blood or blood-clotting factors. Another consideration regarding HIV transmission is that a person who has had another sexually transmitted disease is more likely to contract AIDS.
Laboratories use a test for HIV-1 that is called Enzyme-linked immunosorbant assay (ELISA). (There is another type of HIV called HIV-2.) First developed in 1985 by Robert Gallo and his research team, the ELISA test is based on the fact that, even though the disease attacks the immune system, B cells begin to produce antibodies to fight the invasion within weeks or months of the infection. The test detects the presence of HIV-1 type antibodies and reacts with a color change. Weaknesses of the test include its inability to detect 1) patients who are infectious but have not yet produced HIV-1 antibodies, and 2) those who are infected with HIV-2. In addition, ELISA may give a false positive result to persons suffering from a disease other than AIDS. Patients that test positive with ELISA are given a second more specialized test to confirm the presence of AIDS. Developed in 1996, this test detects HIV antigens, proteins produced by the virus, and can therefore identify HIV before the patient's body produces antibodies. In addition, separate tests for HIV-1 and HIV-2 have been developed.
After HIV invades the body, the disease passes through different phases, culminating in AIDS. During the earliest phase the infected individual may experience general flu-like symptoms such as fever and headache within one to three weeks after exposure; then he or she remains relatively healthy while the virus replicates and the immune system produces antibodies. This stage continues for as long as the body's immune response keeps HIV in check. Progression of the disease is monitored by the declining number of particular antibodies called CD4-T lymphocytes. HIV attacks these immune cells by attaching to their CD4 receptor site. The virus also attacks macrophages, the cells that pass the antigen to helper T lymphocytes. The progress of HIV can also be determined by the amount of HIV in the patient's blood. After several months to several years, the disease progresses to the next stage in which the CD4-T cell count declines, and non-life-threatening symptoms such as weakness or swollen lymph glands may appear. The CDC has established a definition for the diagnosis of AIDS in which the CD4 T-cell count is below 200 cells per cubic mm of blood, or an opportunistic disease has set in.
Although progress has been made in the treatment of AIDS, a cure has yet to be found. In 1995 scientists developed a potent cocktail of drugs that help stop the progress of HIV. Among other substances, the cocktail combines zidovudine (AZT), didanosine (ddi), and a protease inhibitor. AZT and ddi are nucleosides that are building blocks of DNA. The enzyme, reverse transcriptase, mistakenly incorporates the drugs into the viral chain, thereby stopping DNA synthesis. Used alone, AZT works temporarily until HIV develops immunity to the nucleoside. Proteases are enzymes that are needed by HIV to reproduce, and when protease inhibitors are administered, HIV replicates are no longer able to infect cells. In 1995 the Federal Drug Administration approved saquinaviras, the first protease inhibitor to be used in combination with nucleoside drugs such as AZT; this was followed in 1996 by approval for the protease inhibitors ritonavir and indinavir to be used alone or in combination with nucleosides. The combination of drugs
brings about a greater increase of antibodies and a greater decrease of fulminant HIV than either type of drug alone. Although patients improve on a regimen of mixed drugs, they are not cured due to the persistence of inactive virus left in the body. Researchers are looking for ways to flush out the remaining HIV. In the battle against AIDS, researchers are also attempting to develop a vaccine. As an adjunct to the classic method of preparing a vaccine from weakened virus, scientists are attempting to create a vaccine from a single virus protein.
In addition to treatment, the battle against AIDS includes preventing transmission of the disease. Infected individuals pass HIV-laden macrophages and T lymphocytes in their bodily fluids to others. Sexual behaviors and drug-related activities are the most common means of transmission. Commonly, the virus gains entry into the bloodstream by way of small abrasions during sexual intercourse or direct injection with an infected needle. In attempting to prevent HIV transmission among the peoples of the world, there has been an unprecedented emphasis on public health education and social programs; it is vitally important to increase public understanding of both the nature of AIDS and the behaviors that put individuals at risk of spreading or contracting the disease.
