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Germs that infect humans

Human immunodeficiency virus (HIV)

Human immunodeficiency virus (HIV)

© Robert Alain, SME, INRS-Institut Armand-Frappier

Microorganism: human immunodeficiency virus (HIV), genus: Lentivirus, family: Retroviridae.

Disease: acquired immunodeficiency syndrome (AIDS)

Occurrence of the disease

History: in 1978, the Centers for Disease Control in Atlanta observed pneumonia and a very rare form of cancer known as Kaposi's sarcoma in some individuals. These patients also exhibited a reduced immune response characterized by a heightened sensitivity to infection and a reduction in the number of cells known as T lymphocytes. In 1981, this disease was identified as AIDS. The disease appears to have begun to spread in central Africa in the 1950s, and a virus that may be the ancestor of the AIDS virus has been isolated from African green monkeys.

Current situation: AIDS is the first worldwide pandemic of the last half of the 20th century. In 1997, it affected 0.13-0.5% of the Canadian population. Every day, 16,000 new cases of HIV infection are recorded around the world. In 1998, 138 cases of AIDS were reported in Quebec.

Projections: the World Health Organization estimates that 50-100 million people will be infected with AIDS by 2003.

Mechanism of action of the microorganism: the virus first binds to immune system cells known as T lymphocytes. It then enters the cells and multiplies, killing the cells. Because T lymphocytes are one of the body's primary defenses against disease, people infected with HIV develop diseases, known as opportunistic infections, which are usually not dangerous for other people.

There are two types of HIV: HIV-1 and HIV-2. HIV-2 is much less dangerous than HIV-1. HIV preferentially infects CD4+ T lymphocytes. The receptor for the virus is CD4+ and its co-receptors are CXCR4 and CCR5. Individuals who have mutated forms of the genes responsible for the CCR5, CXCR4, CCR2 or SDF-1 molecules are resistant to the disease's development or progression.

Although the virus has occasionally been found in saliva, tears and urine, there have been no reported cases of transmission by contact with these fluids.

Symptoms of the disease: in 1996, 67% of patients diagnosed with a typical AIDS opportunistic infection had survived at least 24 months. By 1998, this rate had increased to 77%, thanks to improvements in the nature and availability of treatments for AIDS and opportunistic diseases.

Incubation of period: variable. Virus antibodies are usually detectable in the blood one to three months after exposure. Symptoms generally appear one to 15 years after HIV infection.

Contagious period: unknown, but several specialists believe that it begins immediately following infection and lasts throughout the infected person's lifetime.

Hosts: humans.

Transmission: heterosexual and homosexual sexual relations, contact with contaminated blood (e.g., during transfusions and the exchange of contaminated syringes by drug users). Transmission from women to children is possible during childbirth or nursing.

Discoverer of the microorganism: Montagnier and Gallo in 1983.

Treatment of the disease: although there is no miracle cure, some drugs may improve patients' quality of life and prolong their lives. These drugs prevent the virus from multiplying, by inhibiting three viral enzymes: inverse transcriptase, protease and integrase. The use of these three inhibitors together is known as triple therapy.

Geographic distribution of the microorganism: worldwide.

Prevention: use of latex condoms with a high-risk partner or with multiple partners. Screening of sexual partners. Use of sterile syringes by intravenous drug users. Screening of pregnant women.

Vaccine: none.