In 1981, a cluster of cases of Pneumocystis pneumonia appeared in the Los Angles area. This extremely rare disease was usually seen only in persons who were immunosuppressed. Investigators soon correlated the appearance of this disease with an unusual incidence of a rare form of cancer of the skin and blood vessels called Kaposi’s sarcoma. The people affected were all young homosexual men, and all showed loss of immune function. By 1983, the pathogen causing the loss of immune function had been identified as a retrovirus that selectively infects certain T cells. This virus is now known as human immunodeficiency (see “AIDS”. Grolier Encyclopedia of Knowledge, pp. 57-61).
There have been several theories about the origin of HIV. It is believed that it arose by mutation of a virus that had been endemic in some areas of central Africa for many years. The virus has been found in blood samples preserved from as early as 1959 in several African nations and in England (see “AIDS”. Grolier Encyclopedia of Knowledge, pp. 57-61).
The intents of this paper are to: (1) understand the HIV infection; (2) know the modes of transmission and; (3) be aware of its vaccines and treatments.
HIV infects helper T cells named T4 cells (for the CD4 antigen on their surface). Once inside the infected cell, the viral RNA is transcribed into DNA that remains incorporated into the genetic material of that cell. HIV may remain latent in the cell or begin replicating. Sometimes, replication results in continuous production of new virus particles that bud from the cell without killing it and infect other cells. In other cases, the cell is quickly killed either by the virus itself or by the action of the immune system in response to viral antigens on the surface of the cell. The virus also infects other types of cells that carry CD4 surface antigens.
These cells include macrophages, antigen-presenting dendritic cells, about 40% of all monocytes, and 5 % of B cells. HIV infection generally does not kill these cells but impairs their normal functioning (see “AIDS”. New Standard Encyclopedia, pp. 34-46).
The ability of the virus to remain latent intracellularly is one reason why the anti-HIV antibodies developed by infected persons fail to prevent the disease. The virus also evades immune defenses by undergoing antigenic changes, even during the course of an infection. There is evidence that a change of a single amino acid in the glycoprotein envelope of the virus may enable it to avoid antibodies against the previous antigenic configuration. HIV also evades the immune system by staying in vesicles within cells so that viral antigens are not displayed on the cell surface and T cells cannot detect the infected cells. Moreover, infected cells displaying bviral antigens can fuse to uninfected cells to spread the virus (see “AIDS”. Wikipedia, the free encyclopedia. Feb.18, 2007).
AIDS is actually only the end stage of an infection by HIV. Shortly after the initial infection, the patient undergoes seroconversation—that is, tests positive for antibodies to HIV. This interval is almost always less than six months. The symptoms at this point are absent or resemble mononucleosis—a mild fever, swollen lymph nodes, and fatigue. Even these symptoms spontaneously disappear in a few weeks (see “AIDS”. Wikipedia, the free encyclopedia. Feb.18, 2007).
A. Modes of Transmission
Transmission of HIV usually requires transfer of bodily fluids. The most important of these are blood, semen, and vaginal secretions that contain the virus, or the transfer of cells, especially macrophages, containing the virus. It has been established that the routes of transmission include intimate sexual contact, breast milk, blood-contaminated needles, and blood-to-blood contact such as transfusions. Heterosexual contact is much more likely transmitting HIV when genital lesions are present.
This is considered a very important factor in the spread of AIDS in central Africa, where sexually transmitted diseases that result in such lesions are prevalent. Heterosexual spread in Africa is so common that the male: female ratio among AIDS patients is about 1:1 as compared with 13:1 in the United States. Heterosexual transmission is increasing in the United States, however, especially via women who are drug abusers and resort to prostitution to support their addiction. The risk of sexual transmission is minimized by the use of condoms. Saliva may contain the virus, but transmission is not known to occur by kissing. There have been recorded instances, though, of sexual transmission of HIV in persons whose only sexual contacts were oral-genital (see “HIV/AIDS Causes”).
Worldwide epidemiological studies indicate three geographic patterns of transmission: (1) Transmission is primarily among homosexual or bisexual males and intravenous drug abusers in North America, Western Europe, Australia, and New Zealand. (2) Heterosexual contact is the primary mode of transmission in Africa and Caribbean countries. (3) Most AIDS cases in Eastern Europe, the Middle East, and Asia have occurred among people who have traveled to endemic areas and had sexual contact with infected homosexual men and female prostitutes. Lax procedures for needle sterilization have resulted in a number of hospital-associated outbreaks of AIDS in the Soviet Union and Romania.
AIDS is not transmitted by insects or by casual social contact such as hugging and sharing household facilities, drinking glasses, or towels. Transmission by blood transfusion in developed countries is unlikely because blood is tested for AIDS antibodies. However, there will always be a light risk, because blood might be donated during interval between infection and appearance of detectable antibodies. Tests for the virus itself are also available, but they have not proved to be superior in screening blood. There is no risk at all in donating blood. AIDS has been transmitted by organ transplants and artificial insemination with donated sperm. HIV-positive women should not become pregnant because of the probability (at least 30%) of transmitting the virus to the fetus (see “HIV Infection and AIDS: An Overview”, March 2005).
B. Vaccines and Treatments
There are two basic areas of AIDS research. Some researchers are working on vaccines to prevent the disease, and others are looking for drugs to treat AIDS.
There are great obstacles to production of an AIDS vaccine, among them the lack of a suitable animal host for the virus. However, researchers are now optimistic that a vaccine, once produced, could be effective. One reason of optimism is that a very few persons who were once HIV-positive have spontaneously become HIV-negative, indicating that the immune system is probably capable in rare instances of eliminating the virus. Because of the extreme virulence of the virus, many think it unlikely that any whole-virus vaccine, either killed or attenuated, would be acceptable for use on uninfected persons. However, such a vaccine would be acceptable for use in attempting to clear the virus after infection (see “HIV/AIDS Treatment: Medication”).
A promising approach to arresting an HIV infection is to flood the body with artificially produced, soluble CD4-type molecules that would bind to circulating viruses before they could locate a CD4 receptor on a T cell. In early experiments the interceptor CD4 molecules were rapidly degraded, requiring repeated injections at an impractical rate. The soluble CD4 would have to be modified in some way so as to remain in circulation for an extended time for this approach to be practical (see “HIV/AIDS Treatment: Medication”).
Most early anti-HIV drugs such as Zidovudine (AZT) are inhibitors of the enzyme reverse transcriptase. HIV is a retrovirus, you will recall, that copies RNA into DNA. The drugs, mostly analogs of nucleic acids, trick the enzyme into terminating the synthesis of viral DNA. Such drugs have slowed the progress of the disease but have not led to a cure. Other than the reverse transcriptase step, there are at least 13 other points at which the production of HIV could be selectively interrupted by drugs (see “AIDS: What is it?”).
The Centers for Disease Control estimate that about 1 million persons in the United States are now infected with HIV and projects that 365,000 AIDS cases will have been diagnosed in the United States and 263,000 patients have died. The annual cost for the care of AIDS patients will be in the billions of dollars. Worldwide, the estimates are that there are more than 1 million total cases of AIDS and that 10 million persons are infected.
The AIDS epidemic gives clear evidence of the value of basic scientific research. It is important to reflect that without the advances of the past few decades in molecular biology, we would have been unable even to identify the agent of AIDS, to develop the tests used to screen donated blood, or to monitor the course of the infection.
“AIDS”. Grolier Encyclopedia of Knowledge, pp. 57-61.
“AIDS”. New Standard Encyclopedia, pp. 34-46.
“AIDS”. Wikipedia, the free encyclopedia. Feb.18, 2007. http://en.wikipedia.org/wiki/AIDS
“HIV/AIDS Treatment: Medication”. http://www.emedicinehealth.com/hivaids/page6_em.htm#HIV/AIDS%20Treatment
“HIV/AIDS Causes”. http://www.emedicinehealth.com/hivaids/page2_em.htm#HIV/AIDS%20Causes
“HIV Infection and AIDS: An Overview”, March 2005. http://www.niaid.nih.gov/factsheets/hivinf.htm