TITLE: From HIV to AIDS and Transmission
AUTHOR: Brianne Spencer
DATE: May 15, 2009

ABSTRACT

The human immunodeficiency virus (HIV) was first recognized in the early 1980s and is thought to have arisen from mutations of the simian immunodeficiency virus (SIV) carried by healthy Mangabey monkeys and chimpanzees living in Africa. Although there is no definitive explanation as to how SIV was transferred to the human population in the mid-20th century, many feel that blood-to-blood contact with infected primates was the primary mode of transmission. This is still of concern today for the human population, as HIV is transferred from one individual to another through direct contact with infected body fluids, such as blood or semen.

HIV is a retrovirus and is considered to be latent, as a person infected with HIV advances from being asymptomatic to being diagnosed with clinical AIDS in a period of about 10 years. In order to replicate, this virus needs to be inside other cells, specifically immune cells consisting of macrophages, helper T cells, and dendritic cells, and enters with the help of its highly functional structure. Although much is known about HIV and AIDS, the high mutation rate and its ability to change rapidly contributes to obstacles in developing vaccines and diagnostic tests.

INTRODUCTION

Although much is known about HIV and AIDS, I only knew of the ways in which this infection could be transmitted; However, I wanted to know more and this is why I decided to research this topic. I wanted to understand the structure of HIV, which was explained to me by a website created by the National Institute of Allergy and Infectious Diseases, as well as know how this virus enters host cells and replicates itself practically undetected by the immune system; I found this information in my Microbiology textbook written by Tortora et al. Also, I, being the curious person that I am, wanted to discover how this virus even showed up in the human population. Was it caused by a mutation of a gene that was already present within humans or did it come from another source? My class textbook helped me answer this question, but a scientific journal, written by Jim Moore and entitled The Puzzling Origins of AIDS, provided more insight and explained the answers to my inquiries in a more thorough fashion. The following discussion is a compilation of the information that I researched and found and I hope it helps make some sense and bring about some understanding in regards to this complicated and serious virus.

DISCUSSION

The Origin of AIDS

The human immunodefiency virus (HIV) is responsible for the development of AIDS and was first recognized in the early 1980s. Through much research, scientists and investigators found that this virus arose from mutations of the simian immunodeficiency virus (SIV) present in Mangabey monkeys in West Africa. In addition, SIV is also carried by chimpanzees in Central Africa; chimpanzees prey on other monkeys and it is likely that a chimpanzee ingested a monkey that carried the simian immunodeficiency virus (Tortora et al. 566).

The virus crossed over into the human population in the mid-20th century and did so through a tainted polio vaccine, or so it has been believed. It is also thought that SIV was transmitted to hunters via blood-to-blood contact with infected primates. Although this last theory is probably the most accepted out of the two that were mentioned, neither one definitively explains how SIV was transferred to the human population (Moore 540-544, 547).

The HIV Infection: Structure, Replication Cycle, and Stages of HIV

Structure of HIV

The Structure of HIV adopted from the National Institute of Allergy and Infectious Diseases
The Structure of HIV adopted from the National Institute of Allergy and Infectious Diseases

HIV is a retrovirus, which indicates that DNA is made from an RNA template with the help of an enzyme called reverse transcriptase, and is in the genus Lentivirus, or "slow" virus. This just means that there is a long interval between the initial infection and the onset of more serious symptoms. As seen in the diagram, two single strands of viral RNA containing the reverse transcriptase enzyme are found within the capsid, or the viral core, which is then surrounded by an envelope that has a lipid bilayer membrane and proteins that protrude through its surface; these are referred to as glycoprotein spikes (HIV/ AIDS).

Replication Cycle of HIV

The Replication Cycle of HIV adopted from the National Institute of Allergy and Infectious Diseases
The Replication Cycle of HIV adopted from the National Institute of Allergy and Infectious Diseases

Viruses are considered to be obligate intracellular parasites meaning that they need to be inside other cells in order to replicate. These host cells, or the main targets of HIV infection, consist of macrphages, which normally function as scavengers that travel through the blood and ingest unwanted invaders, helper T cells, and dendritic cells, which elicit and begin the immune response in healthy individuals (Weiss A13). The following steps describe how HIV enters and replicates within these three different cell types:
  • The spikes on the envelope of the virus allow the virus to attach to cluster destination 4 (CD4) receptors and two co-receptors known as CCR5 and CXCR4.
  • The envelope of the virus and the cell membrane of the host cell fuse together and the virus enters the host cell.
  • In the cytoplasm of the host cell, reverse transcriptase transcribes viral RNA into viral DNA.
  • The newly made HIV DNA travels to the host cell's nucleus where it is spliced into the host's DNA with an enzyme called HIV integrase. The integrated viral DNA is referred to as a provirus, which can be considered a latent virus because its location within the host's DNA shelters it from the immune system.
  • New viruses bud from the host cell and the viral enzyme protease turns the bud into infectious viral particles (HIV/ AIDS).

The replication cycle of HIV makes this virus dangerous because it evades immune responses. In addition to being a provirus, HIV undergoes rapid change and has a high mutation rate. Retroviruses, such as this one, lack the proofreading characteristic, which means mutations occur in every step of the replication cycle. In fact, there are 1 million variants of the virus in an asymptomatic person and 100 million variants during the final stages of infection. This leads to obstacles in the development of vaccines and diagnostic tests (Tortora et al. 568-569).

Stages of HIV
The progression of HIV infection is divided into three clinical categories, which include:
  • Category A
    • The infection may be asymptomatic, which means that no symptoms have appeared, or it may cause the lymph nodes to swell.
  • Category B
    • This stage is characterized by infections by the yeast Candida albicans, which appear in the mouth, throat, or vagina. Other symptoms visible at this stage include shingles, diarrhea and fever, white patches on the oral mucosa, and cancerous or precancerous conditions of the cervix.
  • Category C
    • This stage is clinical AIDS and is characterized by tuberculosis, toxoplasmosis of the brain, and Kaposi's sarcoma.

A person infected with HIV advances from being asymptomatic to being diagnosed with clinical AIDS in a period of about 10 years (Tortora et al. 569).

The Transmission of HIV

In order for HIV to be transmitted from one individual to another, there needs to be a transfer of, or direct contact with, infected body fluids. These fluids consist of semen, which infects an individual during sexual intercourse and carries about 10 - 50 viruses/ mL, and blood. Sharing needles or syringes contaminated with blood containing HIV can lead to its transmission, as blood carries 1,000 - 100,000 viruses/ mL (Tortora et al. 572-573).

Other routes of transmission include organ transplants, artificial insemination, and blood transfusions, but this is now very rare as more countries are starting to screen blood for HIV antibodies (HIV and Its Transmission). Also, children can get HIV from infected mothers before, during, or after birth through transplacental infections and breastfeeding (Tortora et al. 573).

There are misconceptions about HIV and its transmission, but scientists have shown that HIV does not survive well in the environment and it is not transmitted through insect bites, which is contrary to what many have heard. Also, individuals cannot contract HIV through kissing infected persons or coming in contact with the saliva, sweat, and tears of a person infected with this virus (HIV and Its Transmission).

LITERATURE CITED

HIV/ AIDS. Home page. National Institute of Allergy and Infectious Diseases. 23 April 2009
<http://www3.niaid.nih.gov/topics/HIVAIDS/Understanding/How+HIV+Causes+AIDS/howhiv.htm>.

HIV and Its Transmission. Home page. Department of Health and Human Services. 11 May 2009
<http://www.cdc.gov/hiv/resources/factsheets/transmission.htm>.

Moore, Jim. "The Puzzling Origin of AIDS." American Scientist 92 (2004): 540-547.

Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. San Francisco: Benjamin Cummings, 2007.

Weiss, Robin A. "Getting to Know HIV." Tropical Medicine and International Health 5 (2000): A10-A15.