What is Human Immunodeficiency Virus (HIV)?
Human immunodeficiency virus (HIV) is a retrovirus that is transmitted through certain bodily fluids and disrupts the immune system by destroying the CD4 T-lymphocytes that work to fight off infection in our bodies. When left untreated, HIV eventually leads to acquired immunodeficiency syndrome (AIDS), an advanced state of immune system failure that leaves the body weak and vulnerable to life-threatening illnesses and cancers. However, with treatment, individuals can live a near-normal life.
Generally, HIV is categorized into two types:
Generally, HIV is categorized into two types:
- HIV-1: Found worldwide
- Classified into 4 groups: M N O and P
- Group M is the most common, it is the strain that is responsible for the global epidemic
- N P and O are uncommon
- Typically, when HIV is mentioned without an associated virus type, individuals are speaking about HIV-1
- HIV-2: Found mainly within West Africa
- Relatively uncommon
- Less infectious than HIV-1
- Progresses slower than HIV-1
Etiology: The Origins of HIV
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Scientists theorize that HIV-1 was transmitted from chimpanzees to humans in Africa as far back as the late 19th century. Simian immunodeficiency virus (SIV) is a virus that attacks the immune system in monkeys, similar to HIV in humans. Scientists believe that two unique strains of SIV spread to chimps that hunted and ate two infected species of monkeys, the red-capped mangabeys and greater spot-nosed monkeys. The two strains fused together to form a new virus, SIVcpz. Humans then became infected with HIV, a mutated version of SIVcpz, after hunting and eating infected chimpanzees.
Similarly, HIV-2 is believed to have derived from SIVsmm, a mutation of SIV found in sooty mangabey monkeys, after humans hunted and ate these primates.
Today, HIV is spread from human to human through contact with bodily fluids such as blood, semen, and vaginal fluids; or from mother to child during birth or through lactation.
Similarly, HIV-2 is believed to have derived from SIVsmm, a mutation of SIV found in sooty mangabey monkeys, after humans hunted and ate these primates.
Today, HIV is spread from human to human through contact with bodily fluids such as blood, semen, and vaginal fluids; or from mother to child during birth or through lactation.
Statistics
HIV is considered a pandemic, which means that it is a disease that is occurring at alarmingly high rates globally. There are currently 37 million people living with HIV and AIDS worldwide. The following are statistics that were reported for Canadians in 2014 (Public Health Agency of Canada, 2015):
2 570
estimated number of people diagnosed with new infections (Incidence rate: 7.2 per 100 000) |
75 500
estimated number of people living with HIV and AIDS (Prevalence rate: 212 per 100 000) |
16 020
estimated number of people living with undiagnosed HIV |
The Public Health Agency of Canada (2015) also found:
- Men who have sex with men represented 53% of new infections
- Heterosexual contact exposure represented 32.6% of new infections
- Females represented 23.3% of new infections
- Injection drug use exposure represented 10.5% of new infections
- Individuals born in endemic countries were overrepresented, accounting for 13.9% of new infections, whereas they make up 2.5% of the Canadian population
- Aboriginal peoples were overrepresented, accounting for 10.8% of new infections, whereas they make up 4.3% of the Canadian population
Mortality: In 2014, approximately 1.2 million people died globally as a result of AIDS-related illnesses (World Health Organization, 2016). The estimated number of deaths caused by HIV/AIDS is shown by region in the image below:
Developmental Considerations
Infants and Toddlers (Birth-12+ months):
- Babies born to infected mothers receive a drug named zidovudine for 6 weeks after birth
- Babies born to infected women should be tested for HIV at 14-21 days, 1-2 months, and at 4-6 months
- In developed countries, breastfeeding is contraindicated in mothers who are infected with HIV; formula is the safe alternative
- In developing countries, breastfeeding is still recommended as the safest method of feeding
- Adolescents and youth are at increased risk due to risky sexual behaviour, and alcohol and drug leading to risky behaviour
- These individuals are less likely to get tested for HIV, and are less likely to use condoms during sex
- They also have high rates of sexually transmitted infections
- Stigma surrounding the topic of HIV is a challenge in HIV prevention for adolescents and youth
- Females can pass HIV on to children at any point during the perinatal period (pregnancy, birth, or lactation)
- Women who are pregnant or are planning to get pregnant should be tested for HIV as early as possible
- With early treatment during pregnancy, mothers can significantly reduce the risk of passing on HIV to their children
Cellular Characteristics of HIV
Since HIV is a retrovirus, it does not have the machinery it needs for self-replication. Instead the virion "hijacks" healthy cells in the human body in order to replicate and spread. Unlike human cells, the virion's genetic material consists of positive-sense, single stranded ribonucleic acid (RNA) molecules, which are transcribed into deoxyribonucleic acid (DNA) with the help of an RNA-dependent DNA polymerase called reverse transcriptase.
Structure of the Virion
Each virion consists of:
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Target Cells
HIV mainly attacks helper T lymphocytes called CD4 T cells that are essential in the immune system (i.e. required for production of antibodies by B lymphocytes and activation of cytolytic T lymphocytes) because HIV is able to attach using gp120 to CD4 receptors on the surface of T cells. Other proteins on the host, called coreceptors, help tighten the bond. After infecting a T cell, the cell dies within 1.25 days. The progressive degradation of the immune system results in an increased risk of opportunistic infections for affected individuals.
HIV also infects macrophages and microglial cells since both carry CD4 proteins. However, these cells are resistant to destruction by HIV and therefore serve as a reservoir of HIV during chronic infection. |
The Replication Cycle
- HIV particle attaches to host cell membrane. The primary connection is between the gp120 protein on the HIV envelope and the CD4 protein on host cell. Other host proteins (co-receptors), such as CCR5 and CXCR4, also act to help tighten the bond between CD4 and HIV.
- Lipid bilayer envelope of HIV fuses with lipid bilayer of host membrane through a process called endocytosis. The HIV RNA is removed and taken up into host cell.
- HIV RNA transcribed into single stranded complimentary DNA by reverse transcriptase. Viral RNA degrades within host cell.
- Reverse transcriptase uses the single strand of HIV DNA as a template to convert it into a double stranded HIV DNA.
- Integrase facilitates integration of double stranded HIV DNA into the host cell's DNA (chromosome) as a provirus.
- HIV DNA transcription into viral RNA. Some RNA become daughter HIV virions, and the rest become messenger RNA coding for HIV proteins.
- Messenger RNA translated into HIV glycoproteins, and HIV enzymes, and structural proteins to form new capsids and envelopes.
- Glycoproteins incorporated into host cell membrane. HIV components migrate to cell surface and assemble into new virus.
- New virus buds off from host. An envelope for the new virion is formed from cell membrane of host.
- Final maturation occurs under influence of protease, which cleaves large proteins into smaller functional forms. If protease fails: HIV remains immature and consequently non-infectious.
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