Human Immunodeficiency Virus (HIV) is a retrovirus that targets and destroys the CD4+ T-helper cells , leading to progressive immune suppression . If untreated, HIV infection progresses to Acquired Immunodeficiency Syndrome (AIDS) .
Transmission of HIV
HIV is primarily transmitted through:
- Sexual contact (most common route globally)
- Mother-to-child transmission (MTCT) :
- During pregnancy , delivery , or breastfeeding
- Parenteral exposure :
- Sharing contaminated needles or syringes
- Transfusion of infected blood products
- Accidental needlestick injuries (healthcare settings)
Note: HIV is not transmitted via casual contact, insect bites, or respiratory droplets.
Classification of HIV
There are two major types of HIV:
1. HIV-1
- Most prevalent worldwide
- Origin: Chimpanzees (Pan troglodytes troglodytes)
- Associated with faster disease progression
- Highly transmissible
2. HIV-2
- Primarily found in West Africa
- Origin: Sooty mangabey monkeys
- Less transmissible
- Slower progression to AIDS
- Lower plasma viral loads
Both HIV-1 and HIV-2 belong to the Retroviridae family, Lentivirus genus.
HIV-1 vs HIV-2: Key Differences
| Feature | HIV-1 | HIV-2 |
|---|---|---|
| Global prevalence | High (worldwide) | Low (mostly West Africa) |
| Disease progression | Faster | Slower |
| Viral load | Higher | Lower |
| Transmission rate | Higher | Lower |
| Detection challenges | Most tests detect it | Some require specific kits |
| Kaposi’s sarcoma risk | Common in AIDS | Rare |
| Dual infections | Possible with HIV-2 | Does not protect against HIV-1 |
Origin and Evolution of HIV
HIV-1 Origin
- Believed to have crossed species from chimpanzees to humans.
- The Simian Immunodeficiency Virus (SIV) in chimpanzees is genetically related to HIV-1.
HIV-2 Origin
- Originated from sooty mangabey monkeys .
- Transmitted to humans through hunting and consumption of bushmeat.
Genetic Groups and Subtypes of HIV-1
HIV-1 Groups
- Group M (Major): ~90% of global infections
- Group O (Outlier): 1–5%, mostly in West-Central Africa
- Group N (Non-M, Non-O): Rare, mostly in Cameroon
- Group P: Extremely rare, discovered in Cameroon
HIV-1 Group M Subtypes (Clades):
- At least 10 known subtypes (A–K, skipping E and I)
- Subtypes differ in geographic distribution , transmissibility , and response to treatment .
| Subtype | Location |
|---|---|
| A | East and Central Africa |
| B | Americas, Europe, Australia, Middle East |
| C | Sub-Saharan Africa, India, Brazil |
| D | Central and Eastern Africa |
| F | South America and Romania |
| G | West and Central Africa |
| H, J, K | Africa, Middle East |
Subtype B is the most common in the United Kingdom, Europe, and the Americas .
Circulating Recombinant Forms (CRFs)
- When a person is infected by two or more subtypes , viral recombination can occur.
- This leads to the formation of CRFs – Circulating Recombinant Forms .
- CRFs are prevalent in regions with high subtype diversity.
- Example: CRF01_AE , CRF02_AG
CRFs can complicate diagnostic testing and antiretroviral therapy due to genetic variability.
Subtypes of HIV-2
HIV-2 is classified into eight subtypes (Groups A–H):
- Group A and B :
- Most prevalent and clinically significant
- Associated with pathogenic infections
- Groups C–H:
- Rare
- Primarily detected in isolated cases
Clinical Implications
- Diagnosis: Many commercial kits are designed to detect HIV-1 only; HIV-2 requires specific assays.
- Treatment: Some antiretroviral agents are less effective against HIV-2 (e.g., NNRTIs).
- Epidemiology: Understanding subtype and group is critical for public health tracking, vaccine design, and therapy.
High-Yield Points
HIV-1: More transmissible, faster progression, globally prevalent
HIV-2: Slower progression, confined mainly to West Africa
HIV is a retrovirus – uses reverse transcriptase to replicate
CD4+ T-cell destruction → immunodeficiency
Subtypes and CRFs impact transmission and treatment response
Group M, subtype B is dominant in the Western world
Always test for both HIV-1 and HIV-2 in endemic areas
