Human immunodeficiency virus

A chronic immunodeficiency resulting from infection with the human immunodeficiency virus (HIV)
Acquired immune deficiency syndrome (AIDS) is a severe form of HIV characterized by low CD4 count resulting in characteristic infections

Microbiology

HIV-1
- M group
  - Clade A: common in East Africa
  - Clade B: is common in Canada, Americas, Europe
HIV-2

Two phases: initial viral attachment, fusion, reverse transcription, and integration; and the following lifetime of the viral infection
Initial cellular infection
1. Binding or attachment of the virion gp120 Env surface protein to the CD4 receptor with CCR5 or CXCR4 coreceptor (on macrophage or T-cell, respectively).
2. Binding the receptor triggers a conformational change that exposes the fusion domain on gp41, which facilitates fusion and viral entry. The proceeding viral disassembly requires viral protein p24 to bind to cellular cyclophilin A.
3. In the cytoplasm, reverse transcriptase converts viral RNA into viral DNA. The RNA is degraded, then the complementary strand of DNA created.
4. The preintegration complex of double-stranded DNA is imported into the nucleus using viral Gag, viral protein R (Vpr), and integrase. Unlike other retroviruses, HIV does not require active replication to enter the nucleus.
Infection of lymphoid cells and lymph nodes, especially gut-associated lymphoid tissue (GALT)
- Infection therefore kills a large proportion of CD4 cells in the gut
HIV enters from the mucosa to infect activated Langerhans macrophages, which then get to the local lymphoid tissue

63,000 Canadians living with HIV in 2016
14% don't know they have it
Methods of acquisition in Canada
- MSM (52% of cases)
- People who inject drugs (17% of cases)
- Heterosexual sex (33% of cases)

Incubation period 2 to 6 weeks
Usually presents as an influenza-like illness
Symptoms include fever, lymphadenopathy, rash, myalgias, arthralgias, headache, diarrhea, oral ulcers, leukopenia, thrombocytopenia, and mild hepatitis

HIV serology
- Can test for HIV antibodies (usually combined IgM/IgG) and p24 antigen
- Window period of 3-4 weeks exists before antibodies are positive, and is shortened to 2-3 weeks with antigen testing
- If concern for acute seroconversion syndrome (within 2-4 weeks of exposure), may need to repeat serology
- Standard testing in Ontario includes HIV 1+2 antigen/antibody ELISA screen, followed by confirmatory p24 antigen ELISA
HIV qualitative PCR
- Indicated in cases of suspected acute seroconversion (with negative serology), previous indeterminate antibody results, or a newborn of an HIV-infected mother
- More specific than quantitative PCR for viral load
- Can be done from dried blood spot
- Generally only done for HIV-1 outside of a reference lab

Ab/Ag	Ab	Ag	RNA	Interpretation
–				HIV negative, or within serologic window period (2-3 weeks)
–			+	HIV positive, within the serologic window period (2-3 weeks)
+	+			HIV positive, following a 3-4 week window period
+	–	+		HIV positive, within the serologic window period for antibodies (3-4 weeks)

See also Routine follow-up for patients with HIV
HIV viral load
- Every 4 to 6 weeks until undetectable
- Then every 3 months until undetectable for 1 year
- Then every 6 months
CD4 count
- Every 3 to 4 months until viral load undetectable and CD4 count >350 for 1 year
- Then every 6 months until viral load undetectable for at least 2 years and CD4 count > 500
- Then stop monitoring routinely unless evidence of treatment failure
- Assess for failure if RNA level remains detectable at 24 weeks or if it increases to above 50 at any time
Repeat RNA level within 4 weeks