Transmission

HIV is present in an infected person’s blood and in other body fluids such as semen, vaginal secretions, rectal secretions and breast milk (1).

The classic modes of transmission are:

• unprotected intercourse between men
• use of infected instruments, in particular intravenous drug abuse with shared needles
• infusion with infected blood products - in the UK, classically haemophiliacs infused with imported factor VIII
• vertical transmission - either at birth, with second twins being infected more often than first, and between 13% and 45% transmission rate, also from breast milk; it is possible that there is a difference in prognosis determined by the timing of vertical transmission

Other modes of transmission:

• unprotected heterosexual intercourse, a mode accounting for most of the AIDS in Africa, and now the largest growth area in the developed world
  • most cases of heterosexually transmitted HIV are acquired outside UK, specially in sub-Saharan Africa (1)

The risk of transmission may be greater in the following situations:

• when the source has a high plasma viral load –
  • specially important in primary HIV infection
  • in low or undetectable plasma viral loads the risk of transmission may be less but the transmission is still possible
  • viral loads in the plasma normally directly implies to that of the genital tract – but their might be instances where there is a detectable viral load in the genital tract with an undetectable plasma viral load
• if there are breeches in the mucosal barrier e.g. – ulcer or trauma in mouth or genital ulcer, menstruation or other bleeding
• in people with sexually transmitted infections (STI’s) – STIs increase HIV transmission and HIV shedding from the genital tract (except in people receiving effective antiretroviral therapy (2).

Factors that are thought to

• increase the risk of transmission include:
  • high viral load
    • number of copies per mL of plasma HIV-1 RNA (viral load)
  • concomitant sexually transmitted infections (STIs)
    • especially genital ulcers of any cause, herpes simplex type-2 infection, and bacterial vaginosis
  • acute- and late-stage infection
  • receptive anal intercourse
  • behavioural factors - many sexual partners and concurrent partnerships
    
    
• decrease the risk of transmission include:
  • condom use
  • antiretroviral treatment
  • male circumcision
  • preexposure prophylaxis (3,4)

Reference:

• (1) The Medical Foundation for AIDS & Sexual Health (MedFASH) 2008. HIV for non-HIV specialists, diagnosing the undiagnosed: A practical guide for healthcare professionals secondary care to support improved detection and diagnosis of HIV in the UK
• (2) British Association for Sexual Health and HIV (BASHH) 2006. UK Guideline for the use of post-exposure prophylaxis for HIV following sexual exposure
• (3) Maartens G, Celum C, Lewin SR. HIV infection: epidemiology, pathogenesis, treatment, and prevention. Lancet. 2014;384(9939):258-71
• (4) Griswold J, Tungsiripat M. HIV for primary care physician. Cleveland clinic, Center for continuing education 2017