By Jared Stern and Paula Cevaal
Since the first reports of AIDS in 1981, HIV and AIDS have among the general public primarily been known as a sexually transmitted infection(STI). Indeed, the majority of new HIV infections can be attributed to unprotected sex, as has been discussed in our previous blog post. In that case, HIV enters the body through the mucosal membrane or via micro tears that allow direct entry into the bloodstream. However, in the global HIV and AIDS response, it is essential to acknowledge all routes of transmission. The two major non-sexual routes of transmission are: transmission via blood-blood contact, mainly among people who inject drugs (PWID) via the sharing of needles; and vertical transmission (mother-to-child- transmission) during pregnancy, during delivery or through breastfeeding. In both cases, the virus has very specific entry points to the body, allowing effective transmission. This means that a transmission through regular skin contact, coughing, using the same toilet or sharing a drinking glass (contact via saliva) is not possible: our body’s natural barriers prevent HIV from entering the body.
Biology behind transmission via blood-blood contact
To understand how HIV is transmitted by needle sharing, we need to see HIV as a blood-borne infection in addition to it being a sexually transmitted infection. HIV can be present in the blood both as a free virus particle as well as inside infected CD4+ cells. Blood-borne infections can be transmitted via direct blood-blood contact. Transmission of HIV via blood-blood contact has a relatively high per-act efficiency compared to sexual transmission1. This is mainly due to the fact that the virus is directly transmitted into the bloodstream, whereas during sexual intercourse the virus in general needs to pass the mucosal barrier of the genital tract. Once within the blood, HIV can freely infect CD4+ white blood cells and immediately disseminate via the bloodstream.
As the virus does not have a barrier to pass, the successful transmission of HIV is dependent on the amount of viable virus that is transmitted. This is again dependent on the concentration of the virus in the blood, the amount of blood that is transferred, and the viability of the virus in the blood2. One of the most efficient (or high-risk) routes of transmitting HIV is via a blood transfusion, due to the high amount of virus-containing blood that is transferred. During the event of needle sharing among PWID, infected blood that remains inside the needle can be injected into the bloodstream of another person. Since the amount of blood transferred via needle sharing is significantly smaller than during a blood transfusion, the per-act risk is correspondingly much lower at only at 0.63% vs 93% for blood transfusions1. Thirdly, blood-blood transmission can also be established during skin-contact when the skin of both persons is wounded, providing access to the bloodstream and thus overcoming the natural barrier of the skin. However, as blood will naturally flow out of an open wound, the amount of infected blood that can eventually enter someone’s bloodstream (by entering the wound) is incredibly small. Therefore, although possible, this route of transmission accounts for a small minority of the new infections worldwide. Similarly, HIV cannot be transmitted via mosquito bites because the volume of blood transferred is far too small.
The reason why HIV transmission via needle sharing accounts for so many transmissions worldwide, lies in the circumstantial situation. Under normal circumstances, HIV can only survive for a short amount of time outside the body. As a result, HIV transmission has not been reported as a consequence of contact with spillages of blood, semen or other body fluids. However, when contained inside a needle, HIV has been shown to survive for longer, thus increasing the risk at transmission. Secondly, due to the often high amount of drug-injections per day, HIV transmission among PWID is unfortunately associated with very explosive outbreaks of HIV in communities that were unaware of the local risk of HIV. For example, in Eastern Europe and Central Asia (EECA), a significant 30% of newly acquired HIV infections originate from PWID3 and the epidemics in these regions are among the fastest growing in the world 4. Lastly, the sharing of needles is often not refined to one partner but rather expanded to multiple people, particularly in so-called “shooting galleries”, again increasing the risk of rapid transmission within a community5. Needle exchange programs, providing clean needles and syringes to PWID, have been effective in reducing HIV infections. However, achieving a sustainable reduction of transmission among PWID will require changes in existing policy so that needles and syringes can be purchased worldwide without prescription and can be legally possessed6.
Vertical transmission
While we have seen that blood-blood transmission is of primary importance among PWID, the most prevalent route of infection of children is by vertical transmission7. In absence of antiretroviral treatment (ART), the risk of a woman living with HIV transmitting the virus to her child ranges between 15%-45%. Transmission can occur at three stages: during pregnancy by HIV passing the placenta; during delivery via exposure of the child to vaginal fluids or blood containing HIV; or after birth via breast feeding. The rate of transmission can be decreased to below 5% with appropriate interventions, among which the most important is initiation of ART for all HIV-infected pregnant or breastfeeding women. Being on ART will decrease the viral load, and thereby decrease the risk of transmission at all three stages. In addition, WHO guidelines have been recommending to provide ART as prophylaxis to the infant to prevent HIV infection after birth. Depending on the estimated risk during pregnancy, this can vary between 4-6 weeks of medication. Risk factors include viral load of the mother, and route of delivery: vaginal delivery is considered of higher risk as compared to cesarean delivery (c-section)8. Lastly, it is important to mention the most recent guidelines on breastfeeding practices. As mentioned, breastfeeding poses a risk at HIV transmission due to repetitive contact of the infant’s gut lining to infected breast milk, especially when the mother is not on ART. However, breastfeeding is considered of such importance for infant nutrition that unless resources allow “acceptable, feasible, affordable, sustainable and safe” replacement formula milk, breastfeeding remains the recommended practice for a minimum of 6 months9. To conclude, between 2009 and 2014, many countries that implemented this accelerated roll-out of ART have seen a drop in vertical transmission by over 60%, meaning we are getting closer than ever to eliminating vertical transmission as a public health concern10.
1 Patel, P. et al. Estimating per-act HIV transmission risk. AIDS 28, 1509–1519 (2014).
2 Baggaley, R. F., Boily, M.-C., White, R. G. & Alary, M. Risk of HIV-1 transmission for parenteral exposure and blood transfusion: a systematic review and meta-analysis. AIDS 20, 805–12 (2006).
3 Joint United Nations Programme on HIV/AIDS. The Gap Report. September 2014. Available at http://www.unaids.org/sites/default/files/media_asset/UNAIDS_Gap_report_en.pdf Accessed 15-03-2018
4 Jolley, E. et al. HIV among people who inject drugs in Central and Eastern Europe and Central Asia: a systematic review with implications for policy. BMJ Open 2, e001465 (2012).
5 Des Jarlais, D. C., Kerr, T., Carrieri, P., Feelemyer, J. & Arasteh, K. HIV infection among persons who inject drugs: ending old epidemics and addressing new outbreaks. AIDS 30, 815–26 (2016).
6 Abdul-Quader, A. S. et al. Effectiveness of Structural-Level Needle/Syringe Programs to Reduce HCV and HIV Infection Among People Who Inject Drugs: A Systematic Review. AIDS Behav. 17, 2878–2892 (2013).
7 Ahmad, Nafees. “Molecular Mechanisms of HIV-1 Mother-to-Child Transmission and Infection in Neonatal Target Cells.” Life sciences 88.21-22 (2011): 980–986. PMC. Web. 15 Mar. 2018.
8 Panel on Treatment of HIV-Infected Pregnant Women and Prevention of Perinatal Transmission. Recommendations for Use of Antiretroviral Drugs in Pregnant HIV-1-Infected Women for Maternal Health and Interventions to Reduce Perinatal HIV Transmission in the United States. Available at http://aidsinfo.nih.gov/contentfiles/lvguidelines/PerinatalGL.pdf. Accessed 15-03-2018.
9 Guidelines on HIV and Infant Feeding: Principles and recommendations on infant feeding in the context of HIV and a summary of evidence. World Health Organization 2010. Available at http://apps.who.int/iris/bitstream/10665/44345/1/9789241599535_eng.pdf. Accessed at 15-03-2018.
10 Chi, B. H., Stringer, J. S. A. & Moodley, D. Antiretroviral drug regimens to prevent mother-to-child transmission of HIV: a review of scientific, program, and policy advances for sub-Saharan Africa. Curr. HIV/AIDS Rep. 10, 124–33 (2013). Accessed at 15-03-2018.