HIV in Canada: A primer for service providers


Treatment of other sexually transmitted infections (STIs)

Key Points

  • The presence of sexually transmitted infections (STIs) may increase the risk of HIV transmission.
  • Evidence shows that if a person with HIV is taking antiretroviral treatment and maintains an undetectable viral load, they will not transmit HIV sexually, even when an STI is present in either partner.
  • Regular STI testing and treatment should be part of any comprehensive sexual health plan.

STIs impact overall health, and several studies show that they can also increase the chance of HIV transmission.

STIs may increase the risk of HIV transmission to an HIV-negative person through two mechanisms. First, STIs (and certain genital conditions such as yeast infections and bacterial vaginosis) cause inflammation, which is a natural immune response. This response brings additional immune cells to the site of the STI and activates these immune cells to help fight the infection. This is meant to be protective; however, HIV targets these immune cells for infection and prefers to target activated immune cells. The more activated immune cells present, the more cells HIV can infect and replicate (or make additional copies of itself) within. This allows HIV to replicate more quickly and cause a permanent infection. Second, some types of STIs (e.g., herpes and syphilis) can damage the mucous membranes making it easier for HIV to enter the body through ulcers in the genitals or rectum.

People who are HIV positive and on antiretroviral treatment with an undetectable viral load do not transmit HIV through sex, even if an STI is present. However, in people with HIV who are not maintaining an undetectable viral load, having an STI may increase the risk of passing HIV. If a person is HIV positive and has an STI, inflammation brings more virus to the site of the STI. More virus is present because inflammation brings more “activated” CD4 cells to the site of the STI and along with it HIV that has infected these cells. HIV also replicates more quickly in CD4 cells that have been “activated” through inflammation. Thus, inflammation can increase the HIV viral load in a particular area, which can increase the risk of passing HIV to someone else.

STI management is only a partially effective way to prevent HIV transmission on its own, because treating STIs can reduce but not eliminate the risk of HIV transmission. HIV does not rely on the presence of STIs in either partner to be passed through sex. However, regular testing and treatment for STIs is important for sexually active people to include in their comprehensive sexual health plan.


Treating Sexually Transmitted Infections to Prevent HIV Transmission

Human immunodeficiency virus (HIV) sexual transmission risk with bacterial sexually transmitted infection (STI) co-infection – Public Health Ontario

Canadian guidelines on sexually transmitted infections – Public Health Agency of Canada

Gonorrhea – CATIE/SIECCAN fact sheet

Chlamydia – CATIE/SIECCAN fact sheet

Syphilis – CATIE fact sheet

Genital herpes – CATIE/SIECAN fact sheet


  1. Ward H, Rönn M. The contribution of STIs to the sexual transmission of HIV. Current Opinion in HIV and AIDS. 2010 Jul;5(4):305–310.
  2. Rodger AJ, Cambiano V, Bruun T, et al. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational study. The Lancet. 2019 Jun 15;393(10189):2428–2438.
  3. Bavinton, BR, Pinto, AN, Phanuphak, N et al. Viral suppression and HIV transmission in serodiscordant male couples: an international, prospective, observational, cohort study. The Lancet HIV. 2018 Aug 1;5(8):E438–E447.
  4. Johnson LF, Lewis DA. The effect of genital tract infections on HIV-1 shedding in the genital tract: a systematic review and meta-analysis. Sexually Transmitted Diseases. 2008 Nov;35(11):946–959.
  5. Grosskurth H, Mosha F, Todd J, et al. Impact of improved treatment of sexually transmitted diseases on HIV infection in rural Tanzania: randomised controlled trial. The Lancet. 1995 Aug 26;346(8974):530–536.
  6. Kaul R, Kimani J, Nagelkerke NJ, et al. Monthly antibiotic chemoprophylaxis and incidence of sexually transmitted infections and HIV-1 infection in Kenyan sex workers: a randomized controlled trial. Journal of the American Medical Association. 2004 Jun 2;291(21):2555–2562.
  7. Wawer MJ, Sewankambo NK, Serwadda D, et al. Control of sexually transmitted diseases for AIDS prevention in Uganda: a randomised community trial. Rakai Project Study Group. The Lancet. 1999 Feb 13;353(9152):525–535.
  8. Watson-Jones D, Weiss HA, Rusizoka M, et al. Effect of herpes simplex suppression on incidence of HIV among women in Tanzania. New England Journal of Medicine. 2008 Apr 10;358(15):1560–1571.
  9. Celum C, Wald A, Hughes J, et al. Effect of aciclovir on HIV-1 acquisition in herpes simplex virus 2 seropositive women and men who have sex with men: a randomised, double-blind, placebo-controlled trial. The Lancet. 2008 Jun 21;371(9630):2109–2119.