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STIs: What Role Do They Play in HIV Transmission?

Summer 2012

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Table of Contents

Rates of sexually transmitted infections (STIs) in Canada are on the rise. Research suggests that STIs can increase both a HIV-negative person's risk of becoming infected with HIV and an HIV-positive person's risk of transmitting HIV to someone else. This article explores how STIs may increase the risk of sexual HIV transmission, how STIs may be undermining our HIV prevention strategies, and what we can do about it.

The Basics

There are many different types of STIs. Some of the more common ones include gonorrhea, chlamydia, Trichomonas vaginalis, human papillomavirus (HPV), herpes and syphilis. Some of these (such as herpes and syphilis) cause ulcers on the genitals or rectum, while others (such as gonorrhea, chlamydia, and Trichomonas vaginalis) can cause painful urination and/or discharge. HPV can cause the growth of genital warts on or around the genitals or rectum. Most STIs can also infect the mouth and throat.

STIs can cause swelling, redness and pain in the infected area. However, many people who have an STI have no symptoms at all -- this is known as being asymptomatic -- so neither they nor their partner may realize they are infected.

Although all STIs are treatable, only some can be cured through treatment and completely cleared from the body. STIs that can be cured through treatment include gonorrhea, chlamydia, syphilis and Trichomonas vaginalis. STIs that can be managed through treatment, but not cured, include HPV, herpes and HIV.


How Do STIs Cause Inflammation?

To understand how STIs increase the risk of HIV transmission, we need to understand what STIs do once they come in contact with our mouth, genitals or rectum.

Sexually transmitted infections are caused by bacteria, viruses and parasites, also known as germs. When germs enter the body, they are recognized by the immune system, which, as part of the body's response to infection, starts a process known as inflammation. This leads to the symptoms associated with many STIs, such as redness, swelling and pain. The inflammatory process "activates" our immune cells to fight germs and recruits more immune cells to the site of the infection, helping the body clear the germs. For example, if someone has a vaginal STI, then the inflammatory response will recruit more immune cells to the lining of the vagina.

How do STIs Increase the Risk of Becoming Infected With HIV?

Research suggests that HIV-negative individuals with an STI may be at increased risk of becoming infected with HIV through anal sex, vaginal sex, frontal sex (a term used by some trans people to refer to sex using genitals on the front of the body) and oral sex.1 But how do STIs increase someone's risk of HIV infection?

Scientists believe that inflammation plays an important role. Inflammation increases the concentration of "activated" immune cells in the area infected with the STI. Although the inflammatory response is meant to help fight the sexually transmitted infection, HIV likes to infect some of these recruited immune cells, also known as CD4 cells. Also, HIV finds it easier to infect, and replicate in, CD4 cells that are "activated". Therefore, if someone has an STI in the mouth, genitals or rectum, and that area is exposed to HIV, the higher concentration of "activated" CD4 cells facilitates HIV infection, replication and spread throughout the body.

All types of STIs cause inflammation and therefore may increase the risk of becoming infected with HIV in this way. Also, some types of STIs increase the risk of HIV infection through ulcers, which create "holes" or ways for HIV to enter the body through the mouth, genitals or rectum.

How Do STIs Increase the Risk of Transmitting HIV?

Research suggests that HIV-positive individuals with an STI may be at increased risk of passing HIV to someone else through anal, vaginal and frontal sex.2 But how do STIs increase someone's risk of transmitting HIV?

Again, it comes down to inflammation. If a person living with HIV has an STI, then inflammation will "activate" and recruit more immune cells to the infected genitals or rectum. Some of the immune cells in a person living with HIV are already infected with HIV, therefore the inflammatory response brings more HIV (contained in the infected immune cells) to the site of the STI in the genitals or rectum. Consequently, more HIV enters the body fluids in that area. For example, a vaginal STI increases the amount of the virus (viral load) in the vaginal fluid. Research shows that the more virus there is in the body fluids of a person living with HIV the higher the risk of passing HIV to someone else.3 HIV also replicates, or makes more HIV, quicker in immune cells that have been "activated" through inflammation, compared to immune cells that are not "activated."

STI Rates on the Rise

In Canada, the rate of new STIs is on the rise.4 According to the latest report published by the Public Health Agency of Canada, over the past decade chlamydia and gonorrhea rates have increased almost twofold, while syphilis rates have increased more than eightfold (see table).

The rate of new STI diagnoses is defined as the number of STIs diagnosed in the Canadian population over the course of a year. For example, if an STI had a rate of 150 in 2009, this means that 150 infections were diagnosed for every 100,000 people in the Canadian population that year.

























Source: Public Health Agency of Canada

Research suggests that STIs are particularly prevalent among people living with HIV. A recent review of 37 studies found that, on average, 16.3% of people living with HIV were co-infected with another STI.5 Included in the review was a Canadian study, which enrolled people living with HIV from infectious disease clinics in Edmonton, Quebec City and Toronto; of those participants, 54% were co-infected with genital herpes.6 Researchers also estimate that up to half of new syphilis cases in Canada occur in people living with HIV.7

Do STIs Undermine Our HIV Prevention Efforts?

As STIs in Canada are increasing among both HIV-positive and HIV-negative individuals, they may be undermining HIV prevention strategies used by individuals and public health agencies in this country.

HIV Prevention Strategies Used by Individuals

STIs may decrease the effectiveness of some prevention strategies that individuals are using to reduce their risk of HIV transmission. Although condoms are a highly effective way of reducing both HIV and STI transmission, they aren't always used. Instead, some people reduce their risk of HIV transmission in other ways, for example, by having oral sex instead of anal and vaginal sex, through serosorting (practice of choosing sexual partners based on their and your HIV status), strategic positioning (adopting the insertive or receptive role during unprotected sex depending on HIV status), pre-exposure prophylaxis (PrEP), post-exposure prophylaxis (PEP), male circumcision, and having unprotected sex with an HIV-positive person only if they have an undetectable viral load. Although these strategies may reduce someone's risk of HIV transmission, they do not reduce the risk of STI transmission. If the use of these strategies leads to the transmission of STIs, people may inadvertently increase their (or their sexual partner's) overall risk of HIV transmission. In other words, STIs may decrease the protection provided by certain HIV risk-reduction strategies.

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See Also
Quiz: Are You at Risk for HIV?
Ten Common Fears About HIV Transmission
Is HIV the Only Incurable Sexually Transmitted Disease?
The HIV-STD Connection
More on Sexually Transmitted Diseases

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