August 2019 Issue #32 August 2019 Issue #32 | Page 154

The use of ART by someone with HIV can significantly reduce the risk of transmitting HIV to others.23,24 Successful ART lowers the amount of virus (also known as viral load) in the bodily fluids, including the rectal fluid, to undetectable levels3,4,5,6 and research shows this can dramatically reduce the risk of HIV transmission.25 The viral load in the blood of a person living with HIV is monitored as part of their routine clinical care. Although the risk of HIV transmission is significantly lowered when the viral load is undetectable in the blood, the risk may not be eliminated. Undetectable does not mean that there is no virus, only that the amount of virus is below the limits that tests can detect. Therefore, HIV transmission may still be possible because there is still virus present. Also, it is possible for people who have an undetectable viral load in the blood to sometimes have detectable (although lowered) levels of virus in their rectal flu- id.2,3,4,5,6 However, this appears to occur infrequently (although rectal fluid has been studied less than semen and vaginal fluid) and it is unclear how significant it is in terms of HIV transmission. The exact risk of HIV transmission through anal sex (where the HIV-negative person is the insertive partner) when the blood viral load of the partner is undetectable is not known and has not been well studied. However, a preliminary analysis of an ongoing study observed that, when the blood viral load was undetectable, no HIV transmissions occurred among stable serodiscordant gay male couples who engaged in approximate- ly 11,000 condomless anal sex acts (where the HIV-negative person was the insertive partner).24 An estimated six to 12 infections would have been expected if the HIV-pos- itive partners in this study were not on ART and had a detectable viral load. Therefore, although this does not mean the risk is zero, it does show that ART definitely reduces the risk of HIV transmission through this type of anal sex; ongoing analysis from this study will show us more clearly exactly how large this reduction is. Factors that cause inflammation in the rectum of a person with HIV may increase the viral load in the rectal fluid (but not in the blood) and subsequently increase the risk of HIV transmission to an HIV-negative insertive partner.2 Rectal trauma, friction and sexually transmitted infections (STIs) – such as gonorrhea, chlamydia, herpes, and syphilis – can all cause rectal inflammation. Promisingly, a recent study found that rec- tal gonorrhea and chlamydia did not increase the rectal fluid viral load among a small group of people with HIV on ART.5 This suggests that ART may reduce the ability of STIs to increase rectal viral load and HIV transmission risk. Key messages It is important that HIV prevention messaging includes rectal fluid as one of the fluids that can contain and transmit HIV. If rectal fluid is excluded, it could lead an HIV-neg- ative person who is the insertive partner during anal sex to underestimate their risk of HIV infection; or a person with HIV who is the receptive partner during anal sex to underestimate their risk of transmitting HIV. 154