Biological Susceptibility

Biological Susceptibility - Women & HIV:

A brief overview of what biological factors put women at risk

­­­­­­­­­­­­What physiological characteristics of the female reproductive system increase a woman’s risk?

When considering a woman’s risk for HIV it is important to consider that she is biologically susceptible to contracting the virus.

Because of the physiological make up of the female reproductive system, a woman is more vulnerable than her male partner in a number of ways:

  • HIV is a virus that cannot survive outside of the body fluid that contains it. If a penis is exposed to vaginal fluids that contain HIV during vaginal sex, the fluid will dry up once the penis has been removed from the vagina. However, semen does not dry out inside the vagina, which means that a woman can be exposed to the virus for a longer period of time.  Also, the vaginal lining has a greater surface area than a penis, so the space where HIV could be transmitted is greater in a woman than a man.
  • The vagina is quite vulnerable to invasion by bacteria and viruses. The vagina is a warm, moist environment which is ideal for the growth of bacteria which can increase the risk of HIV transmission.
  • Semen generally contains more HIV than vaginal fluids do. How much virus an HIV positive person has can depend on many things (such as whether they are on treatment or not), but for the most part a woman is exposed to more HIV from semen than a man will be from vaginal fluid.
  • Wet linings, called mucous membranes, line the openings of the body including the mouth, nose, vagina, cervix, urethra, foreskin of the penis and the anus/rectum.  In sexual transmission of HIV the virus must pass through the mucous membrane for a new infection to occur.  Some linings, such as those in the anus/rectum and the cervix, are very thin and it is easier for HIV to pass through.  Young women are especially vulnerable because their cervix has not fully developed.  At this time it is even easier for HIV to pass through the cervix.
  • As mentioned in Module 2, HIV targets CD4 cells. A large number of these cells are found in the cervix because it acts as a barrier to protect a potential fetus. For this reason it is easier for the HIV virus to find the cells it will infect inside a woman, compared to a man.  On the penis, most of these cells are found in the foreskin and for this reason circumcision helps to protect men.

What about other Sexually Transmitted Infections?

Having a sexually transmitted infection will also increase the risk of HIV transmission. If a woman has an active STI the risk of HIV transmission is increased because open lesions or ulcers can act as points of entry for the virus, and it will also cause weakening of the mucous membranes.

STI’s cause inflammation of the mucous membranes which means there is an increase in the number of immune cells, including CD4cells, at the infected area. This not only increases a person’s vulnerability to HIV infection, but also increases an HIV positive person’s chance of transmitting the virus.

In many cases, women with STIs will generally have unpronounced symptoms so the infection often goes unnoticed.  Women who do not have regular PAP tests or other STI screening tests are also at a greater risk because they are at an increased risk of having an STI without knowing it.

Does taking birth control change a woman’s biological risk?

Animal studies have also shown that progesterone-based birth control such as Depo-Provera, seem to reduce the thickness of the vaginal lining. If this is also the case in humans, it would mean that taking oral contraceptives could make it easier for HIV to pass through the wet lining of the vagina. Research about the direct effects of birth control on a woman’s risk for HIV is still ongoing.

Even though it is still to be determined if birth control directly increases biological risk, it is known that using Depo-Provera increases the risk of other sexually transmitted infections such as gonorrhea. As previously mentioned, having an STI significantly increases the risk of HIV transmission

What biological factors make women experience HIV differently?

If a woman becomes infected, her experience with HIV can also be very different than a man’s. There are a number of opportunistic infections that are specific to, or more likely to occur in, women - such as pelvic inflammatory disease, cervical cancer, breast cancer or vaginal fungal infections.

When women present these symptoms to their doctor, they may not be recognized as opportunistic infections and be misdiagnosed as general gynecological conditions. This could mean that women who are potentially HIV positive are left undiagnosed, and without proper treatment.

Women are also uniquely impacted by HIV because of the implications it can have for a pregnancy. If a woman with HIV becomes pregnant, she can potentially pass the virus to her baby in the uterus, during birth, or through her breast milk. As mentioned in module 2, this is referred to as vertical transmission. This being said, should an HIV positive woman become pregnant, she is able to take precautions to prevent transmission to her child if she is aware of her HIV status.   Before treatment was made available about 25% of babies born to a HIV positive mother were HIV positive. Since the development of effective treatment, risk of vertical transmission in Canada is reduced to less than 1%. It is important that a pregnant woman’s healthcare team is aware of her HIV positive status and that she follow all treatments and precautions during and after her pregnancy.

For more information about pregnancy guidelines or other HIV related conditions specific to women see the fact sheets available at