AIDS at 25

I mentioned yesterday that one way to help prevent new HIV infections is to treat people who are infected with herpes, another sexually-transmittted virus that infects as much as 20% of the population in the United States. That may seem odd; how does treating one viral infection prevent infection with a second virus?

Studies have shown that, generally, HIV isn't one of the more highly infectious viruses out there. For example, the hepatitis B and C viruses are transmitted via many of the same routes as HIV (such as sexual activity shared needles), but vary in their infectivity. If, for example, a health care worker gets stuck with a needle from an HIV+ patient, they have a very low probability of becoming infected with the virus; so low that less than 150 occupationally-acquired HIV transmissions have been positively documented in the literature. With hepatitis B virus, however, the risk is orders of magnitude greater: chance of infection following a needlestick is almost 1 in 3. The hepatitis C virus is between them, with about a 1 in 50 chance of becoming infected following such a needlestick.

The reasons for the differences in infectivity vary. Some viruses are present at higher levels in the blood or other body fluids, meaning that more particles are transmitted during each contact. Some have a lower infectious dose, meaning that it takes fewer viral particles to actually start an infection. However, host factors are also involved in transmission of pathogens, and one of these is the presence of other pathogens in the body.

This brings us to herpes. As I mentioned, HIV is not very efficiently transmitted between individuals in most cases. However, many studies examining the epidemiology of HIV transmission have shown that the presence of a pre-existing sexually transmitted infection is a big risk factor for the acquisition of HIV, and herpes is particularly important:

The open sores of herpes do play a role in the spread of HIV. A person with a herpes sore is three to five times more likely to acquire HIV if exposed to an HIV-positive sex partner. Also, people with HIV and herpes have an increased amount of HIV fluid in their open herpes sores, which increases the risk of transmitting both diseases to a partner during unprotected sex.

This allows HIV to be transmitted much more effectively. People with herpes who are co-infected with HIV are more likely to transmit the virus, especially during outbreaks, and people who are HIV negative but infected with herpes are more likely to be effectively exposed to the virus when having sex with an HIV-positive partner by virtue of the broken skin resulting from a viral outbreak. Other STDs also increase the likelihood of HIV transmission, but because of the physical effects of herpes virus infection on the skin of the genitals, treating infected individuals can potentially play a significant role in controlling the spread of HIV.