HIV's Hidden Partner
Stopping STD's is critical to HIV control.
By Anne-christine d'Adesky

If you've been hiding under a rock these past two years, you might have missed the other epidemic that's been grabbing the media spotlight: sexually-transmitted diseases (STDs). The United States now has highest rates of STD infection among industrialized nations in the world, with 10 million to 12 million new cases annually. Just behind us are southeast Asia, sub-Saharan Africa, Latin America and the Caribbean, places in the developing world where the HIV epidemic is also hitting hard.

In the U.S., STDs like herpes, chlamydia, and genital warts (human papillomavirus, or HPV) are spreading like wildfire among women and teens in particular. There's been a 30 percent increase in the rate of genital herpes and one in five Americans over the age of 12 now carry the virus, with more women than men infected and a far greater number of African-Americans than other groups. Many people are often unaware that they have been exposed and may be passing these infections on to their sexual partners, a troubling trend that also has serious implications for HIV prevention and treatment. For example, a new five-year Centers for Disease Control study conducted at 12 STD clinics in seven cities found that heterosexual men made up the majority of new HIV cases in these clinics; many had a history of drug abuse. Another large California study found that half the people who got tested for HIV in STD clinics didn't come back for their HIV test results.

A number of studies show that STDs can greatly increase the risk of HIV transmission and that co-infection with STDs can speed up the rate of HIV activity in exposed individuals. Because HIV harms the immune system, STDs are often harder to treat in HIV-positive individuals, causing chronic and drug-resistant infections. These may in turn lead to serious illnesses like cervical or anal cancers that are linked to certain types of human papillomavirus, for example (see table).

Today, the issue of STD control has become even more urgent as new research shows that some STDs increase the amount of HIV found in genital secretions, and that different strains of HIV are found in semen and the female genital tract, than in blood. These findings suggest that the virus may hide and grow in different reservoirs in the body, a discovery that has serious implications for current HIV treatment. In people taking potent HIV drug cocktails, the virus may be undetectable in blood, but supersensitive tests can detect viral particles called provirus lying inside dormant or latently-infected immune cells. A recent small study in the journal AIDS found, for example, that in 11 HIV-positive men, protease inhibitor drugs either did not penetrate the testes or did so with difficulty. Most of the men also had different strains of HIV in their blood and semen.

What happens then if someone on therapy gets a new or recurring STD infection? At least theoretically, STD co-infection could reignite a dormant HIV infection in the male or female genital tract, providing the virus with an opportunity to take off in these protected cell reservoirs. That in turn increases the risk of HIV transmission to a sexual partner and underscores our need to aggressively screen and treat STDs as part of routine HIV care.

These recent observations have significant implications for global public health policy. It's far less expensive to treat STDs with standard antibiotics than to treat new cases of HIV with expensive drug cocktails (see "The Other War On Drugs"). Today, there's a new push to integrate HIV and STD prevention in clinics, schools, and community groups, and to combine screening and treatment by medical providers. New vaccines for herpes, chlamydia, and genital warts are also being tested that stand to make a huge impact on these mini-STD epidemics.

STD-HIV Co-infection
A battery of global studies in recent years have documented that STDs facilitate HIV transmission. STDs like HPV and genital herpes, for example, cause disruptions of the mucosal lining of the genital tract, making it easier for HIV to infect immune cells. That means if you have an STD, you have a higher chance of contracting HIV. In people with HIV, STDs like Treponema pallidum, the bug that causes syphilis, have also been shown to increase the rate of HIV replication, based on test-tube studies. In one Zaire study, researchers concluded that chlamydia and gonorrhea increased the amount of HIV (viral load) found in genital secretions of both men and women. Treating these STDS decreased the amount of virus found in these sexual fluids.

In the U.S., a 1997 study found that men who are HIV positive and contract other STDs have eight times as much HIV in their semen than men without these co-infections. Higher levels of HIV have also been found in HIV-positive men co-infected with Trichomonas vaginalis.

A similar pattern has been seen in women with HIV who have a high risk of getting vaginal infections and various STDs that are hard to treat and may become chronic infections. HIV-positive women have approximately a tenfold increase in abnormal Pap smears and a high risk of developing serious diseases like cervical cancer and pelvic inflammatory disease. New studies also show replicating HIV can be found in the female genital tract of women on anti-HIV combination therapy. As with men, STDs appear to increase the amount of HIV found in vaginal secretions, increasing the risk of infection to a sexual partner.

But not all studies show a clear causal link between STD's and increased HIV progression. In one 1994 Ugandan study, researchers found that treatment of curable STDs had no effect on the rate of new HIV infections. That contrasted sharply with a 1995 Tanzanian study where entire communities were treated for symptomatic STDs. There, the rate of new HIV infections fell by 38 percent in those receiving STD treatment, compared with control groups. The main difference in these two studies may be that in Uganda, HIV infection was well underway, affecting 16 percent of the population. But in Tanzania, only 4 percent of the population carried the virus. (It's important to note that these numbers may be low, since many people in the world haven't been tested for HIV. That's true here, too.)

The bad news about the STD-HIV link has obscured the breakthrough progress we've made in stopping two major diseases. Here in the US., rates of syphilis and gonorrhea have fallen to an all time low, although 15 cities still have elevated rates, with Baltimore leading the nation in both diseases. Today, there's talk of trying to completely eradicate syphilis within a few years, and trying to do the same for gonorrhea. While we still lack the tools to cure other STDs like herpes and hepatitis, the gains we've made against the great sexual scourges of the Victorian era show us that with adequate funding and the right public health priorities, we can alter the course of STDs and in turn, stem the tide of HIV.

There's also the promise of new treatments and vaccines. We've already got vaccines against hepatitis A and B, and new drugs to treat hepatitis B and C. Now we're making inroads using combination antiviral strategies involving drugs that also work against HIV like lamivudine (3TC). (see Anti-HIV Drugs). Novel vaccines are also in the works for genital herpes, HPV and chlamydia, among other STDs. The first generation of longer-lasting "intra-vaginal" DNA-based vaccines are being tested in animals. Anal vaccines are also being dreamed up. These kinds of vaccines can stimulate a genital immune response to STDs that, again theoretically, could work to thwart new or latent HIV infection in genital reservoirs.