NEW YORK (Reuters Health) - The cancerous skin lesions known as Kaposi's sarcoma are closely related to HIV/AIDS, but the virus that causes Kaposi's was prevalent among US gay men before the HIV epidemic ever started, California researchers report.

Their study also revealed that far into the HIV era in San Francisco--when more men were practicing safe anal sex but largely not protecting themselves during oral sex--the Kaposi's-associated virus remained common.

They speculate that transmission routes other than anal sex, including oral sex, may be key in the spread of the cancer-causing virus, called Kaposi's sarcoma-associated herpesvirus (KSHV).

``We think the data strongly suggest that avoiding unprotected anal intercourse is not enough to avoid acquiring this viral infection,'' the study's lead author, Dr. Dennis H. Osmond, told Reuters Health. However, he added, this study cannot establish that definitively because it did not look at each man's individual sexual behavior and his risk of KSHV infection.

Osmond, a researcher at the University of California, San Francisco, and his colleagues report the findings in the January 9th issue of The Journal of the American Medical Association.

KSHV, also known as human herpesvirus 8, is believed to be necessary for Kaposi's sarcoma to develop. In most healthy people the virus is unlikely to lead to Kaposi's sarcoma, but the immune system damage caused by HIV makes infected individuals vulnerable to the cancer.

Because of the cancer's close relationship to HIV and AIDS, experts have suspected that the HIV epidemic triggered the similar one seen with KSHV infection.

But in looking at blood samples from homosexual men involved in three San Francisco studies from the late 1970s to the 1990s, Osmond's team found that KSHV was prevalent when HIV infections were still ``close to zero.'' What's more, it remained stable over the two decades--even after both HIV prevalence and unprotected anal sex dropped substantially among men in the study.

From 1978 through 1980, the researchers report, about 28% of the men were positive for KSHV. That percentage held steady through 1996, when about 26% of men had the virus. Yet during the same period, HIV prevalence varied greatly, peaking in the mid-1980s at nearly half of men in one study, then declining to around 17% among men in another study in the early-1990s.

In addition, the proportion of men reporting unprotected anal sex fell from more than half in the mid-80s to 11% in 1993, according to one study the researchers analyzed.

On the other hand, unprotected oral sex remained common throughout the study periods, ranging from 60% to 90%.

``The inference,'' the researchers write, ``is that behaviors remaining highly prevalent during this period, such as unprotected penile-oral intercourse...seem more likely transmission routes (of KSHV).''

However, Osmond's team points out that there is ``no consensus'' on how KSHV is typically transmitted among gay men, and this study cannot answer that question.

They speculate that KSHV--an apparently ``ancient virus''--may have long been endemic among homosexual men without being noticed. In the US, they note, KSHV-infected people without HIV rarely develop Kaposi's sarcoma.

Osmond pointed out that the cancer has seen a recent decline in the US HIV-positive population. Some, he said, have suggested that this might be due to a decline in KSHV transmission--a belief that seems to be wrong based on these findings.

Instead, Osmond said, the decline in Kaposi's sarcoma appears to be the result of patients' better immune function since the introduction of powerful drug ``cocktails'' containing protease inhibitors.

SOURCE: The Journal of the American Medical Association 2002;287:221-225.

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