This illustration shows how HIV particles slip between loosely connected skin cells to reach immune system targets such as Langerhans cells (orange), macrophages (purple), dendritic cells (green) and CD4 positive T cells (blue). (Ann Carias)

HIV can travel through a healthy woman's genital skin and reach its immune cell targets in four hours, researchers said Tuesday.

"This is an unexpected and important result," said the study's principal investigator, Thomas Hope, a professor of cell and molecular biology at Northwestern University in Chicago. "We have a new understanding of how HIV can invade the female vaginal tract."

Scientist had long believed that the normal lining of the female genital tract worked as a barrier against HIV during sexual intercourse, but the new results suggest otherwise.

Hope presented his findings at Tuesday's annual meeting of the American Society for Cell Biology in San Francisco.

In the study, researchers developed a new method to label HIV viruses with special fluorescent tags. The method allowed them to track the viruses as they entered the outermost lining of the female genital tract, a process that took four hours.

The experiments were done using human tissue obtained during hysterectomies and tissue from rhesus macaque monkeys.

The virus tended to penetrate the barrier when skin cells were shed and were no longer bound as tightly, the researchers said.

HIV reached 50 microns beneath the skin, a depth where immune cells targeted by HIV are found, they reported.

More than 1 way to transmit virus?

Previously, the single layer of skin lining the cervical canal was thought to be the weak point for the HIV virus to invade a woman's immune system, Hope said.

"A big mistake in this field is the idea that transmission only takes place one way," Hope said. "We say one of those ways that needs to be in the equation is that the virus can be transmitted directly through the skin."

If the findings are confirmed in further studies, it could help researchers develop new prevention strategies such as microbicides that block the entry of HIV through a woman's genital skin.

Before that could happen, researchers would need to show that the virus actually infects immune cells in the vaginal tract.

Women account for nearly 70 per cent of HIV-AIDS cases worldwide, according to the World Health Organization.

In Canada in 2005, women accounted for one-fifth of people with HIV/AIDS, up from one-tenth in 1995, the Public Health Agency of Canada estimated.