WASHINGTON (Reuters) - Researchers say they have developed a cream that might prevent herpes infection for as long as a week -- a potentially big step in protecting women from the sexually transmitted infection.

The cream uses a new kind of therapy called RNA interference to turn off genes that the virus uses to invade cells, the researchers reported on Wednesday.

The cream, being developed by Massachusetts-based Alnylam Pharmaceuticals Inc, protected mice from herpes simplex 2, the virus that causes genital herpes, Deborah Palliser of Harvard Medical School in Boston and Albert Einstein College of Medicine in New York and colleagues reported.

“A vaginal microbicide able to protect against HSV-2 transmission could contribute significantly to controlling sexually transmitted diseases,” Palliser’s team wrote in the journal Cell Host & Microbe.

The World Health Organization estimates that 536 million people worldwide are infected with genital herpes, a painful and incurable virus which is highly infectious and can kill newborns.

Herpes viruses head straight to nerve cells, where they stay latent for the life of an animal or person, often causing periodic outbreaks.

Related viruses are herpes simplex 1, which causes cold sores, and varicella, which causes chicken pox and returns in middle or old age as herpes zoster to cause shingles.

Acyclovir and related drugs can suppress symptoms and are available as both creams or pills but have not been shown to prevent infection.

An estimated one in five Americans have genital herpes, according to the U.S. Centers for Disease Control and Prevention, while 100 million have HSV-1.

As a new route to making better drugs, several companies have locked onto technology using small, interfering RNAs or siRNAs. These are molecules that can silence microRNAs -- tiny strands of RNA, or ribonucleic acid, that help turn genes into proteins.

The Alnylam drug targets a gene called nectin-1. Mice genetically engineered to lack this gene are less likely to be infected with HSV-2.

But the cream “silencing” nectin-1 took a day or so to become effective. Attacking a second gene called UL29, found in the virus itself, provided immediate protection, they wrote.

Using both provided about a week of protection, they said. They used a type of cholesterol to help carry the siRNAs and the resulting cream did not irritate the vaginas of the mice, they said.

“Topically applied siRNAs might be useful to treat and prevent reactivation and sexual transmission of clinically latent HSV-2 infection,” they wrote.

More work is needed, they stressed, but the approach might work for other sexually transmitted infections -- notably the AIDS virus.