Oct. 2 (Bloomberg) -- Genital herpes may infect 28 percent of women by age 49, according to the World Health Organization's first global estimate of the prevalence of the incurable disease.

Women are more likely than men to have been infected with the main type of herpes simplex virus that causes the sexually transmitted infection, according to a report in the October edition of the WHO's monthly bulletin. In some areas of sub- Saharan Africa, prevalence among women is as high as 70 percent.

The report is the first to estimate the global burden of the ulcer-causing disease, which increases the risk of HIV, can kill the babies of newly infected mothers, and causes distress and anxiety among sufferers. The reasons for the higher prevalence among women are unclear, and may suggest they are more susceptible to infection, authors Katharine Looker, Geoffrey Garnett and George Schmid wrote.

``Before HIV came along, genital herpes was everybody's biggest fear,'' Schmid, a medical officer with the WHO's HIV/AIDS department in Geneva, said in a telephone interview today. ``But it still is a disease with significant emotional and physical consequences.''

About 536 million people, or 16 percent of those aged 15 to 49 years, were living with HSV-2 in 2003, the report by WHO and researchers at London's Imperial College said.

The estimate is based on a review of international blood-test data reported in at least 318 studies conducted since 1966. The authors estimate 12.8 million women and 10.8 million men aged 15 to 49 years were infected worldwide with the virus in 2003.

Infection Risk

``Anyone who has had more than a handful of partners, particularly in a part of the world where these infections are common, is pretty likely to have the infection,'' said Adrian Mindel, a professor of medicine at the University of Sydney who has written more than 50 published studies on sexually transmitted diseases.

The majority of people infected ``don't have any signs or symptoms of herpes,'' Mindel said in a telephone interview today. Transmission can still occur from an infected partner who doesn't have a visible sore.

When signs do occur, they typically appear as one or more blisters on or around the genitals or rectum, according to the Centers for Disease Control and Prevention in Atlanta. The blisters break, leaving tender ulcers that may take two to four weeks to heal the first time they occur.

Cold Sores

Herpes simplex virus type-1 can also cause genital herpes although it more commonly causes infections of the mouth and lips known as ``cold sores'' or ``fever blisters,'' according to the CDC. Genital HSV-1 outbreaks recur less regularly than genital HSV-2 outbreaks.

Antiviral medicines such as GlaxoSmithKline Plc's Valtrex and Zovirax and Novartis AG's Famvir can shorten and stave off outbreaks. The medicines don't kill the virus, which lingers in nerve roots. London-based Glaxo is also working on a vaccine to control the spread of genital herpes.

HSV-2 is the leading cause of genital ulcer disease in developing countries, where it may be driving HIV epidemics. Studies haven't shown that suppressing HSV disease with antiviral treatment prevents HIV infection.

``Individuals who are infected with herpes simplex virus are 2-3 times as likely to have HIV as those who are not infected,'' Schmid said. ``HSV control does offer a potential means of lessening the rate of acquisition of HIV, although we don't at this point know exactly how to do that.''

Prevalence of HSV among women aged 15 to 49 years was 19 percent, with rates reaching 28 percent among 45 to 49-year-olds, according to the WHO study. Rates among men peaked at 22 percent in the same age group and averaged 13 percent in all males aged 15 to 49 years.

China Rates

In developing Asian countries, prevalence ranges from 10 to 30 percent, according to a separate WHO report published this year. A study in China showed a prevalence rate of more than 65 percent in 505 sex workers tested and a rate of 11 percent among people attending an antenatal clinic.

In the U.S., at least 45 million people older than 12, or one out of five adolescents and adults, have been infected with genital HSV, according to the CDC.

``Estimates of the number of people with incident and prevalent infection are useful to get a general impression of who is infected and in which areas of the world, to guide public- health policy to those groups most at need,'' the authors of the WHO report said.

To contact the reporter on this story: Jason Gale in Singapore at j.gale@bloomberg.net

Last Updated: October 2, 2008 05:59 EDT