Two-thirds of the world’s population under 50, or more than 3.7 billion people, have herpes in its oral form, according to the World Health Organization’s first-ever global estimate for the prevalence of the virus.

The UN health agency estimates 140 million people have been infected genitally by the oral herpes virus, many of them in wealthier countries such as Canada.

This suggests that more people are now getting genital herpes from oral sex.

The findings, published Wednesday in the journal Plos One, reflect something front-line health workers often observe, but rarely see investigated: that herpes simplex type 1, commonly associated with cold sores on and around the lips, is becoming an important cause of genital herpes.

“We see so much of it,” said Jane Greer, director of Hassle Free Clinic, a sexual health medical centre in Toronto.

“It’s so great that somebody’s finally looked at it.

“It’s just something that people don’t understand necessarily.”

Herpes simplex type 1 is a highly infectious virus typically spread by skin-on-skin contact (such as kissing). It can cause cold sores around the mouth, or no symptoms at all.

While it is an incurable infection, the virus provokes far less anxiety than herpes simplex type 2, a sexually transmitted disease that causes genital herpes.

But the traditional belief that herpes type 1 is above the waist and type 2 is below is changing; a 2012 study in the Canadian Journal of Infectious Diseases and Medical Microbiology found that 63 per cent of genital herpes cases analyzed were caused by the type 1 virus. Another 2003 study, also in Nova Scotia, found that herpes type 1 was the dominant virus causing genital herpes.

“I think what’s really interesting about this is it really reflects behavioural changes,” said Dr. Vanessa Allen, chief of medical microbiology with Public Health Ontario.

“As sexual practices change — and part of that is because people are trying to have safer and safer sex — there’s some hypothesizing that this may lead to more oral sex.”

The WHO paper is the first to estimate herpes type 1 prevalence around the world and arrived at its conclusions after combing through nearly 3,000 studies and using mathematical models.

Many assumptions had to be used, because data is lacking for many regions.

Of the 3.709 billion people infected worldwide in 2012, the highest prevalence overall was in Africa, Southeast Asia and the Western Pacific, where populations are also highest. Most of the new infections in 2012 occurred in the first five years of life.

When it comes to genital herpes caused by the type 1 virus, the Americas and Western Pacific regions had the most cases.

In the Americas, around half of herpes type 1 infections in 2012 occurred between the ages of 15 and 49, perhaps because better hygiene and less crowded living conditions are protecting more people from contracting the virus in childhood.

This also means that more people don’t have any immunity to the virus when they become sexually active, said study co-author Dr. Sami Gottlieb, a medical officer with the WHO’s department of reproductive health and research.

“One of the things [that could be contributing to the increase in type 1 genital infections] is an increasing number of people susceptible to infection when they start sexual activity,” said Gottlieb.

“The other thing is an increase in oral sex behaviour.

“There have been some studies in certain settings that show adolescents may be opting for oral sex before having sexual intercourse for the first time.”

A number of factors could be driving an increase in oral sex, including the HIV epidemic (which promoted safe sex awareness and, in some cases, a preference for oral sex, which only very rarely spreads the HIV virus) and pregnancy concerns amongst teens, who, in turn, may have more casual attitudes about oral sex.

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Gottlieb hopes the new WHO study will encourage efforts to develop a vaccine or microbicide that will protect against both herpes type 1 and 2.

But until such a thing is available, the next best weapon is awareness, especially for young people, she said.

“It’s very important that young people are given full access to education and information on both types of HSV and also all sexually transmitted infections,” she said.

“Not to be alarmist, but to really empower people to understand the full range of things that might affect their sexual health.”