Herpes simplex epidemiology and demographics

, Associate Editor-In-Chief:

Overview
Genital herpes is a chronic, life-long viral infection. Two types of HSV have been identified as causing genital herpes: HSV-1 and HSV-2. Most cases of recurrent genital herpes are caused by HSV-2, and at least 50 million persons in the United States are infected with this type of genital herpes. Nationwide, 16.2%, or about one out of six, people 14 to 49 years of age have genital HSV-2 infection. Over the past decade, the percentage of Americans with genital herpes infection in the U.S. have remained stable. Genital HSV-2 infection is more common in women (approximately one out of five women 14 to 49 years of age) than in men (about one out of nine men 14 to 49 years of age). Transmission from an infected male to his female partner is more likely than from an infected female to her male partner.

Epidemiology
Although many people infected with HSV develop labial or genital lesions, many more are either undiagnosed or display no physical symptoms - individuals with no symptoms are described as asymptomatic or with subclinical herpes. Since asymptomatic individuals are often are unaware of their infection, they are considered at high risk for spreading HSV. Many studies have been performed around the world to estimate the numbers of individuals infected with HSV-1 and HSV-2 by determining if they have developed antibodies against either viral species. This information provides population prevalence of HSV viral infections in individuals with or without active disease.

Europe
Large differences in HSV-1 seroprevalence are seen in different European countries. HSV-1 seroprevalence is high in Bulgaria (83.9%) and The Czech Republic (80.6%) and lower in Belgium (67.4%), The Netherlands (56.7%) and Finland (52.4%). The typical age at which HSV-1 infection is acquired ranges from 5–9 years in Eastern European countries like Bulgaria and the Czech Republic to over 25 years of age in Northern European countries such as Finland, The Netherlands, Germany, and England and Wales. Young adults in Northern European countries are less likely to be infected with HSV-1. However, European women are more likely to be HSV-1 seropositive than men.

HSV-2 seropositivity is widely distributed in Europeans older than 12, although there are large differences in the percentage of the population that had been exposed to HSV-2. Bulgaria has a high (23.9%) HSV-2 seroprevalence relative to other European countries: Germany (13.9%), Finland (13.4%), Belgium (11.1%), The Netherlands (8.8%), the Czech Republic (6.0%) and England and Wales (4.2%). Women are more likely to be seropositive than men, and likely acquire the virus at an earlier age. In each country of Europe, HSV-2 seropositivity becomes more common from adolescence onwards and increases in the population with age, with a decline in the older age groups in some countries.

North America
United States

In healthy adults, HSV-2 infection occurs more frequently in the USA than in Europe, and appears to be increasing; in individuals over 12 years old, HSV-2 seroprevalence has increased from 16.4% in 1976 to 21.8% from in 1994 and is still rising. Thus, the current incidence of genital herpes caused by HSV-2 in the U.S. is roughly one in four or five adults, with approximately 50 million people infected with genital herpes and an estimated 0.5 million new genital herpes infections occurring each year. African Americans appear more susceptible to HSV-2, although the presence of active genital symptoms are more likely in Caucasian Americans. The largest increase in HSV-2 acquisition during the past few years is in white adolescents. People with many lifetime sexual partners and those who are sexually active from a young age are also at higher-risk for the transmission of HSV-2 in the U.S.   Women are at higher risk than men for acquiring HSV-2 infection, and the chance of being infected increases with age.

African Americans and immigrants from developing countries typically have an HSV-1 seroprevalance in their adolescent population that is two or three times higher than that of Caucasian Americans, possibly reflecting differences in their socioeconomic backgrounds. Many white Americans enter sexual activity, marriage and child bearing years seronegative for HSV-1. The absence of antibodies from a prior oral HSV-1 infection leaves these individuals susceptible to primary HSV-1 genital infections. This brings with it a risk of vertical transmission to the neonate if the mother contracts a primary infection during the third trimester of pregnancy. A seronegative mother has up to a 57% chance of conveying an HSV infection to her baby during childbirth whereas a woman seropositive for both HSV-1 and HSV-2 has around a 1-3% chance of transmitting infection to her infant. Women that are seropositive for only one type of HSV fall somewhere in between but are still only half as likely to transmit HSV as the seronegative mother. Genital infection caused by HSV-1, in the U.S. is now thought to be about 50% and contributes to a rate of 6 to 20 cases of neonatal herpes per 100,000 live births in the U.S. depending on region and demographics.

Canada

Following a study in Ontario, up to 55% of Canadians age of 15 to 16, and 89% of individuals in their early forties are estimated have antibodies to HSV-1. Teenagers are less likely to be seropositive for HSV-2 - antibodies against this virus is only found in 0-3.8% of 15-16 year olds. However, 21% of individuals in their early forties have antibodies against HSV-2 reflecting the sexually transmitted nature of this virus. When standardising for age, HSV-2 seroprevalence in Ontario, for individuals between the ages of 15 to 44, was 9.1%. This is much lower than estimated levels of HSV-2 seroprevalence in people of a similar age range in the United States. HSV-2 seroprevalence in pregnant women, between the ages of 15-44, in British Columbia is similar, with 57% having antibodies for HSV-1 and 13% having antibodies for HSV-2.

Africa
Sub-Saharan Africa

HSV-2 in more common in some countries, such as those of Sub-Saharan Africa, than in Europe or the North America. Up to 82% of women, and 53% of men in Sub-Saharan Africa are seropositive for HSV-2 (see table), representing the highest levels of HSV-2 infection in the world, although exact levels vary from country to country in this continent. In most African countries, HSV-2 prevalence increases with age. However, age-associated decreases in HSV-2 seroprevalence has been observed for women in Uganda and Zambia, an in men in Ethiopia, Benin and Uganda.

Northern Africa

Genital herpes appears less common in Northern Africa compared to Sub-Saharan Africa, with only 26% of middle-aged women having antibodies for HSV-2 in Morocco. Woman are more likely to be infected with HSV-2 once they are over the age of 40. Children in Egypt are often infected with HSV from a young age - HSV-1 or HSV-2 antibodies are estimated in 54% in children under the age of 5 years and 77% in children over 10 years of age. Algerian children are also likely to acquire HSV-1 infection at a young age (under 6) and 81.25% of the population has antibodies to HSV-1 by the age of 15.

Central and South America
HSV-2 seroprevalency is high in Central and South America, relative to rates in Europe and North America with levels estimated between 20-60%. During the mid 1980s, HSV-2 prevalence was 33% in 25–29 years old women and 45% in those aged 40 and over in Costa Rica, and, in the early 1990s, was approximately 45% among women over 60 in Mexico. The highest HSV-2 prevalence (60%) in Central or South America has been found Colombian middle-aged women although similar HSV-2 prevalence (54%) has been observed in younger women in Haiti. HSV-2 infects about 30% in women more than 30 years old from Colombia, Costa Rica, Mexico, and Panama and steadily increases to 52% in an age-associated manner in those aged 50–59. HSV-2 antibodies were found in more than 41% of women of childbearing age in Brazil. However, no increase in seroprevalence was associated with age in women over 40 years old in this country - HSV-2 prevalence was estimated at 50% among women aged 40–49 years, 33% among women 50–59, and 42% among women over 60. Women in Brazil are more likely to acquire an HSV-2 infection if their male partners had history of anal sex and had many sexual partners in his lifetime. In Peru, HSV-2 prevalence is also high among women in their 30s but is lower in men.

Asia
Eastern and South East Asia

HSV-2 seroprevalence in developing Asian countries is comparable (10-30%) to that observed in North America and Northern Europe. HSV-1 seroprevalence in some Asian countries is low, relative to other countries worldwide, with only 51% women in Thailand and between 50-60% in Japan possessing antibodies against this virus. However, estimates of HSV-2 infectivity, in Thailand, is higher than observed in other Eastern Asian countries - total HSV-2 seroprevalence is approximately 37% in this country. HSV-2 seroprevalence is low in women in the Philippines (9%), although commencing activity while young is associated with an increase risk of acquiring HSV-2 infection; woman starting sexual activity by the time they reach 17 are seven times more likely to be HSV-2 seropositive that those starting sexual activity when over 21. In South Korea, incidence of HSV-2 infection in those under the age of 20 is low at only 2.7% in men and 3.0% in women. Seroprevalence levels increase in older South Koreans, however, such that the population over 20 that has antibodies against HSV-2 is 21.7% of men and 28% of women, with increasing HSV-2 prevelence becoming significant once individuals reached their 30's.

Southern Asia

In India, 33.3% of individual are seropositive for just HSV-1 and 16.6% are seropositive for only HSV-2. Those with both HSV-1 and HSV-2 antibodies are estimated at 13.3% of the population. Indian men are more likely to be infected with HSV-2 than women, and increasing seroprevalence of this virus is associated with an increasing age.

Middle East

High levels of HSV-2 (42%) and HSV-1 (97%) were found amongst pregnant women in the city of Erzurum in Eastern Anatolia Region, Turkey. In Istanbul, a city in the Marmara Region in North West Turkey, however, lower HSV-2 seroprevalence was observed; HSV-2 antibodies were found in 4.8% of sexually active adults, and HSV-1 antibodies were found in 85.3%. Only 5% of pregnant women were infected with HSV-2, and 98% were infected with HSV-1. Prevalence of these viruses was higher in sex workers of Istanbul, reaching levels of 99% and 60% for HSV-1 and HSV-2 prevalence respectively. The prevalence of HSV-2 in Jordan is 52.8% for men and 41.5% for women. HSV-1 seroprevalence is 59.8% in the population of Israel and increases with age in both genders. An estimated 9.2% of Israelian adults are infected with HSV-2. Infection of either HSV-1 or HSV-2 is higher in females; HSV-2 seroprevalence reaches 20.5% in females in their 40s. These values are similar to levels in HSV infection in Europe. Antibodies for HSV-1 or HSV-2 are also more likely to be found individuals born outside of Israel, and individuals residing in Jerusalem and Southern Israel. People from jewish origin, living in Israel, are less likely to possess antibodies against herpes. HSV-1 causes 66.3% of genital herpes in individuals living in Tel Aviv, Israel. Genital herpes infection from HSV-2 is predicted to be low in Syria although HSV-1 levels are high. HSV-1 infections is common (95%) among healthy Syrians over the age of 30, whilst HSV-2 prevalence is low in healthy individuals (0.15%), and persons infected with other sexually transmitted diseases (9.5%). High risk groups for acquiring HSV-2, in Syria, include prostitutes and bar girls that have 34% and 20% seroprevalence respectively.

Australasia
In Australia, the seroprevalence of HSV-1 is 76%, with differences associated with age, gender and Indigenous status. An estimated 12% of Australian adults are seropositive for HSV-2, with higher prevalence in women (16%) than in men (8%). Larger cities have higher HSV-2 seroprevalence (13%) than rural populations (9%) in this country. Higher prevalence is found in Indigenous Australians (18%) than non-Indigenous Australians (12%) but is lower than HSV-2 prevalence observed in the United States. As in the U.S., HSV-1 is increasingly identified as the cause of genital herpes in Australians; HSV-1 was identified in the anogenital area of only 3% of the population in 1980, but had risen to 41% in 2001. This was most common in females and persons under 25.

The number of genital herpes infections appears to be rising in New Zealand with three times more cases in 1993 compared to 1977. In this country, HSV-2 affects 60% more women than men of similar age.