Figure 1: | WHO Initiative on HIV/AIDS and Sexually Transmitted Infections (HSI) |
This document contains WHO estimates of the prevalence and incidence of some of the curable sexually transmitted diseases (STDs), based on information published in the world scientific literature and in WHO archives. The methodology used was developed by the Office of STD of the WHO Global Programme on AIDS in collaboration with the Rockefeller Foundation, and is the first WHO attempt to estimate STD incidence based on epidemiological modelling. As more data on STD become available, the WHO database will be updated and estimates will be refined.
Sexually transmitted diseases (STDs) are among the most common causes of illness in the world and have farreaching health, social and economic consequences. In addition to their sheer magnitude, STDs are a major public health problem for two additional reasons: their serious sequelae, and the fact that they facilitate transmission of HIV.
STDs take a great toll on health through their sequelae, i.e. conditions resulting from the spread of STD pathogens from the point of infection, usually the genital region, to another part of the reproductive tract, such as the fallopian tubes in women. Sequelae of some sexually transmitted diseases, in particular gonorrhoea and chlamydial infection which cause pelvic inflammatory disease in women, impair the fertility of both men and women. Another sequela of these same STDs is increased risk of ectopic pregnancy, a condition that can kill from sudden and severe internal bleeding following rupture of the fallopian tube.
Some STDs attack the fetus and infant as well. In twothirds or more of pregnant women with early syphilis, for example, the infection spreads through the placenta and infects the fetus and because of this up to one-half of syphilis-infected pregnancies end in spontaneous abortion, stillbirth, or perinatal death. Gonorrhoea or chlamydial infection may likewise infect the eyes of babies as they pass through the cervix and vagina during birth, while chlamydial infection may spread to the lungs of newborns, resulting in chlamydial pneumonia.
Young adults between 15 and 19 years of age often have high rates of STD and present a particularly important problem because of their lack of easy access to STD services and condoms, and their frequent and multiple casual sex partners.
Because sexually transmitted diseases and their sequelae have such a widespread effect on men, women, youth and newborns the problem of curable STDs is costly to individuals and the health care system. The World Bank has estimated that STDs collectively rank second in importance among diseases for which intervention is possible among women 1544 years of age worldwide,1 and that four curable STDs gonorrhoea, chlamydial infection, syphilis and chancroid rank among the top 25 causes of healthy days of life lost in subSaharan Africa.2
Biological factors of the curable STDs which increase the risk of HIV transmission and infection include disruption of the normal epithelial barrier by genital ulceration and/or microulceration; and by accumulation of pools of HIV-susceptible or HIV-infected cells (lymphocytes and macrophages) in semen and vaginal secretions. Epidemiological studies from subSaharan Africa, Europe and North America have suggested that there is approximately, a four times greater risk of becoming HIV-infected in the presence of a genital ulcer such as caused by syphilis and/or chancroid; and a significant though lesser increased risk in the presence of STDs such as gonorrhoea, chlamydial infection and trichomoniasis which cause local accumulations of lymphocytes and macrophages.3
An estimate of the impact of curing or preventing each of the curable STDs has been made by Over and Piot.2 The model used suggests that by curing or preventing one hundred cases of syphilis among an STD high-risk (core) group, approximately 1,200 HIV infections linked to those one hundred episodes of syphilis could be prevented over the coming 10-year period (Figure 1). For other curable STDs the impact of treatment and prevention is significant, though somewhat less.
Figure 1:
In 1990, using a modified Delphi technique, WHO estimated that in that year there were over 250 million new cases of sexually transmitted diseases. The Delphi technique was chosen at that time because of the lack of information on STDs in many regions. Recently there has been an increase in publications on STD prevalence in developing countries in Africa, Asia, Latin America and the Caribbean. Using this information, and information from other sources such as official STD prevalence estimates from industrialized countries and WHO archival information from countryspecific reports, prevalence rates of gonorrhoea, chlamydial infection, syphilis and trichomoniasis were estimated by sex and by (UN standard) region.4
Regional adult prevalence for 1995 was calculated using midyear population estimates of adults 1549 years of age (Figure 2). Because of a lack of published and archival information on chancroid, no estimates of this disease using this methodology could be made. Likewise, estimates were not made for the viral STDs such as herpes, human papillomavirus and hepatitis B.
Figure 2:
The next step was to estimate the duration of each infection by sex and by region. These estimates were based on the probability of a symptomatic or an asymptomatic person getting treatment for his/her STD. Regional adult STD incidence for 1995 was calculated by dividing the estimated prevalence by the estimated duration of each disease. The results are shown in Figure 3. Table 1 summarizes population, prevalence and incidence data by region.
Figure 3:
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Region Population 15-49 Prevalence Prevalence Annual Annual
(millions) (millions) per/1000 Incidence Incidence
________________________________________________________________________________________________________
North America 153 8 52 14 91
Western Europe 211 10 45 16 77
Australasia 11 0.6 52 1 91
Latin America and the Caribbean 251 24 95 36 145
Sub-Saharan Africa 254 53 208 65 254
Northern Africa and Middle East 163 6.5 40 10 60
Eastern Europe and Central Asia 158 12 75 18 112
East Asia and Pacific 803 16 19 23 28
South and South East-Asia 943 120 128 150 160
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Total 2,946 250 85 333 113
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A more complete description of the methodology used to estimate STD prevalence and incidence is available upon request from the WHO Global Programme on AIDS.
During 1995 it is estimated that there will be approximately 12 million new cases of syphilis among adults worldwide, with the greatest number of cases occurring in South and SouthEast Asia followed by subSaharan Africa (Figure 4).
Figure 4:
Syphilis was first described in the sixteenth century. In industrialized countries syphilis apparently declined during the latter half of the nineteenth century. In these same countries, however, there was a sharp rise in incidence after the First World War, but following the Second World War the incidence fell rapidly, coinciding with the availability of improved diagnostic tests and antibiotics. In some industrialized countries syphilis began to rise again in the 1960s and has been increasing steadily in some industrialized and developing countries since then.
Syphilis is the classic example of an STD which can be successfully controlled by public health measures: a simpletouse and highly sensitive diagnostic test is available, as is a highly effective antibiotic to which resistance has not developed. If untreated, however, syphilis may lead to nerve damage, arterial wall damage, and mental disorientation, and eventually to death.
Treponema pallidum, the causative agent of syphilis, can cross the placental barrier and infect the fetus. There is evidence that in approximately twothirds of pregnancies, infection spreads across the placental barrier, and that many of these pregnancies end in spontaneous abortion, stillbirth, or perinatal death. Congenital defects may occur in those fetuses which go to term and are delivered. In a study in Ethiopia, for example, pregnant women with a positive serological test for syphilis were shown to have a five times greater chance of having a spontaneous abortion or stillbirth than those who were serologically negative;5 while in Zambia, 24% of stillbirths could be attributed to syphilis, and congenital syphilis was implicated in 30% of all perinatal infant mortality.6
It is generally observed that the incidence of syphilis, as reported by the number of cases treated each year, is highest among the 1530-year-old group and those persons with the greatest sexual activity, and that incidence decreases with age. Based on reports of new cases of syphilis treated in Chile in 1993, for example, the highest incidence was among 2024-year-olds, followed by those 2529 years of age; 15-24-year-olds represented 40% of all cases (Figure 5).7
Figure 5:
During 1995 it is estimated that there will be approximately 62 million new cases of gonorrhoea among adults worldwide, with the greatest number in South and SouthEast Asia followed, as for syphilis, by subSaharan Africa (Figure 6)
Figure 6:
Gonorrhoea is a common adult disease, though a significant proportion of those with infection (up to 80% among women, 10% among men) are asymptomatic, i.e., they do not have symptoms and therefore they are neither aware of the need for treatment nor of the risk of transmitting the disease to others.
Infected men usually have symptoms and seek treatment spontaneously. Women frequently have only minor symptoms or are asymptomatic, so detection of infection depends mainly on screening by culture, which is costly and requires relatively sophisticated facilities. Few countries operate regular screening programmes and therefore gonorrhoea reporting seldom reflects true levels of infection. Coinfection with chlamydia is very common and treatment with antibiotics is simple and highly effective.
WHO estimates that by successfully treating 100 women for gonorrhoea, among whom one-quarter are pregnant, 25 would be prevented from developing pelvic inflammatory disease and 7 of their newborns would be spared from ophthalmia neonatorum, an eye infection acquired during passage through the birth canal which, if untreated, can result in blindness (Table 2). Serious sequelae such as ectopic pregnancy, chronic pelvic pain and infertility could also be averted by treatment of these women.
Figure 7:
The complications and sequelae of gonorrhoeal infection have been clearly demonstrated in reports from Cameroon in the mid-1980s, when up to 30% of newborns to women infected with gonorrhoea at the time of delivery developed ophthalmia neonatorum, leading to an overall rate of ophthalmia neonatorum of 4% among all births (Figure 7)8. The lighter section of the map in Figure 8 shows the infertility belt in West and Central Africa, where up to 40% of women over the age of 45 years have been unable to conceive, due in large part to STDinduced infertility among young women, the potential of which is demonstrated by the 17% rate of gonorrhoeal infection among women attending antenatal clinics in Cameroon in 1984.
Figure 8:
During 1995 it is estimated that there will be approximately 89 million new adult chlamydial infections worldwide, again with the greatest number in South and SouthEast Asia followed, as for syphilis and gonorrhoea, by subSaharan Africa (Figure 9).
Figure 9:
Chlamydial infection, like gonorrhoea, is a common adult disease which has asymptomatic rates in women similar to those for gonorrhoea, but higher rates of asymptomatic infection than gonorrhoea in men. Like gonorrhoea, chlamydia has serious sequelae such as pelvic inflamatory disease and infertility. Diagnosis of chlamydial infection is costly and those diagnostic tests which are most sensitive are not generally available in developing country laboratories. Even in industrialized countries laboratory testing is not available in all health facilities, and screening programmes in women at risk of infection are therefore not consistently conducted. As many infections are neither detected nor treated, prevalence rates are high.
Recently developed laboratory tests have, however, made screening programmes for chlamydial infection possible in some industrialized countries. Rates of infection among women attending family planning clinics from 1989 to 1993 in the United States of America, for example, have been shown to vary from 4.5% to 8.5%. (Figure 10)9
Figure 10:
During 1995 it is estimated that there will be approximately 170 million new cases of trichomoniasis among adults worldwide, with the greatest numbers in developing countries and higher prevalence and incidence rates than for any other STDs in both developing and industrialized countries (Figure 11).
Figure 11:
Trichomoniasis is one of the most common STDs. It causes symptoms in approximately 50% of infected women. In men, infection is usually urethral and of short duration, but men easily transmit the parasite to women during the short period when they are infected. Diagnosis is made by a wet mount preparation of vaginal or urethral discharge using an ordinary light microscope, and can also be made by culture, a more costly and less widely available test.
There is little recently published information on trichomoniasis, and that which is available is mainly from studies in subSaharan Africa. Prospective studies in Malawi and in Zaire have shown an association of the disease with HIV seroconversion in women. These findings, along with its high prevalence worldwide, indicate the attention which must given to trichomoniasis, a disease for which diagnosis is simple and treatment effective.
Prevalence rates of trichomoniasis among African women attending antenatal clinics, range from 12% in Kenya to 47% in Botswana (Figure 12).10-16
Figure 12:
Prevalence studies of trichomoniasis among male and female secondary school students in Kenya (1991) and Nigeria (1993) ranged from a low of 1.3% in Kenya17 to a high of 24.7% in Nigeria.18
No estimates of chancroid were made using the methodology developed for syphilis, gonorrhoea, chlamydial infection and trichomoniasis. Poor understanding of the epidemiology and natural history of the disease and the absence of a good test make it difficult to undertake prevalence studies and to estimate prevalence and duration of infection.
An idea of the magnitude of the chancroid problem can, however, be provided based on the ratio of syphilis to chancroid in the previous WHO (Delphi) estimates for syphilis and chancroid, and the 1995 estimate for syphilis. Such a comparison suggests that there may be approximately 7 million new cases of chancroid during 1995. As more published data on chancroid becomes available these estimates will be refined.
The genital ulcers produced by chancroid are a major risk factor for HIV transmission, and the incidence of chancroid varies greatly between countries and regions. For example in Swaziland and Kenya 44% (1979)19 and 62% (1980)20 respectively of genital ulcers were diagnosed as chancroid in STD clinics. In western Algeria chancroid is the most common STD observed and the primary cause of genital ulcer disease.21
Studies among female commercial sex workers in Kenya in 1992 showed that 30% have evidence of having had chancroid infection, as compared to 2% of women attending antenatal clinics.22 In India in 1989, chancroid represented 26% of all reported STD. In Latin America and the Caribbean the prevalence of chancroid varies from region to region. In some areas, e.g. French Guyana and northern Brazil, it is reportedly a common cause of genital ulcers. In most industrialized countries chancroid has become rare. With the development of new tests, the diagnosis of chancroid will be easier and more information on its prevalence will become available.
Though STDs remain a severe public health problem, especially in developing countries, progress is being made in their control in each region. For example, reported gonorrhoea in Sweden and Norway, each with over ten thousand reported cases of gonorrhoea in 1981, is now approaching zero (Figure 13).23, 24
Figure 13:
In Costa Rica reported gonorrhoea for both men and women began a steady and sustained decline in 1982, and in Chile reported STDs began to decline in the mid-1980s (Figures 13 25 and 14 26)
Figure 14:
Figure 15:
In Zimbabwe, reported STD in the capital city Harare began a sustained decline in 1991, and in Thailand, declines in reported STD began in 1990 and continue to the present (Figures 15 27 and 16 28).
Figure 16:
Figure 17:
In other regions, however, such as Eastern Europe and Central Asia, the incidence of STDs is increasing. In Estonia, Latvia, Lithuania, and Russia, for example, reported gonorrhoea has increased since 1991. However, in neighbouring Poland, reported gonorrhoea has declined (Figure 18).29-33
Figure 18:
The figures and references which follow are all derived from information contained in the WHO STD database. The figures reflect the variety of sample sizes and of methodologies used to collect this information. As more standardized and refined information becomes available, the WHO estimates will become more accurate.
Information gathered from studies in pregnant women provides a good idea of the prevalence of syphilis in the sexually active general population, and such data are readily available from antenatal syphilis screening programmes. In Latin American and Caribbean countries from which information is available, for example, syphilis prevalence among pregnant women ranged from 1.3% in Honduras to 6.3% in Paraguay in 1991 (Figure 19).34-36
Figure 19:
In three countries of East Asia, the Pacific and South and SouthEast Asia for which data are available, prevalence rates of syphilis among pregnant women range from 0.6% in Korea (1986) to 14.2% in Fiji (1987) (Figure 20).37-39
Figure 20:
Data showing syphilis prevalence in men representing the general population are scarce. Figure 20 34, 40-42 shows results from studies carried out in males which also provide, though probably not as reliably as figures for pregnant women, an approximation of syphilis prevalence in the general population.
Figure 21:
In Figure 21 43-45 syphilis prevalence in men attending STD clinics for any reason and screened for syphilis is shown. In this group, in which the main complaint is not necessarily a genital ulcer, the syphilis prevalence is relatively high, reflecting the frequent association of syphilis with other STDs.
Figure 22:
Persons with certain occupations, such as longdistance truck driving and commercial sex work, appear to be at especially high risk of syphilis and other STD. African longdistance truck drivers, for example, have been reported to have prevalence rates of syphilis as high as 15%, placing them at high risk for HIV infection or, if they are already infected, increasing their risk of transmission to others (Figure 23).13, 46-50
Figure 23:
Likewise, commercial sex workers in North Africa and the Middle East have been reported to have high prevalence rates of syphilis: 47% in Somalia (1991), 46% in Djibouti (1994) and 23% in Sudan (1988) (Figure 24).51-53
Figure 24:
Recent information from studies of pregnant women and family planning clinic attenders in Cameroon and three other subSaharan African countries suggests that gonorrhoea remains a priority health problem among women. During the period 19901993, for example, prevalence rates of symptomatic and asymptomatic gonorrhoea among women attending antenatal clinics in Cameroon was approximately 12%, and ranged from 3.8% in Côte d'Ivoire to 14% in Botswana (Figure 25).50, 54-56
Figure 25:
Gonorrhoeal infection among pregnant women is a significant public health problem in South and SouthEast Asia and the Pacific, where rates in three countries during the 1980s ranged from less than 1% in Malaysia to 12% in Thailand (Figure 26).57-58
Figure 26:
As for syphilis and the other STDs, rates of gonorrhoeal infection are often higher among certain occupational groups. In Kenya, for example, rates of gonorrhoea among female sex workers were shown to be high in 1984, and varied from 20% for sex workers who solicited upper class clients in hotels to 63% among those who solicited clients on the streets (Figure 27).59
Figure 27:
Rates of chlamydial infection in South and SouthEast Asia, Latin America and the Caribbean, and subSaharan Africa are likewise relatively high. Among young women in Thailand, for example, rates have been found to be approximately 3%, much higher than the rate of 0.1% for gonorrhoeal infection and 0.6% for positive syphilis serology, while rates among pregnant women in El Salvador (1991) have been shown to be as high as 44% and those in rural Botswana (1990) as high as 49% (Figure 28).28, 55, 60
Figure 28:
Rates of chlamydial infection among commercial sex workers are also high in Asia, as shown in Nanjing Province, China, in 1993, where 21% of commercial sex workers were infected as compared to 10% of adults attending STD clinics, 3% of antenatal clinic attenders and 1% of sexually active men (Figure 29).61
Figure 29:
Screening in men using ELISA tests also reveals relatively high prevalence rates among different populations and regions, varying from 1% in Canada to 12% in Austria (Figure 30).11, 61-69
Figure 30:
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