2 Epidemiology of HIV infection in women A N N E M. J O H N S O N

Since the acquired immune deficiency syndrome (AIDS) was first described in homosexual men in the US in 1981, the pandemic of human immunodeficiency virus (HIV) infection has come to be recognized as a problem which affects women as much as men, particularly in the countries of sub-saharan Africa. Thus, less than a decade after the first description of AIDS, the World Health Organization (WHO) estimated that by late 1990 a cumulative total of nearly nine million cases of HIV infection had occurred amongst adults world-wide and that approximately one third of these cases had occurred in women (Chin, 1990a). Beyond the clinical manifestations of disease, HIV infection affects women in all the fundamental areas of their personal lives. Because HIV is predominantly sexually transmitted, it affects women in choices about their sexual partners and in the expression of their sexuality. It affects them as mothers in choices about pregnancy, contraception and abortion, and it affects them in their working lives whether caring for the home and family, or in the medical and nursing professions when caring for the sick. This chapter describes the world-wide epidemiology of HIV in women, considering trends in HIV infection and AIDS cases, and the relative contribution of different routes of transmission to the overall pattern of infection. It addresses the risks of and risk factors for heterosexual transmission of HIV infection and discusses research on behavioural and biological co-factors which may influence the probability of heterosexual transmission of HIV. The chapter considers parameters which will determine the extent of future viral spread and reviews recent advances in the study of patterns of sexual behaviour.

GLOBAL EPIDEMIOLOGY OF AIDS/HIV Epidemic patterns The WHO now describes four different epidemiological patterns of HIV infection world-wide (Figure 1) (Sato et al, 1989). Pattern I describes the situation in western Europe, North America, Australia and New Zealand where more than 80% of cases of HIV infection have thus far occurred in homosexual or bisexual men and injecting drug users, with heterosexually Baillibre' s Clinical Obstetrics and Gynaecology--

Vol. 6, No. 1, March1992 ISBN0-7020--1632-2

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• ,,,,e~o, []Patlernl/If[] Pattern11 []Pattern III Figure 1. Four global epidemiological patterns of HIV infection and AIDS are apparent as of 1989. Pattern I is found in North America, western Europe, Australia and New Zealand: about 80-90% of the cases in these areas are homosexual and bisexual men, or injecting drug users. Pattern II is found in sub-saharan Africa and parts of the Caribbean. The primary mode of transmission in these areas is heterosexual, and the ratio of infected males to infected females is approximately equal. Latin America is in evolution from pattern I to pattern II epidemiology, and is now classified separately as pattern I/II. Pattern III consists of areas where few cases or infections have occurred to date. Boundaries on this map do not imply the expression of any opinion whatsoever on the part of WHO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delineation of its frontiers or boundaries. From Sato et al (1989), with permission.

a c q u i r e d i n f e c t i o n s still a c c o u n t i n g for a small p r o p o r t i o n of the total n u m b e r of cases. P a t t e r n II is f o u n d p r i m a r i l y in s u b - s a h a r a n A f r i c a a n d parts of the C a r i b b e a n w h e r e the p r i m a r y m o d e of t r a n s m i s s i o n is t h r o u g h h e t e r o s e x u a l v a g i n a l i n t e r c o u r s e . I n these c o u n t r i e s the ratio of infected f e m a l e s to m a l e s is a p p r o x i m a t e l y o n e to one. This is in m a r k e d c o n t r a s t to p a t t e r n I c o u n t r i e s w h e r e the ratio of i n f e c t e d m a l e s to i n f e c t e d f e m a l e s is closer to t e n to one. P a t t e r n I I I occurs in areas of the world w h e r e thus far relatively few cases of A I D S have b e e n r e p o r t e d a n d w h e r e the virus has b e e n i n t r o d u c e d m o r e recently. I n these c o u n t r i e s i n f e c t i o n t h r o u g h i n j e c t ing drug use, h o m o s e x u a l a n d h e t e r o s e x u a l c o n t a c t are all o b s e r v e d b u t at low rates a n d with the first cases o f t e n seen a m o n g s t those who have t r a v e l l e d to h i g h e r p r e v a l e n c e areas. H o w e v e r , these initial categorizations of p a t t e r n s of i n f e c t i o n are b y n o m e a n s static. I n L a t i n A m e r i c a , w h e r e the first cases of A I D S were seen in h o m o s e x u a l m e n a n d i n j e c t i n g drug users, t h e r e is n o w e v i d e n c e of a n i n c r e a s i n g p r o p o r t i o n of h e t e r o s e x u a l l y a c q u i r e d i n f e c t i o n o c c u r r i n g since the m i d - t o - l a t e 1980s with a c o r r e s p o n d i n g decrease in the m a l e to f e m a l e

EPIDEMIOLOGY OF HIV INFECTION

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ratio of infections. This region has become classified as a pattern I/II area (Sato et al, 1989). In other areas initially classified as pattern III, and particularly in India and Thailand, there is now evidence of rapid viral spread particularly amongst urban female prostitutes and injecting drug users (Kandaswami et al, 1991; Ungshusak et al, 1991). ESTIMATING THE MAGNITUDE OF THE AIDS PANDEMIC IN WOMEN

Global statistics on the magnitude of the AIDS epidemic are collected by the WHO and are based on surveillance systems in more than 180 participating countries. Most of the industrialized countries of North America, Europe and Oceania base reports on the 1987 Centers for Disease Control AIDS definitions (Centers for Disease Control, 1987). The majority of African countries use the WHO clinical Bangui definition (World Health Organization, 1985) and in Latin America a modified clinical Caracas definition is used (Pan American Health Organization, 1990). By the end of June 1991, 371802 cases of AIDS had been reported to the WHO (World Health Organization, 1991). These, however, are recognized to be an underestimate of the true number of cases world-wide due both to under-reporting of cases and delays in reporting (Chin, 1990a; Evans et al, 1991). It is believed that in developing countries for a variety of reasons there is considerable under-reporting of AIDS cases, such that in Africa only 10-20% of cases are thought to be reported, and less than half in Latin America. The WHO therefore estimates that the true number of adult AIDS cases occurring up to the end of 1990 was in the order of 1 million with approximately one third of these cases occurring in women (Chin, 1990a). PREVALENCE OF HIV INFECTION The median incubation period between infection with HIV and the development of AIDS is estimated to be in the order of 8--I0 years. Studies of cohorts of infected persons (mainly homosexual men in the US) indicate that approximately 50% of persons with HIV infection will progress to AIDS after 10 years (Moss and Bacchetti, 1989). There are currently many more people infected with HIV world-wide than cases of AIDS. The prevalence of HIV infection in women shows marked geographical variation. Thousands of seroprevalence studies have been undertaken throughout the world to estimate the prevalence of infection. These studies vary widely in their methodology and in the selection of study samples ranging from surveys within hospital, surveys of those at particularly high risk such as urban prostitutes, to women attending antenatal clinics, blood donors and community-based surveys using random sample techniques. Taken together, these surveys provide a global picture of the epidemic, but at the same time emphasize the marked heterogeneity of infection amongst women both within and between countries.

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Y

Figure 2. Estimated rates of HIV infection for early 1990; one case of infection per number of adults. One in 300 adults infected worldwide. From Chin (1990a), with permission.

Figure 2 shows WHO estimates of the rate of infection for early 1990, demonstrating the predominance of infection in women in sub-saharan Africa (1 in 40 women infected) with intermediate rates in the Americas (from 1 in 500 to 1 in 700) and lower rates in Europe and Australasia (1 in 1400). Sub-saharan Africa

HIV has spread rapidly in many urban centres in sub-saharan Africa through the 1980s. In this area at least 80% of virus transmission is estimated to have occurred through heterosexual transmission. Some of the earliest indicators of a rapidly developing epidemic came from studies of urban female prostitutes. Piot et al (1987), for example, demonstrated the rapid rise in prevalence of HIV infection in a cohort of Nairobi prostitutes. Prevalence of 4% was recorded in 1981 rising to 61% in 1985. Over the same period, prevalence in men attending sexually transmitted diseases (STD) clinics in the same area rose from 3% to 15%, and by 1985 evidence of HIV infection had also emerged in women attending antenatal clinics (2%). Extensive sero-surveys have been carried out in some countries, notably Rwanda (Rwanda HIV Seroprevalence Study Group, 1989). Studies such as these have demonstrated a consistent age distribution of HIV infection amongst women in sub-saharan Africa. The highest rates of infection are seen in sexually active young women with a peak at age 35 years. Women in the age group of 15-24 years have higher rates of infection than men of the same age group, possibly reflecting differential patterns of age mixing in sexual partnerships, such that women may experience their first intercourse earlier and tend to have sex partners older than themselves.

EPIDEMIOLOGY OF HIV INFECTION

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While sero-surveys in central African cities have reported rates of up to 40% infection amongst 30-34 year olds, there is marked heterogeneity in levels of infection in central Africa, and in many rural areas infection rates are much lower (perhaps 10-20% those of urban rates). In comparison with the rapid increase of prevalence in many cities in central Africa, in some rural areas prevalence has remained fairly stable over the course of a decade (Nzilambi et al, 1988). S. E. Asia There is evidence of recent introduction of HIV to Thailand and India. In Thailand the epidemic has been characterized by rapid spread amongst injecting drug users and female prostitutes (Ungshusak et al, 1991). In India, the epidemic mirrors the early stages of the epidemic in Africa, with rapid spread of infection reported amongst urban prostitutes in some cities and emergence of infection amongst heterosexual STD clinic attenders (Bhave et al, 1990; Jayapaul et al, 1990; Kandaswami et al, 1991). US, UK and Europe Estimates of the prevalence of HIV infection in industrialized countries are now largely based on the results of unlinked anonymous seroprevalence studies undertaken in women attending antenatal clinics and STD clinic attenders. In these studies, blood left over from clinical screening samples (e,g. for rubella or syphilis) are unlinked from patient identifiers and tested anonymously for HIV antibodies. This technique allows unbiased estimates of prevalence to be made and to be monitored over time without introducing the participation bias inherent in estimates of prevalence from named clinical testing programmes. US

In the US an extensive programme of unlinked anonymous surveillance is in progress (Centers for Disease Control, 1990). Studies in women attending antenatal clinics have emphasized the substantial variability in HIV prevalence in different areas. The mid-Atlantic, south-eastern States and Puerto Rico have the highest rates, while urban populations generally have higher levels of infections than rural populations (Landesman et al, 1987; Hoff et al, 1988; Centers for Disease Control, 1990). In a programme testing 1-2 million specimens from newborns for maternal antibody between January 1988 and February 1990, the median state seroprevalence was 1 positive for every 2000 tested, with the highest rate of more than 1 in 200 (Centers for Disease Control, 1990). In inner city areas such as New York, prevalence of 1.25% has been recorded (Novick et al, 1989). In general, prevalence in the US has been found to be higher in blacks and Hispanics than among whites. High prevalence in New York is also associated with areas where there are high rates of injecting drug use (Novick et al, 1989).

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UK

While HIV prevalence rates amongst women attending antenatal clinics are lower in the UK than the US, the same geographical variability is evident. The first results of the unlinked anonymous testing programme in the UK were published in 1991. In women attending antenatal clinics in England and Wales prevalence ranged from 1 in 500 in inner London through 1 in 1440 for the rest of the Thames regions to 1 in 16 000 in a region outside London (Public Health Laboratory Service AIDS Centre, 1991). Similar results in inner London were obtained by testing Guthrie card blood spots from neonates for maternal HIV antibody. Of great concern is evidence of the rising prevalence of HIV infection in pregnant women in inner London, which rose from 1 in 2000 in 1989 to i in 500 in 1991 (Ades et al, 1991). From these results it is estimated that obstetricians were aware of maternal infection in only one in five infected pregnancies. In Scotland, testing of Guthrie cards showed prevalences ranging from approximately 1 in 1500 in Aberdeen to 1 in 400 in Edinburgh (Tappin et al, 1991). RISK FACTORS FOR AIDS/HIV INFECTION IN WOMEN

The relative contributions of the three modes of HIV transmission (sexual, parenteral and perinatal) vary throughout the world as discussed previously. Globally, 70-80% of HIV infection is estimated to be acquired sexually (mostly heterosexually), 8-15% is acquired parenterally through blood transfusion and injecting drug use, and 5-10% through perinatal transmission (Chin, 1991). Similarly, the relative contribution of heterosexual and parenteral transmission to AIDS amongst adult women varies considerably. Sub-saharan Africa

In sub-saharan Africa it is estimated that more than 80% of HIV infection amongst women is acquired heterosexually. Studies have shown that the risk of seropositivity in African populations is associated with the number of heterosexual partners per year, a history of contact with prostitutes, the practice of prostitution and a history of STD (Clumeck et al, 1985a,b; van de Perre et al, 1985; Melbye et al, 1986; Quinn et al, 1986). Further support for a primarily heterosexual epidemic in Africa comes from the nearly equal sex ratio amongst cases, and the observation that the highest rates of HIV infection are seen in the sexually active age groups, with very low rates of infection in children aged 0-14 years and in those over 60 years (Clumeck et al, 1985b; Quinn et al, 1986; Rwanda HIV Seroprevalence Study Group, 1989). US, UK and Europe

Figure 3 shows the exposure categories for HIV infection amongst adult women with AIDS in the UK, US and Europe. In the US and Europe as a whole, more than half of the women with AIDS have a history of injecting

pmdu~ Other

~

IDU

Other

~x-~-~ Heterosexualcontact product~

Cumulative total cases = 18,201

Bbod

Exposure categories % to June 1991

AIDS in Adult Women-US

Cumulative total cases = 239

Blood

contact

use.

Figure 3. Exposure categories for women with AIDS in the UK, US and Europe. Cumulative cases to end of June 1991. IDU, injecting drug

Cumulative total cases = 7188

tDU

~k,LI 1.1 / I I'~/} Other

Bloodprod~eterosexual

Exposure categories % to June 1991

Heterosexual contact

AIDS in Adult Women-Europe

AIDS in Adult Women-UK

Exposure Categories % to June 1991

7 N

©

0 ©

.=

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A. M. JOHNSON

drug use with approximately one third of cases attributed to heterosexual contact. In contrast, more than half of the cases of AIDS reported in women in the UK are attributed to heterosexual contact and less than one quarter to injecting drug use. This reflects the relatively low prevalence of HIV infection amongst injecting drug users in many UK cities (with the exception of Edinburgh) in contrast to high levels of infection in US cities and in southern Europe (Des Jarlais and Friedman, 1987). Furthermore, in European and US populations, female prostitutes have thus far been relatively protected from sexual acquisition of HIV from their clients, probably as a result of the high rates of condom use reported in the sex industry (Padian, 1988). Infection amongst female prostitutes has generally been associated with a history of injecting drug use, and a proportion of drug users may use prostitution to finance their drug habit. Both the number of women and the proportion of cases occurring in women are increasing in the US and Europe. Data for the US are shown in Figure 4. Thousands

Percent

6

12

5

10

4-

8

3-

~

6

2

4

1

2

0

~

,

1981

1982

"';'" 1983

,

,

1984

1985

,

1986

,

,

1987

1988

0

1989

1990

Year Number

i

Percentage of t o t a l

Figure 4. Number and percentage of cases of AIDS in women, US 1981-1990, by year of reporting, From Ellerbrock et al (1991).

In 1990, 17% of newly reported cases of AIDS in Europe occurred in women, 8% in the UK and 11.5% in the US (Centers for Disease Control, 1991a; Communicable Disease Surveillance Centre, 1991; European Centre for the Epidemiological Monitoring of AIDS, •99•). Increasing concern has been expressed at the growing number of heterosexually acquired cases of HIV and AIDS in industrialized countries. Since the first descriptions of the epidemic, both the absolute numbers and the proportion of cases of AIDS attributed to heterosexual contact has increased annually. In the US a decreasing proportion of new cases of AIDS is occurring amongst women with a history of injecting drug use and an increasing proportion is attributed to heterosexual transmission (Ellerbrock et al, 1991). In the US heterosexual infection in women is currently substantially driven by transmission from injecting drug users. Thus more

21

EPIDEMIOLOGY OF HIV INFECTION

Sex with IDU

Pattern II country Sex with bisexual

Other

Figure 5. Heterosexually acquired cases of AIDS in women, US: risk factors for infection. Cumulative cases to end of June 1991 (n = 6035). IDU, injecting drug user. Source: Centers for Disease Control (1991a).

than 60% of women with heterosexually acquired AIDS are partners of injecting drug users (Figure 5) and a decreasing proportion of heterosexually acquired AIDS is seen in those who are 'born in pattern II' countries. In southern Europe, where the prevalence of HIV amongst injecting drug users is high, heterosexual transmission is similarly driven by injecting drug use. In the UK the pattern is currently somewhat different, with more than 65% of heterosexually acquired AIDS cases occurring amongst those who are from, or who have lived in, countries where the major route of HIV-! transmission is through sexual intercourse between men and women. EFFICIENCY OF FEMALE TO MALE AND MALE TO FEMALE TRANSMISSION The efficiency of heterosexual transmission has been studied through heterosexual partners studies. In these studies heterosexual contacts without other risk factors for infection but who have had sexual intercourse with an HIV-positive person with a known risk factor for infection (e.g. haemophiliac, injecting drug use, bisexuality) are interviewed and their HIV serostatus assessed. These studies have demonstrated that HIV can be transmitted from man to woman and woman to man through unprotected sexual intercourse and they provide estimates of the magnitude of risk. These estimates can be applied both in clinical counselling for couples as well as in epidemiological models of the potential for virus transmission. The results of the major heterosexual partners studies in theUS and Europe are summarized in Table 1. The proportions of positive female contacts range from 18% to 53%, but the majority of the large studies fall in the range of 18-30%. Of the three studies reporting proportions above 30%, two studied populations with contact in sub-saharan Africa and one studied mainly those with advanced HIV disease (Laga et al, 1989; Staszewski et al, 1988; Steigbigel et al, 1988).

Blood transfusion Bisexual/IDU Bisexual/IDU Bisexual/IDU

IDU Mixed (50% African connections) IDU (stage IV disease) 75% African contacts

Peterman et al (1988) Padian et al (1987; 1991) Johnson et al (1989a)* European Study Group (1989; 1991)*

Nicolosi (1990) Staszewski et al (1988) Steigbigel et al (1988) Laga et al (1989)

55 219 115 155 404 368 60 114 64

18 20 26 27 20 27 33 45 53

Percentage of contacts with HIV antibodies

* Couples with female index cases are included in European Study Group data. IDU, injecting drug use.

Major risk factors in index case

Reference

No. of couples studied

Male to female

Table 1. European and American partners studies of heterosexual transmission.

25 72 22 -159 90 30 14 16

No. of couples studied

8 1 14 -12 9 9 50 13

Percentage of contacts with HIV antibodies

Female to male

to

EPIDEMIOLOGY OF HIV INFECTION

23

Studies of female to male transmission have been based on smaller sample sizes than those of male to female transmission, reflecting the difficulty of recruiting suitable female index cases. The largest data-set is available from the European Study Group (1991) which found 19 of 159 (12%) male contacts of seropositive women to be infected. This proportion is significantly lower than the proportion of female contacts infected in the recent data from the study (20%). Furthermore, despite the variability in the proportions of positive male contacts in the studies summarized in Table 1, within each of the studies reported, lower rates of female to male than of male to female transmission are consistently found. This suggests that female to male transmission may be slightly less efficient than male to female transmission. This is consistent with the higher efficiency of male to female transmission observed for other STDs (Handsfield, 1984).

CO-FACTORS FOR HETEROSEXUAL TRANSMISSION

Partners studies have clearly demonstrated that heterosexual transmission occurs through vaginal intercourse and that both symptomatic and asymptomatic persons may transmit the virus to their partners. World-wide it is estimated that vaginal intercourse accounts for 60--70% of all HIV infections. However, a number of factors, in addition to vaginal intercourse, have been found to increase the risk of transmission between couples. Partners studies have consistently identified an increased risk of male to female transmission associated with the practice of anal intercourse. Studies have also reported an increased transmission risk in association with the presence of symptomatic disease (Centers for Disease Control stage IV) in the index case; the presence of other STDs in either index or contact case; and the absence of circumcision in the male for female to male transmission. Several studies have identified an increased risk of transmission in association with late stage disease, although this is not a consistent finding in all studies (Padian et al, 1987; European Study Group, 1989; 1991; Johnson et al, 1989a; Laga et al, 1989; Ragni et al, 1989). For example, Laga et al (1989) found a significantly increased risk of transmission as T4 lymphocyte counts fell below 300/mm 3 in the index case and the European Study Group (1989) found an increased risk in association with Centers for Disease Control stage IV disease (Centers for Disease Control, 1986). A postulated biological mechanism for increased probability of transmission from individuals with late stage disease is through increased infectivity in the index case. Virological research in this area has been limited, largely because laboratory techniques for assessing infectivity have only recently become available. The development of improved viral culture techniques for cell-free virus in plasma, indicate that Centers for Disease Control stage IV HIV disease is associated with increased plasma viraemia (Coombs et al, 1989). Quantitative plasma assays indicate that there is wide variability between individuals in viral load in plasma, but that mean titres of infectious virus in both plasma and peripheral blood mononuclear cells are

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A.M. JOHNSON

higher in those with stage IV disease compared with those with asymptomarie infection (Ho et al, 1989). More recently, quantitative R N A polymerase chain reaction (PCR) techniques have become available which may provide useful tools for the examination of the influence of viral load on infectivity as well as the impact of antiviral therapy on free virus in plasma (Ou et al, 1988; Qu Zhang et al, 1991). It remains uncertain whether levels of infectious virus in blood correlate with viral load in semen and vaginal or cervical secretions. Virus has now been isolated both from the cellular and from the cell-free component of semen, but the relationship between plasma viraemia and semen viraemia is still under investigation (Borzy et al, 1988; Anderson et al, 1990). Relatively little attention has been focused on the determinants of viral shedding in cervical and vaginal secretions. HIV was first isolated from cervical secretions in 1986 (Wofsy et al, 1986; Vogt et al, 1986). Virus can be isolated throughout the menstrual cycle (Vogt et al, 1987). However, the influence of cervical inflammation or disease stage on levels of viral shedding has not been extensively investigated (Bernstein, 1990). Other STDs

Evidence is now emerging, primarily from studies in sub-saharan Africa, that the presence of other STDs, particularly genital ulceration, may increase both the infectivity of an HIV-seropositive individual and the susceptibility of an uninfected individual. Studying men who reported contact with female prostitutes in Nairobi, Cameron et al (1989) reported an increased risk of female to male transmission of HIV associated with the acquisition of genital ulcer disease and with the absence of circumcision in the male. More recently, Laga et al (1990) reported an association between risk of H I V infection and the presence of non-ulcerative STD. The role of STD as co-factors in transmission is biologically plausible because the inflammatory process which increases the number of lymphocytes in genital secretions may increase infectivity and because epithelial disruption may increase susceptibility (Wolff and Anderson, 1988). Measurement of the magnitude of any causal effect of other STDs in fuelling HIV transmission is problematic because of confounding by patterns of sexual activity which are associated both with the risk of HIV acquisition and the risk of infection with other STD. Furthermore, the possible effect of H I V exacerbating the clinical severity of STD may further bias estimates of the independent influence of STD on HIV transmission (Mertens et al, 1990). In order to overcome the biases of observational studies, randomized community intervention trials are being undertaken to assess the impact of STD control programmes on HIV incidence in sub-saharan Africa. The protective effect of male circumcision in reducing female to male transmission reported in African populations (Cameron et al, 1989) requires further investigation in European populations. This has policy implications for countries where routine male circumcision is undertaken in only a minority of the population.

EPIDEMIOLOGY OF HIV INFECTION

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BEHAVIOURAL CHANGE IN HETEROSEXUAL COUPLES The prevention of further spread of HIV currently depends upon education and behavioural change. Mass media education campaigns have achieved high levels of awareness of HIV, its modes of transmission and understanding of safer sex, both in developed and in some developing countries (Health Education Authority, 1988; Lindan et al, 1991). Less well understood is the overall impact on population patterns of heterosexual behaviour. Within clinical services, and particularly in STD clinics, considerable resources have been invested in counselling for safer sex, particularly for patients considering HIV antibody testing. The goals, content and effectiveness of HIV counselling are the subject of Chapter 4, and details are not provided here. Although further research is required on the impact of counselling on behavioural change, some evidence for sexual behavioural change after counselling emerges from follow-up studies of the seronegative partners of infected men and women. Changes in sexual practice are characterized by an increase in the use of condoms and a reduction in the frequency of sexual intercourse (Johnson et al, 1990). Moore et al (1991), in a partners study in San Francisco, reported on 86 couples followed for a mean of 14 months in whom no further seroconversions occurred and in whom there was a marked increase in the use of condoms. In the European Study, no further seroconversions were observed in 100 couples who always used condoms, while ten were observed in 104 couples with inconsistent condom use (de Vincenzi et al, 1991). These findings indicate that knowledge of the partner's HIV status with counselling on behavioural change can achieve sustained uptake of safer sex practices and that consistent condom use can reduce the risk of further HIV transmission. Nevertheless, in all these follow-up studies, approximately half of the couples continued to practise some unprotected intercourse. In many couples, a desire for pregnancy may motivate the decision not to use condoms (Wyld et al, 1991). F O R E C A S T I N G THE FUTURE EPIDEMIC

Future spread of HIV world-wide will depend on a number of key parameters. These are the rate of sexual partner change and patterns of sexual mixing in the population; the prevalence of injecting drug use; the probability of transmission through sexual intercourse, parenteral transmission and perinatal transmission; and the duration of infectiousness of an infected individual (Anderson and May, 1988). A number of techniques has been used to forecast the future epidemic. These include extrapolation of trends in existing data; back-calculation methods (which provides estimates using the number of reported AIDS cases and knowledge of the incubation period); and transmission models based on knowledge of population patterns of sexual behaviour, transmission probabilities and knowledge of the incubation period for AIDS (Public Health Laboratory Service working group, 1990).

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h. M. JOHNSON

W H O estimates of the future epidemic indicate that during 1990s the pandemic will kill a further three million women and children (Chin, 1990b). Particularly in sub-saharan Africa, AIDS is expected to have a major impact on both child (5 years old or less) and adult (particularly in the 20--49-years age group) mortality rates. In the most severely affected sub-saharan African cities, adult deaths from AIDS could equal or exceed deaths from all other causes in the 1990s (Chin, 1990a). In the US, AIDS has already become established as a leading cause of death amongst men and women aged 25-44 years, and in 1991 it is likely to rank amongst the five leading causes of death for both men and women in this age group (Centers for Disease Control, 1991b). As a result of behavioural change in homosexual men and injecting drug users in pattern I countries, the incidence of infection through these routes may have now declined in comparison with rates in the early to mid-1980s. Nevertheless, as those infected in the 1980s progress towards late stage disease, the number of individuals with late stage disease is estimated to increase by 40% or more (Brookmeyer, 1991). However, while rates of heterosexually acquired infection remain low, there is little evidence of a slowing in the rate of new heterosexually acquired HIV infection and heterosexually acquired AIDS cases are expected to continue to increase to the mid-1990s (Brookmeyer, 1991). Similar increases in reports of heterosexually acquired AIDS are anticipated in the UK (Public Health Laboratory Service working group, 1990), but great uncertainty surrounds the potential for future heterosexual spread without better data on population prevalence and patterns of risk behaviour in the population. SURVEYING SEXUAL LIFE-STYLES In an attempt to understand the marked geographical variability in patterns of HIV transmission world-wide, many countries are now undertaking survey research to obtain reliable data on sexual practices, partnership patterns and sexual preferences in representative population samples (Centers for Disease Control, 1988; Carballo et al, 1989; Johnson et al, 1989b; Wellings et al, 1990; Bajos et al, 1991; Catania et al, 1991; Biggar and Melbye, 1991). These studies have emphasized the marked heterogeneity in rates of sexual partner change reported by individuals (Johnson et al, 1989b). In a UK pilot study, substantial generational changes were reported in the age of first heterosexual intercourse (Johnson et al, 1989b). The median age at first intercourse fell from 20 years for men (22 years for women) in the age cohort of 55-454 years, to 17 years for men (17 years for women) in the age cohort of 16-24 years. The highest rates of partner change in the last 5 years were seen amongst the youngest age cohort (16-24 years), a group who are also at the greatest risk of STD and vulnerable to commencing injecting drug use. Data from this study have been used for exploratory analysis to examine the potential spread of HIV in heterosexual populations in the UK (Blower et al, 1990). They indicate a wide range of possible estimates for the doubling

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time of the epidemic in the UK, ranging from 2 to 14 years. While this analysis from pilot data must be interpreted cautiously, it indicates that heterosexual virus transmission may continue to increase slowly over the course of decades and be detectable only through long-term monitoring of HIV prevalence in large population samples. Surveys of sexual behaviour in a number of African countries are underway, co-ordinated by the W H O (Carballo et al, 1989). These examine patterns of sexual partnerships, rates of sexual partner change, including the influence of age mixing and contact between male clients and female prostitutes. Mathematical models have been used to examine the influence of different patterns of sexual contact and mixing in African societies. These indicate that AIDS may change population growth rates from positive to negative values in the course of a few decades in African countries (Anderson et al, 1991). CONCLUSION In conclusion, as we move into the 1990s, HIV is likely to be a growing problem for women and their children in all parts of the world. It will probably have a major impact on population growth in Africa and increase adult and infant mortality rates throughout the world. It is anticipated that continuing spread, particularly through heterosexual transmission, will be seen in parts of the world previously relatively spared from HIV infection, such as India and Thailand, and that the incidence of heterosexually acquired cases of AIDS will continue to increase in industrialized countries in the foreseeable future. It is likely that in the future both the prevention and management of HIV infection will become an increasingly important problem in gynaecological practice. In clinical studies, the manifestations and natural history of HIV in women have, until recently, received relatively little attention. Further research is required into this area as well as into the gynaecological problems associated with HIV infection. These aspects are the subject of Chapter 12. As facilities for HIV testing and counselling become available in antenatal services and contraception clinics, clients may increasingly request wellinformed and non-judgemental advice on the transmission, clinical manifestations and prevention of HIV and other STDs. Sound knowledge of the epidemiology, clinical manifestations and management of the physical, social and ethical problems raised by the HIV pandemic has rapidly become an essential requirement for the obstetrician and gynaecologist. REFERENCES Ades AE, Parker S, Berry T et al (1991) Prevalence of maternal HIV-1 infection in Thames Regions: results from anonymous unlinked testing. Lancet 337: 1562-1564. Anderson D, Wolff H, Pudney J e t al (1990) Presence of HIV in semen. In Alexander NJ, Gabelnick HF & Spieler JM (eds) Heterosexual Transmission ofAIDS, pp 167-180. New York: Alan R. Liss.

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