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HIV/AIDS |
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During the 20th century, improvements in sanitation and hygiene, reduced crowding, and the development of antimicrobials and vaccines have greatly reduced infectious disease morbidity in industrialized countries. In developing countries, smallpox has been eradicated, polio eliminated from the Western Hemisphere, and child survival prolonged through the expanded use of childhood vaccines, and improved management of diarrheal and acute respiratory infections of children. However, there is growing concern about the persistence, emergence or reemergence of a host of other infectious disease threats. Sexually transmitted infections, including HIV, figure prominently among these emerging infectious diseases. The number of microbial pathogens identified as causes of STD, and the morbidity attributable to STD, relative to that caused by other infectious diseases, have continued to increase throughout the 20th century. In fact, recent analyses show that STDs collectively rank among the five most important causes of years of healthy productive life lost1,2 in developing countries. Changes in human behavior and ecology, including socioeconomic factors and related economic development policies, the population explosion and the demographic transition (with rapid increases in the number of adolescents and young adults), rural-to-urban migration, war and attendant sociocultural disruption all have led to epidemic increases in STDs.3 Sexually transmitted pathogens are increasingly being linked to various common disease syndromes and new sexually transmitted pathogens (such as HIV) are emerging, causing new sexually transmitted diseases (like AIDS). Unrestrained population growth and urbanization leave every reason to expect the continued emergence of new STD pathogens and syndromes into the next century. In addition, improved diagnostic methods have led to the identification of previously unrecognized pathogens (e.g., human T lymphotrophic virus [HTLV], and a growing number of human papilloma viruses [HPV]). Careful clinical epidemiologic studies have demonstrated that these pathogens are sexually transmitted and associated with diseases that were formerly considered idiopathic (e.g., tropical spastic paraparesis caused by HTLV, genital cancers caused by HPV).
Why Aggressive STD Control Efforts are Necessary The World Health Organization estimates that the global incidence in 1995 of new cases of selected curable STDs–gonorrhea, chlamydial infection, syphilis and trichomoniasis–was 333 million.5 The global prevalence of active or latent infection with the common chronic viral STDs such as genital herpes simplex virus, genital HPV, hepatitis B virus and, increasingly, HIV, could be estimated in the billions of cases, since in some populations the majority of adults become infected with one or more of these pathogens. The consequences of being infected with an STD can be divided into two categories: complications and sequelae seen in STDs other than HIV/AIDS, and those directly related to HIV/AIDS. Social and economic consequences of these conditions as well as the direct health costs need to be considered. The most serious health consequences of STDs, other than HIV/AIDS, tend to occur in women and newborn children. Complications in women include cervical cancer, pelvic inflammatory disease with resulting infertility, chronic pain, ectopic pregnancy and related maternal mortality. In developing countries maternal complications directly related to untreated STDs are a major cause of infant and maternal morbidity and mortality. Complications in newborns include congenital syphilis, gonococcal infection of the conjunctivae–a potentially blinding condition–and perinatal hepatitis B infection. Measures for preventing these three manifestations of STD in newborns have been judged among the most cost effective measures for preventing neonatal infectious morbidity (see Chapter 9).6 A remarkably large and growing number of malignancies is now partially attributed to pathogens that can be sexually transmitted. These malignancies include cervical cancer and hepatocellular carcinoma, two of the most common malignancies in developing countries; squamous epithelial cell cancers of the vagina, vulva, penis, and anus; Kaposi's sarcoma; and T lymphocyte malignancies. Serious neurologic diseases caused by STD pathogens include those attributable to syphilis, as well as neonatal herpes encephalitis, tropical spastic paraparesis/HTLV-I-associated myelopathy, and various HIV-associated neurological disorders. Thus, effective prevention and control of STDs is now recognized as (1) a high priority for preserving reproductive health, especially among women; (2) an important approach to preventing cancer and neurologic disease; and (3) a very cost-effective approach to preventing neonatal morbidity. Nonetheless, public awareness of the spectrum of adverse health consequences of sexually transmitted infections has been slow to develop–perhaps because the link between infection and consequences is obscured by the frequently subclinical nature of early infection, especially in women. Other reasons include the long delay until recognition of adverse reproductive health consequences or the occurrence of malignancy or neurologic sequelae. And, finally, there has not been a major effort to educate the public on the potentially serious effects of sexually transmitted infections. The Role of STDs in Increasing Sexual Transmission of HIV The impact of other STDs on sexual transmission of HIV was initially suspected on the basis of epidemiologic studies, showing that persons with an ulcerative or nonulcerative STD appear more susceptible to acquiring HIV infection. Subsequent studies showed that urethral and endocervical inflammation caused by non-ulcerative STDs increases genital shedding of HIV-infected cells, and thus probably increases infectivity of the person with HIV infection.7,8 The importance of STD in promoting HIV transmission and the potential effectiveness of STD control in preventing sexual transmission of HIV were confirmed in Mwanza, Tanzania, where a community randomization trial of strengthened syndromic management of STD was associated with a 42 percent reduction in HIV incidence over a two-year period.9 Thus, strengthened STD prevention and management have become essential components of HIV prevention programs. How STD Epidemiology Defines the Basic Strategies for Preventive Intervention The rate of spread of STDs, including HIV, is determined by three factors: (1) the average rate of exposure of susceptible people to infected people; (2) the average probability that an exposed susceptible person will acquire the infection (the "mean efficiency of transmission"); and (3) the average time that newly-infected persons remain infectious and able to continue spreading infection. Anderson and May10 have expressed these relationships in a mathematical model, in which R0, the reproductive rate of infection, represents the number of secondary cases of STD arising from a new case. In this model, R0 = c _ ß _ D, where c is the "effective mean rate of sexual partner change" within the population, ß is the mean efficiency of transmission per exposure, and D is the mean duration of infectiousness after acquisition of a new infection. Thus, interventions can prevent spread of an STD within a population by (1) reducing the rate of exposure to STD by lowering the rate of partner change; (2) by reducing the efficiency of transmission; or (3) by shortening the duration of infectiousness for that STD. Figure 1 presents a simple conceptual approach to STD prevention, illustrating these three primary targets of various individual and population-level interventions. For HIV infection and most other viral STDs, therapy has not yet been clearly shown to shorten the duration of infectiousness, so interventions currently target the rate of exposure and the efficiency of transmission. For the curable STD, interventions also target the duration of infectiousness. "Behavioral" vs. "Biomedical" Approaches to STD Prevention
It is not reasonable to focus solely on preventing exposure, or acquisition, or transmission. While it is true that reduction of exposure, efficiency of transmission, or duration of infectiousness to zero would eliminate transmission altogether, these are not realistic goals. Focusing on one goal, to the exclusion of the others, would have diminishing returns. Conversely, equal allocation of resources to each of these three targets is neither feasible in every setting, nor necessarily the most cost-effective approach even when feasible. Nonetheless, a balanced effort initially seems desirable, in which resources are increasingly committed to each potential point of intervention, as the marginal yield of increasing efforts in each area is monitored. To reduce exposure to STD, behavioral interventions can be designed to delay the onset of sexual activity among youth, to decrease rates of new sex partner acquisition, to encourage choice of a safe sexual partner, and to foster faithful monogamy or polygamy. In addition, a lower prevalence of STD in the population will lower the risk that a sexually active individual will be exposed to an STD. To reduce the efficiency of STD transmission per sexual exposure to infection, behavioral and biomedical interventions can include promoting a delay in the sexual debut of females (young women are anatomically more susceptible than older women to acquiring certain endocervical infections if they are exposed), safer sexual practices, use of condoms, use of topical microbicides and immunization (however, currently the only available vaccine against an STD is hepatitis B vaccine) should be promoted. Finally, to reduce the duration of infectiousness accessible, acceptable and effective services for early STD detection and treatment are necessary together with promotion of their use when they are needed. Thus, each step in the intervention pathway, from avoiding exposure, to preventing acquisition of infection, to preventing further transmission of infection, involves both behavioral and biomedical interventions. A coordination of such interventions is essential. It is important to emphasize that interventions that reduce risky sexual behaviors or result in increased condom use, can prevent most, if not all, important STDs; whereas those designed to improve only early detection and curative treatment of STDs can reduce transmission only of those specific diseases that are cured or suppressed. Unique Issues in STD Prevention Unique aspects of the epidemiology of communicable diseases have important implications for STD prevention. Patterns of intimate contact between members of the population generally do not directly determine the incidence of non-communicable diseases, but they do directly determine the incidence of communicable diseases. Roy Anderson and his colleagues at Oxford have demonstrated how the variance in rates of partner change within the population greatly influence the rates of spread of STD.11 Those individuals with highest rates of partner change, the so called "core groups," disproportionately increase the rate of spread of STD within a population. Research on sexual behavior mixing becomes critical in order to determine the identity and priority of various possible "core groups." Further, it can be shown by mathematical models that patterns of sexual partner mixing, and the characteristics of sexual networks, are important determinants of the rates of spread of STD.12,13 For example, if those with many partners tend to have sex with others who have many partners ("assortative mixing"), infection spreads rapidly at first; and infection spreads fastest within "dense" sexual networks, which have sexual links between many people over a short period of time. Thus, interventions should have greatest impact if they are effectively focused and delivered among individuals who have many partners, and in dense sexual networks (see Chapter 13). To the extent that interventions provided through the private or public health-care sectors do not reach such populations, the impact on STD spread in the population may be quite limited. Finally, for communicable diseases in general, and for STD in particular, there is a threshold rate of transmission required to sustain spread of an infection in the population. For STD, when the reproductive rate of infection is greater than one (R0 > 1), more than one secondary case on average arises from each new case, and the infection spreads. Whereas when the reproductive rate of infection remains less than one (R0 < 1), the infection eventually disappears from the population. As a result, when the value of R0 is 1 or more, then interventions that reduce the value of R0 to less than 1, if sustained, could drive the infection to extinction in the entire population. This is true even when these interventions are provided only to those individuals and networks that experience highest rates of transmission. For example, this could explain how a modest reduction in STD incidence or prevalence in the Mwanza, Tanzania trial of STD syndromic management could have led to a 42 percent reduction in sexual transmission of HIV. For communicable diseases, curative treatment provides secondary prevention at the individual level, and primary prevention at the population level. In public health, it is customary to distinguish between primary and secondary levels of prevention. A leading textbook on public health and preventive medicine14 states that "in epidemiologic terms, the aim of primary prevention is to reduce the incidence of disease and injury...by removing the precipitating causes and determinants of departures from good health;" whereas, "the aim of secondary prevention is to detect and correct departures from good health as early as possible; in other words, to reduce the prevalence of disease and disability." Although these distinctions apply well to the noncommunicable diseases, like hypertension and trauma, the distinctions become blurred for the curable communicable diseases, for which early detection and treatment reduce the infectivity duration, and thus lower the incidence of infection. Thus, for the bacterial STDs, which are communicable from person-to-person, early detection and curative treatment of recently infected individuals provide secondary prevention at the individual level by preventing complications in the individual; and provide primary prevention at the population level by preventing further transmission. Thus, the terms "primary" and "secondary" prevention, when applied to STD prevention, are better replaced by terms more specifically descriptive of the intervention used when applied to STD prevention. Even when applied to the chronic incurable viral STDs, such as HIV infection, for which the duration of infectivity may or may not be influenced by antiviral therapy, use of the terms "primary" and "secondary" prevention may mislead. When "primary prevention" refers to preventing acquisition of infection, interventions focus on uninfected persons; whereas when primary prevention also encompasses prevention of transmission of infection from those already infected, interventions also can focus on those who are already infected. In research and public health programs on prevention of curable and incurable STDs, both behavioral and biomedical scientists have tended to underemphasize the importance of preventing the transmission of infection from those who are already infected. On the Importance and Use of this Handbook for STD Control This handbook has been designed to fill an important unmet need in the field of STD prevention and control. Existing texts address in detail the etiology, epidemiology, pathogenesis, clinical manifestations, diagnosis and treatment of STDs, but none provides a comprehensive and integrated approach to the design and management of STD control programs, particularly for resource-limited settings.15,16 This has been a glaring deficiency in the field. Many of the essential technical tools for STD prevention and control are now in hand, but what has been lacking are pragmatic guidelines for the effective use of these tools in an integrated program. Such a guide has been long overdue, and this handbook should now prove the catalyst to finally begin mobilizing technically sound programs in developing countries in an effective way. It will complement existing textbooks for STD management in industrialized countries as well. The handbook is for STD program managers and for all levels of service providers. It includes sections covering program management issues (Chapters 2, 3, 6, 7, I4, I5); case management issues (Chapters 8, 9, 11, 12); other service issues (Chapters 4, 5, 10); and innovative approaches suggested by authors who have a wealth of experience in STD program development (Chapter 13). Thus, health managers, including categorical STD program managers and primary health-care managers, will find the book an essential training, planning and management tool. Service providers will find practical and up-to-date guidelines for preventive and clinical services. The book's chapters are autonomous and modular, with extensive cross-referencing to other chapters throughout the book.
We would like to express our gratitude and appreciation to the authors and reviewers who, despite their busy schedules, found time to contribute to the Handbook. A special thanks goes to contributors from the World Health Organization and the Joint United Nations Programme on AIDS (UNAIDS). The most difficult tasks were performed by the AIDSCAP/Family Health International editorial and support staff. We owe a very special expression of gratitude to Jim Cassell, the consulting editor, who worked tirelessly to bring together the works of different authors on very different subject areas. The Handbook was supported by a dedicated and hard-working team made up of Tracy Smarrella, Associate Program Officer, Sexually Transmitted Disease Unit; Diana Randall, Staff Assistant, Information Programs; Mary O'Grady, Associate Director, Information Programs; Kathleen Henry, Senior Writer/Editor, Information Programs; Michael Buja, Program Assistant, Information Programs; Anjum Husain, Research Assistant, Behavior Change Communication Unit; Robert Gringle, Senior Editor/Writer; Linda Francis, Procurement; and Dever Designs, Inc., independent design firm. |
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