Sexually transmitted diseases (STDs) are caused by a group of infectious microorganisms that are transmitted mainly through sexual activity. These agents represent a costly, burdensome global public health problem. STDs can cause harmful, often irreversible, clinical complications, including reproductive health problems, fetal and perinatal health problems, and cancer, and they are also linked in a causal chain of events to the sexual transmission of human immunodeficiency virus (HIV) infection. Although STDs are largely preventable through behavior modification and sound primary health care, they are under-recognized and under-appreciated as a public health problem by most healthcare providers, the general public, and healthcare policy makers. In 1997, the Institute of Medicine characterized STDs as "hidden epidemics of tremendous health and economic consequence" in the United States and advocated urgent national preventive action.
An estimated 333 million curable STDs occur annually worldwide. In the United States, STDs are among the most frequently reported infectious diseases nationwide. Each year an estimated 15 million new cases of STDs occur in Americans, including nearly 4 million infections in U.S. teenagers. The annual direct and indirect costs of the principal STDs, including sexually transmitted HIV infection, and their complications are estimated at $17 billion.
More than twenty-five bacteria, viruses, protozoa, and yeasts are considered sexually transmissible. Bacterial STDs include those caused by Chlamydia trachomatis (chlamydia), Neisseria gonorrhoeae (gonorrhea), Treponema pallidum (syphilis), Haemophilus ducreyi (chancroid), and other common sexually transmitted organisms. Chlamydia and gonorrhea cause inflammatory reactions in the host. In women, these organisms can ascend into the upper reproductive tract where pelvic inflammatory disease (PID) can cause irreparable damage to the reproductive organs and result in infertility, ectopic pregnancy, and chronic pelvic pain. In its early stages, syphilis causes painless genital ulcers and other infectious lesions. Left untreated, syphilis moves through the body in stages, damaging many organs over time. Chancroid is associated with painful genital lesions. In pregnant women, acute bacterial STDs can cause potentially fatal congenital infections or perinatal complications, such as eye and lung infections in the newborn. Effective single-dose antimicrobials can cure chlamydia, gonorrhea, syphilis, and chancroid.
Viral STDs include the sexually transmitted viral infections caused by human immunodeficiency virus (HIV infection), herpes simplex virus type 2 (genital herpes), and human papillomavirus (HPV infection). Initial infections with these organisms may be asymptomatic or cause only mild symptoms. Treatable but not curable, viral STDs appear to be lifelong infections. HIV is the virus that causes acquired immunodeficiency syndrome (AIDS). Herpes causes periodic outbreaks of painful genital lesions. Some strains of HPV cause genital warts, and others are important risk factors for cervical dysplasia and invasive cervical cancer. Hepatitis B virus (HBV) is another acute viral illness that can be transmitted through sexual activity. Most persons who acquire HBV infection recover and have no complications, but it can sometimes become a chronic health problem.
Trichomonas vaginalis (trichomoniasis) is a common protozoal STD, and Candida species (candidiasis) are sexually transmitted yeasts. Both are frequently associated with vaginal discharge.
STDs are behavior-linked diseases that result from unprotected sex. Nonetheless, several biological factors contribute to their spread. These include the asymptomatic nature of STDs, the long lag time between infections and complications, the higher susceptibility of women to STDs, and the way that STDs facilitate the transmission of HIV infection.
The silent nature of STDs represents their greatest public health threat. Most STDs cause some symptomatic illness, but many produce symptoms so mild or nonspecific that infected persons are not alerted to seek medical care. As many as one in three men and two in three women with chlamydia infection have no obvious signs of infection. Without treatment or other interventions, infected persons can continue to infect new sex partners. Moreover, serious complications that cause irreversible damage can occur "silently" before any symptoms are apparent. A related problem is the long interval that can elapse between acquiring an STD and recognizing a clinically significant health problem. Women can develop cervical cancer many years after infection with some strains of HPV. A woman may first suspect she had an asymptomatic infection with chlamydia or gonorrhea when she finds out later in life that she is infertile or has an ectopic pregnancy. Because the original infection was likely to have been asymptomatic, there is frequently no perceived connection between the original sexually acquired infection and the resulting health problem. The lack of awareness of this connection leads people to underestimate their risk and to forego preventive precautions.
Gender and age are also associated with increased risk for STDs. Women are at higher risk than men for most STDs, and young women are more susceptible to certain infections than older women. Due to cervical ectopy that is extremely common in adolescent females, the immature cervix of adolescent females is covered with cells that are especially susceptible to STDs such as chlamydia.
The presence of other STDs, especially those that cause genital ulcers or inflammation, influences the sexual transmission and acquisition of HIV infection. Studies have repeatedly demonstrated that people are two to five times more likely to become infected with HIV through sexual contact when other STDs are present. In addition, dually infected persons (persons who are infected with both HIV and another STD) are more likely to transmit HIV infection during sexual contact. Conversely, effective STD detection and treatment can slow the spread of HIV infection at the individual and community levels. For example, in a study in Malawi in the mid-1990s, treatment of gonorrhea in HIV-infected men returned the frequency and concentration of HIV genetic material in semen to levels comparable to levels found in HIV-infected men who were not infected with other STDs. Similarly, a community trial in Tanzania in the mid-1990s demonstrated that treatment of symptomatic STDs resulted in a 42-percent decrease in new heterosexually transmitted HIV infections.
Some social factors directly affect STD spread especially in vulnerable populations. In addition, the stigma that continues to surround STDs in the United States indirectly interferes with establishing new social norms pertaining to sex and sexuality.
When there are barriers to health care, it is difficult to detect and treat STDs early. Infected persons also miss an opportunity for behavioral change counseling. Health care access barriers keep infected persons in the community where they continue to spread STDs. In the United States, groups with the highest rates of STDs are the same groups in which access to health care services is limited or absent.
Perhaps the greatest social factor contributing to the spread of STDs, and the factor that most significantly separates the United States from industrialized countries with low STD rates, is the stigma that continues to be associated with sexually transmitted infections. Although sex and sexuality pervade many aspects of American culture, most Americans are secretive and private about their sexual behavior. Talking openly and comfortably about sex and sexuality is difficult even in intimate relationships. This secrecy about sexuality and STDs adversely affects STD prevention in the United States by thwarting sexuality and STD education programs for adolescents, hindering communication between parents and children and between sex partners, promoting unbalanced sexual messages in the media, obstructing education and counseling activities, and impeding research on sexual behaviors.
All racial, cultural, economic, and religious groups are affected by STDs, and people in all communities and sexual networks are at risk. Nevertheless, some persons are disproportionately affected by STDs and their complications.
STDs disproportionately affect disenfranchised persons and individuals who are in social networks characterized by high-risk sexual behaviors, substance abuse, and limited access to health care. Some notable disproportionately affected groups include sex workers, homeless persons and runaways, adolescents and adults in detention, and migrant workers. Many studies document the association of substance use, especially alcohol and drug use, with STDs. The introduction of illicit substances into communities can dramatically alter sexual behavior in high-risk sexual networks leading to epidemic spread of STDs. The national U.S. syphilis epidemic of the late 1980s was fueled by the effect of increased crack cocaine use, especially in minority communities. Crack cocaine led to increases in sex exchanged for drugs and in the number of anonymous sex partners and decreased health care-seeking behavior and motivation to use barrier protection—all factors that can increase STD transmission in a community. Other substances, including alcohol, can also affect a person's cognitive and negotiating skills before and during sex, lowering the likelihood that preventive action will be taken to protect against STDs and pregnancy.
Gender disparities are an important aspect of the epidemiology of STDs. Compared to men, women suffer more frequent and serious STD complications, including PID, ectopic pregnancy, infertility, and chronic pelvic pain. Women are biologically more susceptible to infection when exposed to a sexually transmitted agent, and STDs are often more easily transmitted from a man to a woman than from a woman to a man. Given that some newly acquired STDs (and even some long-term complications) are only mildly symptomatic or completely asymptomatic in women, the combination of increased susceptibility and silent infection frequently results in delayed STD diagnosis and treatment. A further complication is that STDs are more difficult to diagnose in women due to the complex anatomy of the female reproductive tract and the frequent need for a speculum examination and diagnostic culture tests.
In pregnant women, STDs can result in serious health problems or death to a developing fetus or newborn. Sexually transmitted pathogens can be transmitted across the placenta, resulting in congenital infection, or can reach the newborn during vaginal childbirth, resulting in perinatal infection. Regardless of the route of infection, these organisms can permanently damage the fetal or newborn brain, spinal cord, eyes, auditory nerves, or immune system. Even when the organisms do not reach the fetus or newborn directly, they can cause spontaneous abortion, stillbirth, premature rupture of the membranes, and preterm delivery.
For a variety of behavioral, social, and biological reasons, STDs also disproportionately affect adolescents. In 1998, U.S. teenagers 15 to 19 years old had the highest reported rate of chlamydia and the second highest rate of gonorrhea. The herpes infection rate among white youth in the United States aged twelve to nineteen increased nearly fivefold from the late 1970s to the early 1990s. Because not all teenagers are sexually active, the actual rate of STDs among teens is even higher than the observed rates suggest. There are several contributing factors. Many teenagers are, in fact, sexually active and at risk for STDs, and they are having sex with partners from sexual networks that are already highly infected with untreated STDs. In 1999, among U.S. high school youth interviewed for the Youth Risk Behavior Surveillance System survey, half (49.9%) indicated they had had sexual intercourse during their lifetimes. Early sexual activity and multiple sexual partners were commonly reported among American high school youth; 8.3 percent of students indicated they had first had sex before age thirteen, and 16.2 percent said they had four or more sex partners during their lifetime. Despite the supposedly easy access to condoms that can lower STD transmission risk considerably, only 58 percent of sexually active students said they used a condom the last time they had intercourse. Sexually active teenagers are often reluctant to seek STD services or face serious obstacles to obtaining such services. In addition, health care providers are often uncomfortable discussing sexuality and risk reduction with young persons.
Some minority racial and ethnic groups (mainly black and Hispanic populations) in the United States have higher rates of STDs compared with rates for whites. Race and ethnicity in the United States are risk markers that correlate with other more fundamental determinants of health status such as poverty, access to quality health care, health care-seeking behavior, illicit drug use, and living in communities with high STD prevalence. Public health data may over-represent STDs among racial and ethnic groups who are more likely to receive STD services from public sector STD clinics characterized by timely and complete reporting of public health statistics. However, even when random sampling techniques are used to study health problems, higher rates of STDs are often found among African Americans and Hispanics compared with whites.
The dynamics of how STDs spread in populations have been studied extensively to derive approaches to prevention and control. Three main factors predict how fast and at what level STDs will spread in a population: the nature of sexual relationships, the degree to which susceptibility to STDs can be modified, and the timeliness and completeness of treatment.
The nature of sexual relationships refers to the decisions people make about when to become and remain sexually active and whom to select as sex partners. The earlier that vaginal, oral, or anal sexual intercourse begins and the greater the number of lifetime sex partners, the more likely a person is to acquire one or more STDs in a lifetime. Behavioral interventions that help delay the initiation of intercourse and reduce the lifetime number of sex partners will have a positive effect on slowing STD transmission.
Susceptibility to STDs can be modified with vaccines or barrier contraceptives such as condoms. If uninfected persons are somehow immune to STDs, then no transmission will occur. The availability of effective vaccines against STDs could dramatically slow increases in or even eliminate some STDs. For example, there is an effective and widely available vaccine for hepatitis B, a viral STD. Current strategies to immunize all children against hepatitis B before they become sexually active could greatly reduce the societal burden of this disease. Susceptibility can also be altered each time sex occurs. The correct and consistent use of condoms can reduce the rate of STD transmission in a population. Persons who choose to engage in sexual behaviors that place them at risk of STDs should use latex or polyurethane condoms every time they have sex. A condom put on the penis before starting sex and worn until the penis is withdrawn can help protect both the male and the female partner from most STDs. When a male condom cannot be used appropriately, sex partners should consider using a female condom. However, condoms do not provide complete protection from all STDs. Sores and lesions of STDs on infected men and women may be present in areas not covered by the condom, resulting in transmission of infection to a new person. This is common with genital warts and other genital HPV infections.
Although condom use has been on the rise in the United States over the past few decades, women who use the most effective forms of contraception (sterilization and hormonal contraception) are less likely than other women to use condoms for STD prevention. The most effective methods of contraception are not the most effective methods of STD prevention; likewise, methods that give a considerable measure of protection against STDs are considered to be good, but not the most effective, methods of pregnancy prevention. This suggests that, especially for young women who are at highest risk for unwanted pregnancy and STDs, using dual protection (condoms and hormonal contraception) will offer the best overall protection against both.
The third factor in STD prevention and control focuses on finding and treating infected persons and their sex partners. The longer someone has an untreated STD (especially if the person is asymptomatic), the longer that person can potentially infect others. If that interval can be shortened for the millions of persons who acquire STDs each year, then transmission would slow appreciably. Screening and treatment are the biomedical approaches that can be applied to this situation. For STDs that are frequently asymptomatic, screening and treatment also benefit those likely to suffer severe complications (especially women) if infections are not detected and treated early. For example, in the early 1990s, chlamydia screening in a large metropolitan managed-care organization reduced the incidence of subsequent PID in the screened group by 40 percent. Identifying and treating partners of persons with curable STDs has always been an integral part of organized control programs. Theoretically, this can break the chain of transmission in a sexual network. Early antibiotic treatment of a sex partner can interfere with an STD taking hold in a recently exposed person. Partner treatment benefits the original patient by reducing the risk of reinfection, and the partner benefits by avoiding acute infection and potential complications. Because future sex partners are protected by treating partners, this strategy also benefits the community. New screening tests (some of which can be performed on urine specimens) that facilitate STD screening in nontraditional settings are now available.
Many examples demonstrate the effectiveness of organized approaches to STD prevention and control that incorporate these strategies on a large scale. When a sustained, collaborative, multifaceted approach to STD prevention and control is undertaken, dramatic results can be achieved. One need only observe the results of sustained STD prevention efforts in many countries in Western and Northern Europe, Canada, Japan, and Australia, where STD rates are many times lower than in the United States, to conclude that STD prevention programs can work on a national scale.
ALLISON L. GREENSPAN
JOEL R. GREENSPAN
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