Sexually Transmitted Diseases in America: How Many Cases and at What Cost?
Part 2
Estimates Of The Incidence And Prevalence Of Stds In The United States
Abstract
Background
Accurate, updated estimates of the incidence and prevalence of sexually transmitted
infections in the United States are needed. The most widely quoted number of annual new STDs is 12
million. However, this figure has not changed in more than a decade despite improvements in detection
methods and the effects of STD control programs.
Goals
To propose a system for assessing the strength of STD surveillance data and to estimate the
incidence and prevalence of STDs in the United States for 1996, using the available published data.
Results
We estimate more than 15 million incident STDs occurred in the United States in 1996.
This number exceeds the earlier 12 million estimate primarily because improved detection techniques
have allowed an assessment of previously undiagnosed infections.
Conclusions
Large numbers of new STDs continue to occur each year in the United States, with
serious health and economic consequences. More than two-thirds of our current estimate of 15 million
STDs annually is contributed by two infections - trichomoniasis and HPV - for which we have only
level III surveillance data. As the quality of our surveillance data improves, we can further refine the
precision of our national estimates.
Introduction
Sexually transmitted diseases (STDs) remain a major public health problem in the United States (Eng
& Butler, 1997). However, estimating the overall incidence and prevalence of STDs is a complex and
elusive task. For the past decade, the most widely quoted figure has been 12 million sexually transmitted
infections occurring annually (Gunn, 1998; ASHA, 1995). The scientific basis for this number has not
been closely examined, and recent variants have been proposed. For example, for 1994, the sum of the
specific STDs listed in the Institute of Medicine's report was 10.5 million new cases annually, while a 1998
NIH Program Announcement cited 14 million STDs annually (NIH, 1998).
Are 12 million cases of STDs still an accurate estimate of STDs for the United States today? Several
developments suggest a readjustment is in order. Because control programs directed against gonorrhea and
syphilis have enjoyed recent successes, the incidence and prevalence of these infections have declined
(DSTDP 1997). Chlamydia control programs, which have emphasized increased screening, have led to a
paradoxical (though predictable) situation where reported cases are increasing despite decreasing incidence
and prevalence (DSTDP 1997). Also, improved detection techniques have made us aware of the
unrecognized extent of genital herpes, human papillomavirus, and trichomoniasis. This review examines
the available published evidence to provide an updated point estimate and range for the incidence and
prevalence of selected STDs in the United States.
Measures of STDs
Estimates of the incidence and prevalence of STDs in the United States vary according to the source
of data and the methods used to detect infections (Eng and Butler, 1997; St. Louis, 1997). Sources
generally include 1) reportable infections (e.g. gonorrhea, syphilis and chlamydia), 2) diagnoses made
during visits to office-based practices, 3) national surveys of representative populations, 4) prevalence
data on individuals attending specialized health facilities (e.g. STD clinics, family planning clinics, etc.),
and 5) data from multinational models of STD natural history.
Case reports for notifiable STDs for the United States are collected by the Centers for Disease
Control and Prevention through formal surveillance systems based in the states (DSTDP, 1997).
Reported data may vary in accuracy, depending upon the surveillance priorities of STD control
programs. These data on notifiable infections tend to be more accurate in states that have laws that
require reporting of positive STD tests. Other data on infections can be derived from five key sources:
1) the National Health and Nutrition Examination Survey (NHANES), conducted by the National
Center for Health Statistics (NCHS), which collects clinical and biologic data on a random sample of
Americans; 2) the Hospital Discharge Survey of NCHS, which includes 7,500 randomly selected
hospitals from throughout the United States; 3) the National Ambulatory Medical Care Survey of
NCHS, which is a probability sampling of the diagnoses of 1,900 physicians; 4) the National Hospital
Ambulatory Medical Care Survey of NCHS, which is a probability sampling of visits to hospital
emergency and outpatient departments; and 5) the National Disease and Therapeutic Index (NDTI),
which is a private survey of a random sample of office visits to U.S. physicians in office-based practices.
Unfortunately, each of these sources has limitations. Data on reported infections are affected by
differences in the completeness of reporting between public and private health care sources, as well as
interstate variation. Because infections diagnosed in public facilities are reported more frequently, these
data are susceptible to biases related to the characteristics of individuals who tend to use public clinics.
A cascading set of circumstances must occur for STDs to be measured accurately by public health
authorities. For symptomatic infections, the symptoms must be initially perceived as abnormal by the
individual and must be severe enough to cause the person to seek health care. The STD must then be
diagnosed and, for reportable infections, it must be communicated to appropriate health authorities by
clinicians. For asymptomatic infections, screening programs must be available at health services
routinely used by infected persons. Likewise, data from private clinicians' practices are often affected by
the absence of diagnostic validation.
States differ markedly in the quality of their surveillance data on specific STDs. Most have ongoing
systems to collect information on syphilis and gonorrhea; however, cutbacks on testing for the latter may
have affected the consistency of reporting (Gershman and Rolfs, 1991). Recently, the reported number
of genital chlamydial infections has been increasing, in large part because a growing array of states has
made genital chlamydia a reportable infection, and wider screening has identified more cases (CDC,
1997). Finally, data from specific health facilities suffer from the problem of patient selection bias, as
well as local geographic variation.
National surveys are limited by their size, sporadicity, and the superficial nature of their analytic
variables. The national data bases include relatively small numbers of STDs in the samples, which lead
to wide confidence intervals in subpopulations. Most national surveys are conducted years apart, which
makes timely interpretation of trends difficult. Several rely on self-reports for identifying a history of
previous STDs, which is limited by a respondent's inability to recognize subclinical infection and
reluctance to admit a stigmatic condition. Growing use of biomarkers as measures of current (i.e. urine
LCR) or past (i.e. antibodies) STD will help overcome the problem of self-reported data.
Data on specific STDs also vary by the type of infection (DSTDP, 1997), depending on whether
current or cumulative infection is being measured. Symptomatic viral infections (measured by physician
visits) occur less frequently than serologic or cytologic indicators of the cumulative number of infected
persons. Thus, care must be used in making comparisons among the different estimates of STDs, and
differences between incident and prevalent infections should be kept in mind.
Finally, the World Health Organization (WHO) has used a simple prevalence model to estimate the
magnitude of curable STDs worldwide (Gerbase, 1998). First, the available information on STD
prevalence from both developed and developing countries was summarized. Then, the prevalence of
gonorrhea, chlamydial infection, syphilis, and trichomoniasis was estimated by gender and by United
Nations region. The 1995 regional "denominator" was calculated using mid-year population estimates
of adults 15-49 years of age. Next, the duration of each curable infection was estimated by gender and
by region. These duration estimates were based on the probability that a symptomatic or asymptomatic
person received treatment for her/his STD. Regional STD incidence in adults was then calculated by
dividing the estimated prevalence by the estimated duration of each disease. Although based on broad
assumptions, this WHO approach provides a standardized mechanism to make global estimates for
public health purposes. It also is the best source of our estimate for trichomoniasis in the United States.
Strength of Surveillance Evidence
Several approaches have been suggested to assess the quality and reliability of the estimates for
specific STDs within the United States. One is based on a characterization of the quality,
generalizability, and precision of the available data (Table 1); a second has listed estimation methods by
source of data (St. Louis, 1997). We have chosen to use the first approach. Even though it relies heavily
on qualitative assessments of the data sources, the concept can be used to give readers an idea of the level
of confidence the panel has in its estimates of the incidence and prevalence of specific STD. We have
categorized the various STDs by strength of evidence according to the levels of "good," "fair," and
"poor," or levels I, II and III, respectively (Table 1).
| Table 1. Strength of Evidence STD Surveillance Systems |
| | Rating Level | Criteria | Example |
|
| I | GOOD | * Representative national surveys
* Complete national reporting | HSV-2, AIDS |
| II | FAIR | * Widespread, consistent prevalence data from convenience samples
* Consistent widespread, though incomplete national reporting | Chlamydia, HIV, HBV, Syphilis, Gonorrhea |
| III | POOR | * Inconsistent, non-representative prevalence data
* Estimates based only on rough extrapolations | HPV Trichomoniasis, Chancroid, BV |
|
| Sources: Zaidi (1996), ASHA Panel to Estimate STD Incidence, Prevalence and Cost (1998) |
Level I surveillance data come either from representative national surveys such as NHANES or
from national reporting systems with nearly complete counts such as AIDS. Level II surveillance data
are derived from composite prevalence figures obtained from multiple populations over time (e.g. for
chlamydia) or from less complete national reporting systems (e.g. for gonorrhea). Finally, level III
surveillance data are based on even weaker evidence and rough extrapolations (e.g. for HPV and
trichomoniasis).
Epidemiology of STDs in the United States
In the United States, the incidence of reported genital chlamydial infections and viral STDs steadily
increased in recent years, while the incidence of gonorrhea generally declined during the same interval.
However, the actual number of chlamydial infections probably fell as control programs expanded.
Levels of syphilis varied among different population subgroups, but have reached record lows since
1995. Vaginal infections such as trichomonas and bacterial vaginosis have probably remained high,
although surveillance for these conditions is rudimentary.
- Chlamydial Infections.
Genital chlamydial
infections became the most prevalent bacterial
STD in the United States during the 1980s, at
the time gonorrhea levels began declining. In
1996, nearly 500,000 cases of genital
chlamydia were reported to CDC, exceeding all
other notifiable diseases in the United States
(DSTDP, 1997). Reported chlamydial
infections in women greatly exceed those in
men, primarily because screening programs
have been directed to that group. Moreover,
chlamydial prevalence is strongly correlated
with younger age and heterosexual behaviors.
A previous estimate of 4 million new
chlamydial infections annually in the United
States was made more than a decade ago, using
a prevalence ratio approach (Washington,
1986). Because the expansion of chlamydia
control programs has probably led to declining
chlamydial prevalence in the interim (DSTDP,
1997), this estimate has been updated. In
1997, between 2.6 and 3.2 million new cases of
genital chlamydia were estimated to have
occurred in persons aged 10-44 years
(Groseclose, 1997). As a point estimate, we
chose 3 million new chlamydia infections
having occurred in 1996 (Table 2).
- Gonorrhea.
Gonorrhea trends have been quite
consistent ever since 1975. The number of
reported gonorrhea cases has generally declined,
starting in the mid-1970s with the introduction
of the national gonorrhea control program. A
disproportionate share of the decline occurred
among older, white populations, with infection
rates remaining relatively high among minority
races and adolescents (Gershman and Rolfs,
1991; Webster, et. al, 1993; Fox, et. al, 1998).
In addition, reported gonorrhea is associated
with a younger mean age than syphilis among
all gender and race categories. In 1996, CDC
reported 325,900 new cases of gonorrhea
(DSTDP, 1997). Because previous
investigations have shown about half of all
diagnosed gonorrhea cases are reported to
public health authorities, an estimated total of
650,000 new gonorrhea infections occurred in
1996 (Table 2).
| Table 2. Estimated Incidence and Prevalence of STDs, United States, 1996, by Strength of Evidence |
| STD | Incidence | Prevalence |
|
| Chlamydia | 3 million -II | 2 million -II |
| Gonorrhea | 650,000 -II | |
| Syphilis | 70,000 -II | |
| Herpes | 1 million -II | 45 million -I |
| Human Papilloma Virus | 5.5 million -III | 20 million -III |
| Hepatitis B | 77,000 -II | 750,000 -I |
| Trichomoniasis | 5 million -III | |
| Bacterial Vaginosis | No Estimates | |
| HIV | 20,000 -II | 560,000 -II |
|
| TOTAL | 15.3 million | |
Source: ASHA Panel to Estimate STD Incidence, Prevalence and Cost.
NOTE: Incidence is the number of new cases in a given time period;
prevalence is the total number of cases in the population. |
- Syphilis.
Syphilis trends have followed a roller
coaster course for the last half-century. Its
incidence rose during World War II, but fell
thereafter, coinciding with the introduction of
penicillin. The lowest levels were observed at
the end of the 1950s, but from the 1960s on,
the incidence of syphilis increased (Nakashima,
1996). A rapidly rising male-to-female ratio
coincided with the spread of syphilis among
men having sex with men throughout the
1970s. However, in the 1980s, indicative of
the safer sexual behaviors stimulated by HIV
prevention messages, syphilis cases in gay males
declined precipitously (Rolfs and Nakashima,
1990). This encouraging trend was directly
countered by the number of climbing syphilis
cases during the late 1980s among
heterosexuals of minority races, in large part
fueled by the crack epidemic. Nonetheless,
during the 1990s, syphilis levels again fell to
numbers seen two decades earlier, leading
public health authorities to entertain notions of
syphilis elimination (CDC, 1998; St. Louis,
1998). In 1996, CDC reported 11,400 new
cases of primary and secondary syphilis and
53,000 new cases of all stages of syphilis
(DSTDP, 1997). Accounting for an estimated
20% underreporting, approximately 70,000
total syphilis infections in 1996 were estimated
to have been diagnosed (Table 2).
- Genital Herpes.
The numbers of symptomatic
genital herpes cases increased eleven fold during
the 1970s and 1980s (DSTDP, 1997). Genital
herpes causes at least ten times more genital
ulcer cases than does syphilis. A comprehensive
analysis of existing national databases estimated
nearly 150,000 clinical visits for genital herpes
in 1992 (Tao, 1998). Moreover, recent
investigations have shown that symptomatic
infections caused by herpes simplex viruses
(HSV) are only the tip of the iceberg (Fleming,
1997). Infection with HSV-2 has occurred
among an estimated 45 million Americans,
even though less than one-quarter perceive
themselves ever to have had genital herpes.
Based on differences between HSV-2 levels
measured cross-sectionally in the late 1970s and
the late 1980s, up to 1 million new HSV-2
infections may be transmitted each year in the
United States (Table 2). This number ignores
the sizable percentage of genital herpes
contributed by HSV-1, and thus might be
considered a minimum estimate.
- Human Papilloma Virus.
Likewise, the
diagnosis of symptomatic genital warts caused
by the human papilloma viruses (HPV) has
been skyrocketing during the last two decades
(DSTDP, 1997). Its asymptomatic
counterparts, HPV infections of the cervix and
vagina, have emerged as the most common
STD among sexually active young populations.
The cumulative three-year incidence of HPV
infection among college-aged students was 43
percent, with a duration of eight months (Ho,
1998). Using conservative assumptions and
extrapolating these data to the general U.S.
population, one obtains an annual estimate of
at least 5.5 million new HPV infections each
year (Ratcliffe, 1998 - Table 2). Likewise, a
conservative estimate of the prevalence of
productive HPV (that is, persons with active
shedding of HPV DNA) is approximately 20
million (Koutsky, 1997 - Table 2).
- Hepatitis B.
Hepatitis B, despite the
availability of a preventive vaccine, still remains
among the main sexually transmitted viral
infections. Approximately two-thirds of the
total incident hepatitis B cases are spread
sexually (Sabin, 1998). Based on CDC
estimates of 128,000 overall HBV infections in
the US in 1995, we count 77,000 incident
sexually transmitted hepatitis B cases (Table 2).
Based on serological measures from NHANES-III,
an estimated total of 1,250,000 prevalent
cases of chronic hepatitis B exist in the United
States. Thus, we estimate a prevalence of
approximately 750,000 currently infectious
persons with sexually acquired HBV (Table 2).
- Trichomoniasis.
Vaginal infections caused by
Trichomonas vaginalis are among the most
common conditions found in women attending
reproductive health facilities. Between 3% and
48% of sexually active young women
requesting routine care at prenatal, family
planning, or college health clinics were
diagnosed with trichomoniasis (Cotch, 1997).
The WHO estimated that this STD accounted
for nearly half of all curable infections
worldwide. Based on WHO estimates for
North America, we extrapolate 5 million cases
of T. vaginalis infection annually in the U.S.
(Table 2).
- Bacterial Vaginosis.
Bacterial vaginosis, a
sexually associated condition, is the most
frequent cause of vaginitis in sexually active
women of reproductive age (Sobel, 1997).
Depending on the population studied, the
prevalence of BV in the United States varies
from 17% in family planning settings to 37%
among selected groups of pregnant women. In
the developing world, BV is even more
common in the general population, affecting
approximately half of all women - including
those with only one reported lifetime sex
partner (Wawer, 1998). The natural history of
untreated BV remains a controversial research
topic, although the presence of this condition
has been linked to pelvic inflammatory disease
and HIV acquisition. Moreover, in high
prevalence areas, BV tends to recur even after
mass treatment of asymptomatic women and
their partners (Wawer, 1998). Because no
established surveillance system exists for BV
and no previous estimates of its incidence or
prevalence have been made, we elected not to
include this condition in the aggregate number
of STDs we have derived.
- Human Immunodeficiency Virus and AIDS.
HIV infection epidemic trends in the U.S. have
been evolving. Beginning in the mid-1970s,
HIV was transmitted primarily among
homosexual and bisexual men, and AIDS was
first diagnosed in this group by the early to
mid-1980s. The virus entered the injection
drug-using (IDU) populations in the early
1980s and rapidly spread during the decade.
Limited heterosexual transmission occurred
until the late 1980s. However, since 1989, the
greatest proportionate increase of reported
AIDS cases has been among heterosexuals, and
this trend is expected to continue (Rosenberg,
1995). In 1993, an estimated 750,000 persons
in the U.S. were infected with HIV, with
approximately 40,000 new infections occurring
each year (Rosenberg, 1995). By 1996, another
approach to estimating HIV incidence and
prevalence yielded an estimate of 41,000 new
HIV infections annually, with between 700,000
to 800,000 prevalent HIV infections
(Holmberg, 1996 - Table 2). The introduction
of protease inhibitors may increase the number
of prevalent infections by extending the life of
HIV-infected people. Approximately half of
the incident and three-quarters of prevalent
infections were estimated to have been sexually
transmitted. Thus, it appears as if the
incidence of HIV has been relatively stable over
the past several years (CDC, 1998). Globally,
the incidence of HIV is much higher, with an
estimated 5.8 million new HIV infections
annually and more than 30 million persons
currently living with HIV (UNAIDS, 1998).
More than 90% of the global total has been
spread sexually.
Conclusion
Our updated estimate of the number of STDs annually is somewhat higher than the 12 million
infections estimated in 1988. The cumulative number of incident infections spread sexually is more
than 15 million cases per year. However, nearly 70% of that total are contributed by two infections -trichomoniasis
and HPV - for which we have only level III surveillance evidence. Thus, this point
estimate is not precise; the true number could be as low as 10 million or as high as 20 million STDs
annually.
A variety of explanations can account for the larger number of STDs. As diagnostic sensitivity
improved through use of amplification techniques, we gained a greater understanding of the magnitude
of asymptomatic HPV infections. Our update of new trichomonal infections, albeit extrapolated from
WHO methods, also raised the total. Finally, use of serologic data from NHANES allowed better
estimates of asymptomatic acquisition of HSV. These increases were only partially offset by decreases in
the number of chlamydia, gonorrhea and syphilis infections resulting from recent successes in STD
control programs.
These estimates of STD incidence and prevalence represent a snapshot in time, based on the available
published data. CDC has a goal of improving the strength of evidence supporting its surveillance of
each STD (St. Louis, 1997). For example, a pilot study of genital chlamydia from urine samples
obtained from NHANES participants suggested reliable national prevalence estimates could be obtained
(Mertz, et. al, 1998). As the methods to detect and measure the magnitude of STDs improve, so will
the precision and accuracy of these estimates. Nonetheless, as both a barometer of the STD burden in
the late 1990s and also a number upon which public policy can be based, the national estimate of 15
million new cases of STDs annually is a useful tool.
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