Background: The mortality associated with AIDS among men may have had an influence on primary and secondary syphilis trends among men in the United States, through the loss of men at high risk for acquisition or transmission of syphilis in this population and/or by prompting safer sexual behaviors in response to the threat of AIDS.
Goal: The goal of this study was to examine the association between AIDS mortality rates and primary and secondary syphilis incidence rates among men in the United States from 1984 to 1997.
Study Design: We used a fixed-effects regression analysis of state-level AIDS mortality rates and primary and secondary syphilis incidence rates for men.
Results: Our analysis showed a significant association between higher AIDS mortality and lower rates of syphilis incidence, after we controlled for confounding factors. Our model estimates suggested that every 20 AIDS deaths per 100,000 adult men are associated with declines of about 7% to 12% in syphilis incidence rates among men.
Conclusion: Increases in AIDS-associated mortality may have accounted for one-third to one-half of the decline in syphilis rates among men in the early 1990s. Recent declines in AIDS mortality in the United States may have contributed to the recent outbreaks of syphilis, particularly among men who have sex with men. Our findings underscore the importance of providing STD prevention services to men with HIV infection and the need for STD surveillance in communities at risk for syphilis outbreaks.
SYPHILIS RATES IN THE UNITED STATES have declined dramatically since peaking in 1990. The 5979 cases of primary and secondary (P&S) syphilis reported in 2000 represent an almost 90% decline from 1990. 1 P&S syphilis rates in recent years are the lowest ever reported, prompting a nationwide syphilis elimination campaign. 2
Although the reasons for this unprecedented decrease in syphilis are not fully understood, at least four important factors may have contributed to the decline: (1) increased support of syphilis control programs and HIV prevention activities, 3,4 (2) acquired immunity to syphilis in the population at risk for syphilis, 3,4 (3) declines in crack cocaine use, a major contributor to the syphilis epidemic of the late 1980s, 3 and (4) sexual behavioral changes in response to the HIV/AIDS epidemic. 4-8
Examples of sexual behavioral responses to the HIV/AIDS epidemic include increases in condom use and reductions in number of sex partners. For example, increased condom use in response to increasing local AIDS prevalence has been observed, 9 and mathematical models have suggested that a decline in the rate of partner change as a result of AIDS awareness could explain the dramatic declines in STD rates among men who have sex with men (MSM) following the advent of the AIDS epidemic. 10
In addition to prompting safer sexual behaviors, the HIV/AIDS epidemic may have contributed to the decline in syphilis through a more direct mechanism: AIDS-associated mortality. It is likely that people at high risk for syphilis acquisition (and, more important, syphilis transmission) were disproportionately included among the hundreds of thousands who have died of AIDS-related causes in the United States, because syphilis is much more common in people with HIV infection than in the general U.S. population, and risk factors for syphilis (such as having unprotected sex or having numerous sex partners) are also risk factors for sexual acquisition of HIV. 11,12
A recent report showed that at least 30% of MSM and 5% of heterosexual men with diagnosed gonorrhea in Colorado Springs, Colorado, in 1981 died in the ensuing 2 decades, and HIV infection likely was the main cause for the higher death rates among MSM. 13
Mathematical modeling has illustrated the possibility that high AIDS death rates among the people at greatest risk for STD acquisition and particularly transmission may influence STD incidence rates. One model showed that the loss of sexually active people from the population through AIDS-related mortality can reduce the rate of partner change in the population, thereby reducing STD rates. 14 Another such model suggested that substantial declines in STDs in MSM since the advent of AIDS could be attributable to sickness and deaths due to AIDS, even if AIDS affected only a relatively small fraction of the population. 15
Theoretical studies also have demonstrated that differential AIDS mortality in a cohort can account for some of the observed reductions in risky sexual behavior in that cohort over time. 16
Thus, HIV/AIDS may have contributed to the decline in syphilis directly (through the loss of people at high risk for acquisition and particularly transmission of syphilis) and/or indirectly (by prompting safer sexual behaviors). In this study we use national STD surveillance data to explore the association between AIDS mortality and syphilis incidence in the United States in more detail, focusing on syphilis incidence rates and AIDS death rates among men.