Rise and Fall of Syphilis Said Normal

The recent rise in US syphilis rates is likely due to the cyclical nature of the infection rather than an increase in high-risk behaviors such as unsafe sex, according to researchers at London's Imperial College. Syphilis has been on the rise in the United States since 2000, when 5,979 cases were reported -- the lowest number in six decades. In 2003, the most recent year for which data are available, 7,177 cases were reported.

Similar spikes in syphilis incidence have been observed before. When syphilis rates peaked in the early 1970s, some researchers attributed the rise to the sexual revolution; when infections rose again in the 1980s, the spread of crack cocaine was blamed.

Epidemiologist Nicholas C. Grassly and colleagues examined syphilis infection trends in 68 US cities over the past 50 years and discovered an oscillating pattern that peaks at eight- to 11-year intervals. But when they compared gonorrhea rates over the same period, no such pattern was detected. Because the diseases spread the same way, if changes in sexual behavior had caused the syphilis spikes, they should have caused a similar pattern in gonorrhea. Instead, gonorrhea rates climbed from the 1950s through the 1970s, and have steadily declined since about 1980.

The researchers concluded that the regular ups and downs in syphilis rates are an intrinsic property of the infection itself. Unlike people who get gonorrhea, those who recover from syphilis can resist reinfection for a period of time. So after syphilis sweeps through a community, a proportion of people are relatively immune to it and infections rates decline.

But as the community's population evolves, the proportion of susceptible people rises, causing infection rates to do the same. The researchers used a computer model to demonstrate that the time from one peak to another is about a decade.

The data revealed another interesting pattern: Until around 1980, syphilis rates in different cities varied, in that rates could be falling in one city and rising in another. But more recently, syphilis rates in US cities have become much more uniform.

The full study, "Host Immunity and Synchronized Epidemics of Syphilis Across the United States," was published today in the journal Nature (2005;433(7024):417-420).

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