updated 1/26/2005 1:43:36 PM ET 2005-01-26T18:43:36

A recent rise in syphilis rates in the United States is probably due to natural cycles rather than an increase in unsafe sex or other behaviors, according to a new study.

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The finding is encouraging to public health authorities who have worried that increasing syphilis infection, especially among gay and bisexual men, is a sign that people at high risk for HIV have grown complacent about practicing safe sex.

Syphilis has been on the rise in the United States since 2000, when the incidence of the disease was at its lowest in six decades. In 2003, the most recent year for which data are available, 7,177 cases were reported, compared with 5,979 in 2000.

“One would expect for the next few years for syphilis incidence to continue to rise,” said Nicholas C. Grassly, one of the study’s authors.

Similar jumps in syphilis incidence have been observed in the past. When syphilis rates peaked in the early 1970s, some researchers blamed the sexual revolution; when infections rose again a decade later, they were attributed to the spread of crack cocaine.

Grassly and his colleagues argue in this week’s issue of the journal Nature that syphilis infection follows a natural cycle that peaks at eight- to 11-year intervals. Though sexual behavior certainly influences the overall number of people infected, the researchers concluded, those regular ups and downs are an intrinsic property of the disease itself.

The researchers discovered the oscillating pattern by examining syphilis infection trends in 68 U.S. cities over the past 50 years. But when they looked at the pattern of gonorrhea infection over the same period, the pattern was absent. Gonorrhea rates tended to rise over the second half of the 20th century, then fell gradually beginning around 1980.

Infection spurs resistance
Because the diseases spread the same way, if changes in sexual behavior had caused the oscillating pattern in syphilis they should have created a similar pattern in gonorrhea. Yet gonorrhea rates show a steady rise from the 1950s through the 1970s, followed by a steady fall.

“You don’t see these repeated, regular epidemics,” said Grassly, an epidemiologist at Imperial College in London. He wrote the Nature paper with Christophe Fraser and Geoffrey P. Garnett, also of Imperial College.

The key to the difference between syphilis and gonorrhea is immunity. Unlike those who catch gonorrhea, people who recover from syphilis can resist re-infection for some time afterward. That means when the disease sweeps through a city, it leaves a relatively immune population in its wake and infection rates fall.

But as the population gradually evolves, the proportion of susceptible individuals rises and so do infection rates. Using a computer model, Grassly and his colleagues showed that the time from one peak to the next should be about a decade.

“It’s a kind of neat piece of detective work,” said Bryan Grenfell, a biologist at Pennsylvania State University who studies childhood infectious diseases. Many of those illnesses, such as measles and chickenpox, also follow oscillating patterns.

The British researchers detected another interesting pattern in the data as well. Until about 1980, syphilis rates in different cities varied independently. Houston might be experiencing a peak at the same time that New York was going through a trough. But more recently, syphilis rates have become much more uniform in the United States.

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