Rise and Fall of Syphilis Said Normal

By MATT CRENSON

AP National Writer

January 27, 2005, 7:26 AM EST

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.

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.

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.