Unsafe Sex Is Not Main Driver of Syphilis Epidemics

Syphilis epidemics are primarily driven by the intrinsic cyclical nature of the disease rather than changes in sexual behaviour, according to the results of a new study published in the January 26th issue of Nature.

Researchers at Imperial College London analyzed data reported to the US Centers for Disease Control and Prevention from 68 cities between 1941 and 2002, and found that rises and falls in the disease followed a distinct pattern that was repeated over a 10-year cycle.

The rises in syphilis cases could be explained by falling population immunity, and not, as previously believed, by changes in sexual behavior. Immediately after an epidemic, immunity was at its highest. It then took time for immunity to drop to a level where a new epidemic could occur, the researchers report.

"While we do not dispute the fact that syphilis is transmitted by unsafe sex, our findings suggest that change in population immunity is the main reason for periodic epidemics of syphilis, not change in sexual behaviour," lead author Dr. Nicholas Grassly commented in a statement.

Dr. Grassly's team compared syphilis case reports with gonorrhea reports from the same cities and found contrasting results. Despite infecting the same groups, cyclical epidemics of gonorrhoea did not occur because a protective immune response associated with this infection is absent.

Changes in the number of new gonorrhea cases are therefore more likely to reflect changes in sexual behaviour rather than the cyclical natural phenomenon seen with syphilis.

Dr. Grassly added: "It is striking how the repeated epidemics of syphilis are predicted by what we know about the natural history of infection."

"As well as analyzing previous epidemics it may also be possible to use these findings to help doctors and sexual health workers predict and prepare for future outbreaks of the disease," he continued.

"Troughs in the number of cases offer an unprecedented opportunity for eradication of the disease," he noted. "However, when this opportunity is missed, an epidemic is likely to follow."

01/31/05

Nature 2005;433:417-421.