US syphilis epidemics not driven by increases in unsafe sex say researchers

A UK based team of researchers has found that regular epidemics of syphilis in the USA are due to the intrinsic cyclical nature of the disease. They show that changes in the immunity of the population cause periodic syphilis outbreaks, rather than changes in sexual behaviour, as was previously thought.

According to research published this week in Nature, the team from Imperial College London and funded by the Medical Research Council and the Royal Society suggest that syphilis outbreaks, previously attributed to social phenomena such as the sexual revolution or the gay liberation movement, are actually caused by a loss of immunity among those at risk of infection.

Dr Nicholas Grassly, from Imperial College London, based at St Mary's Hospital and one of the researchers said: "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."

After analysing data from 68 cities across the USA since the 1940s, the team found that rises and falls in the disease followed a distinct pattern that was repeated over a 10-year cycle. Rises in syphilis cases can be explained by falling population immunity. Immediately after an epidemic, immunity is at its highest. It then takes time for immunity to drop to a level when an epidemic can occur again.

The team compared syphilis case reports with gonorrhoea reports from the same cities and found contrasting results. Despite infecting the same groups, cyclical epidemics of gonorrhoea did not occur because there is no immunity to re-infection with this disease. Change in the numbers of new gonorrhoea cases is therefore more likely to reflect change 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 analysing 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. Troughs in the number of cases offer an unprecedented opportunity for eradication of the disease. However, when this opportunity is missed, an epidemic is likely to follow."

The researchers believe the role of population immunity highlights the need to carefully interpret data from routine surveillance studies, where changes in the rate of new infections may not always be attributable to changes in behaviour or the environment.