Contributed by Jai A. Dennison| 27 January, 2005 02:55 GMT
"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.
Regular epidemics of syphilis in the US are due to the intrinsic cyclical nature of the disease, contends a UK based team of researchers. Changes in the immunity of the population cause periodic syphilis outbreaks, rather than changes in sexual behavior, as previously was thought, they say.In the past, syphilis outbreaks often were attributed to such social phenomena as the sexual revolution or the gay liberation movement. In fact, they are caused by a loss of immunity among those at risk of infection, according to research published Wednesday in Nature.
Ten-Year Cycle for Syphilis Outbreaks
"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 behavior," says one of the researchers, Dr. Nicholas Grassly of Imperial College London, based at St. Mary's Hospital.
After analysing data from 68 cities across the US 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 gonorrhea reports from the same cities and found contrasting results. Despite infecting the same groups, cyclical epidemics of gonorrhea did not occur, because there is no immunity to re-infection with this disease. Change in the numbers of new gonorrhea cases therefore is more likely to reflect change in sexual behavior than the cyclical natural phenomenon seen with syphilis.
Plan for Eradication Possible
"It is striking how the repeated epidemics of syphilis are predicted by what we know about the natural history of infection," adds Dr. Grassly, who also is a Royal Society University Research Fellow. "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 suggests.
"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," Dr. Grassly observes.
The researchers believe the role of population immunity highlights the need for careful interpretation of data from routine surveillance studies, where changes in the rate of new infections may not always be attributable to changes in behavior or the environment. |
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