Immunity Changes, Not Sex, Drive Syphilis Outbreaks

Outbreaks of syphilis in the United States were not driven by the sexual revolution or the gay liberation movement but by changes in people’s immunity, research by scientists from Imperial College in London showed.

Although syphilis is a sexually transmitted infection, epidemics in the United States were due to the cyclical nature of the disease, not changes in sexual behavior, said researchers, who studied data going back to the 1940s. “When there is an outbreak of syphilis one should not immediately say people are adopting unsafe sexual practices,” Dr Nicholas Grassly, who headed the research team, told Reuters on Wednesday. “It is actually the fact that syphilis generates protective immunity that drives these epidemics.” Sexual behavior does play a role but Grassly and his colleagues suggest the main reason is changes in the immunity of the population. Syphilis, which is caused by the bacterium Treponema pallidum, conveys a natural immunity to re-infection but scientists are not sure exactly how long it lasts. Grassly, an infectious disease epidemiologist, and his colleagues studied data from the Centers of Disease Control (CDC) in Atlanta, Georgia, from 68 cities in the United States dating back decades. Their findings are reported in the science journal Nature. They looked at infections of syphilis and gonorrhea, another sexually transmitted disease. There was a distinct pattern of rises and falls in syphilis that was repeated in 10-year cycles but not in gonorrhea, which has no protective immunity. “Syphilis shows these repeated epidemics every 10 years but gonorrhea does not. You would expect them to be very much the same given they are spread by the same route, they tend to infect the same people, they last the same length of time and are treatable by antibiotics,” Grassly explained “It led to this alternative explanation.” The scientists also noticed that epidemics in the different cities started to happen at the same time and became more synchronized over time. Although syphilis had declined by 89.2 percent in the United States from 1990-2000, the number of cases rose from 5,972 in 2000 to 6,103 in 2001. It was the first increase since 1990, according to the CDC. Grassly said periods of low levels of infections would be an ideal time to try to eliminate the disease by increasing access to treatment and promoting the use of condoms and safe sex. (Source: Reuters Health, January 2005)