PARIS, Jan 26 (AFP) - Worries that syphilis is spreading rapidly in America and Europe, mainly as a result of unsafe sex among gay men, are probably unfounded, according to a probe into the long-term cycles of this disease.
Health authorities in the United States, Britain, France and Germany issued a series of alerts in 2003 and 2004 after noting double-digit annual rises in syphilis cases.
Most of the cases occurred among homosexuals and bisexuals, sparking worries of a return to unprotected sex after years of campaigning for condom use against HIV and other sexually-transmissible diseases.
But the latest research, published on Thursday in the British journal Nature, places a big question mark over this assumption.
A trio of London epidemiologists looked at data for syphilis and gonorrhoea, reported by doctors in 68 American cities to the US Centers for Disease Control and Prevention (CDC) from 1941 to 2002.
They found that syphilis cases rose and fell in cycles lasting eight to 11 years, whereas the trend for gonorrhoea had none of these unusual oscillations.
The reason lies with the "natural dynamics" of syphilis infection, the study suggests.
People who get syphilis derive a partial immunity from exposure to the bacterium, whether they are treated or not.
That phenomenon has a major impact on the spread of the disease.
The more syphilis cases there are in the community, the more this partial immunity builds up. This pushes the rates of detected syphilis down.
But as time goes by, the proportion of the population with the partial immunity falls. That increases the overall vulnerability to syphilis, so rates start to rise again.
These wide amplifications do not exclude mini-epidemics, such as an outbreak that happened in the United States in the late 1980s, mainly among women who had fallen into prostitution as a result of addiction to crack cocaine.
However, gonorrhoea, which does not have such regular oscillations, is therefore likely to be a far more accurate indicator of unsafe sexual behaviour, suggests the study, authored by three epidemiologists at Imperial College London.
US cases of gonorrhoea rose in the late 1960s, coinciding with the advent of sexual permissiveness.
But the rate fell back significantly in the 1990s, most probably in response to the safe-sex message of the anti-AIDS campaign, the authors say.
"Recent synchronised increases in syphilis incidence across the United States have sparked concern that there is growing complacency about the risk of HIV and returns to unsafe sexual behaviour, particularly among men who report sex with men," they say.
"Our analyses, however, suggest that these synchronised increases may largely reflect natural dynamics after a period of buildup of non-immune individuals, a conclusion supported by the lack of any correlated increases in gonorrhoea case reports."
01/26/2005 16:17 GMT - AFP