One in five syphilis cases in Chicago gay men due to oral sex

The Chicago Department of Public Health (CDPH) has found that at least one in five cases of primary and secondary syphilis among gay men in the city appears to have been caught through oral sex. In contrast, only one in 16 of cases among heterosexuals seems to have been orally transmitted. This finding was published in the October 22^nd edition of Morbidity and Mortality Weekly Report.

Between 1998 and 2002, Chicago had the highest syphilis incidence of any city in the United States. During this period 1582 cases of primary and secondary syphilis were recorded by the CDPH. A successful prevention campaign has since begun to reduce the incidence of syphilis.

The overall rate has remained remarkably steady during this time, varying between 338 and 353 cases annually, or 11.8 to 12.2 per 100,000 people.

However a major shift in the makeup of this population happened over the five years. Rates declined by 68% among women, from 9.2 to 2.9 per 100,000, but they increased by 50% among men, from 14.7 to 22.1 per 100,000. Among black men, rates decreased by 31%: the majority (54%) of gay male cases were among white men whereas 90% of heterosexual cases were among African-Americans.

The number of cases among gay men increased suddenly in the year 2001, from around 30 to 40 cases annually in the preceding three years to around 180 in 2001 and over 200 in 2002. This represents a 469% increase in white gay men and a 462% increase among Hispanic gay men.

Many gay men interviewed by the CDPH indicated surprise they had been infected, and the investigators decided to interview 627 of the 962 people diagnosed with syphilis in 2001 and 2002 to establish risk behaviour.

Of the gay men, 20% indicated that they had no unprotected sex other than oral during the period they were likely to have acquired syphilis. The maximum incubation period for primary syphilis is three months (median three weeks) and for secondary syphilis it is six months. Fourteen per cent of the entire sample reported oral sex as their only risk behaviour.

There was no association between HIV status and the likelihood of transmission through oral sex: 22% of HIV-positive gay men reported oral sex as their only risk factor and 20% of those who had tested HIV-negative. Thirty per cent of Hispanic gay men reported oral sex as the only risk factor.

When compared with heterosexual men and women, respectively, gay men were 3.8 and 4.3 times more likely to report oral sex only during the likely period of infection.

These figures for oral sex transmission must be regarded as minima because the survey could not determine the route of infection among people who had had anal or vaginal as well as oral sex. In a survey from the United Kingdom Communicable Disease Surveillance Centre, the proportion of cases of syphilis acquired through oral sex was estimated as one in three.

The authors comment that secondary syphilis can manifest itself as infectious patches on the oral mucosa, which may be mistaken by patients for ordinary mouth ulcers or herpes sores.