Increase in syphilis doesn't mean more new HIV in SF and LA - yet

The number of new HIV infections amongst gay men in San Francisco and Los Angeles remained stable between 1999 and 2002 despite a massive increase in the incidence of syphilis, according to a report published in the July 9^th edition of Morbidity and Mortality Weekly Report (MMWR).

However, the report’s authors caution that the continued spread of syphilis could contribute to an increase in HIV infections, and note that 50% of the cases of syphilis seen between 1999 and 2002 were in men who were already chronically infected with HIV.

In several cities in the UK, Europe and the US there have been recent outbreaks of syphilis amongst gay men. These have coincided with data suggesting that gay men may be taking more sexual risks. Infection with syphilis can facilitate both infection with HIV and the transmission of HIV.

Investigators wished to establish if the recent increases in cases of primary and secondary syphilis amongst gay men in San Francisco and Los Angeles had had an impact on new cases of HIV.

Data were gathered on the number of cases of syphilis between 1998 and 2002 in the two US west coast cities. Rates of syphilis per 100,000 gay men were than calculated per year based upon the estimate that there are just over 50,000 gay men living in San Francisco.

Stored blood samples obtained from gay men undergoing HIV testing at public health facilities were tested using an assays able to determine if an individual had been recently infected with HIV.

In San Francisco the number of syphilis cases in gay men increased from four in 1998 to 260 in 2002. The rate per 100,000 gay men increased from eight in 1998 to 512 per 100,000 in 2002.

There were no cases of syphilis diagnosed in gay men in Los Angeles before 2000, when the infection was seen in 67 men, and this increased to 299 cases in 2002. In total, 47% of gay men in San Francisco and 18% of gay men in Los Angeles had their syphilis diagnosed at a public health facility.

Despite this significant increase in new cases of syphilis, the number of new HIV infections seen at San Francisco’s AIDS Health Project Clinic and the City Clinic remained stable between 1998 and 2002 (AIDS Health Project incident infections 1998 1.9% and 2.4% 2002, City Clinic incident infections 4.8% in 1998 and 4% in 2002).

Similarly, in Los Angeles, numbers of new HIV infections diagnosed at state health facilities remained stable between 1998 and 2002 (4.8% incident infections in 1998 and 4.1% in 2002).

“The number of new HIV infections among [gay men] at public HIV testing sites in San Francisco and Los Angeles did not increase during 1999-2002, a period when syphilis cases among [gay men] increased substantially in both cities” note the MMWR editors.

However, the editors add that there were limitations with the investigators’ report. First, data was only obtained from public health facilities. The majority of syphilis cases amongst gay men were diagnosed at private practices and the editors caution that individuals using private health care might have different demographics and risk behaviour. In addition, the investigators’ analysis did not include a consideration of men undergoing oral HIV testing. At one of the San Francisco public health facilities the proportion of men having an oral HIV test increased from 11% in 1998 to 36% in 2002. It was not possible to determine if the men who tested positive using oral tests had recent or chronic HIV infection.

The editors believe that the large increase in cases of syphilis amongst gay men in San Francisco and Los Angeles did not translate into an increase in HIV incidence because the number of syphilis cases remained low relative to the number of gay men at risk of HIV. In addition, in both cities, over half the syphilis cases involved men with long-standing HIV infection.

“If the outbreaks of syphilis continue unabated, HIV incidence among [gay men] at public HIV-testing sites and in the larger [gay] community might increase,” conclude the editors.