San Francisco health officials reported an alarming increase in new syphilis cases last year after years of declines.

New syphilis cases rose 55.8 percent in 2008 over the previous year, San Francisco public health officials reported.

The increase was significantly lower than the dramatic spikes reported early in the decade, when new infections increased 167 percent from 2001 to 2002. But last year's rise would seem to indicate an increase in unsafe sexual practices, which could lead to a rise in HIV infection rates.

San Francisco health experts said they have not seen an increase in new HIV cases and cannot explain the reason for it.

"We don't have a single explanation to tie it to, but we do know reported risk behaviors haven't changed dramatically," said Dr. Susan Philip, acting director of the sexually transmitted disease prevention and control services unit of San Francisco's Health Department, which published the new findings in its 2008 summary of sexually transmitted diseases.

One possible explanation, Philip suggested, is that people who are already infected with HIV are having sex with others who are HIV positive.

The vast majority of early syphilis cases - about 97 percent - occurred in men, and more than 90 percent involved men who had sex with men, according to the Health Department. More than 70 percent of cases occurred among men who are HIV positive.

"If you have HIV-positive men having unprotected sex with other HIV-positive men as a conscious decision to reduce the rate of HIV transmission ... and if they're not aware of syphilis, you have the unintended consequence of rising rates of syphilis," said Steve Gibson, director of Magnet, a sexual health center for gay men in the Castro and a program of the San Francisco AIDS Foundation.

Gibson said the center has seen an uptick in syphilis cases since 2006, but said 2007 and 2008 rates of the disease diagnosed at the clinic remained stable. He said awareness of syphilis tends to ebb and flow with changes in the population and in funding for sexually transmitted disease education.

The number of early-stage cases reported in San Francisco in 2008 was 548, just shy of the all-time record of 554 new cases in 2004, according to health officials.

The good news is that new syphilis cases have decreased by 3.5 percent for the first nine months of 2009 compared to the same period last year, according to the Health Department.

While San Francisco health officials couldn't explain the slight decline, they said the city has stepped up efforts to promote syphilis awareness among the public and health providers, particularly dentists and hygienists. This includes the Healthy Penis campaign, complete with appearances this year of the 6-foot-tall costumed penis at fairs, parades and public events and his buddy, Phil the Syphilis Sore.

"We're not sure if what we're doing is having any impact, but we're gratified to see it (the syphilis rate) didn't increase," Philip said.