Syphilis lowers CD4 cell count and increases viral load in HIV-positive men

Syphilis infection in HIV-positive men is associated with an increase in viral load and reduction in CD4 cell count, according to an American study published in the October 21^st edition of AIDS. The study investigators believe that their findings indicate that HIV-positive men with syphilis are potentially more infectious and call for integrated public health campaigns to prevent the spread of both HIV and syphilis.

Although it is known that syphilitic ulcers can facilitate HIV transmission, the effect of syphilis on HIV viral load and CD4 cell count has been little studied. Other sexually transmitted infections have been linked to changes in immune status, and investigators in San Francisco wished to see if infection with syphilis had an impact on the viral load and CD4 cell counts of HIV-positive men.

In common with many other cities in the United States and Europe, there has been a substantial increase in the number of cases of primary and secondary syphilis diagnosed in gay men in San Francisco in recent years. Surveillance data for other sexually transmitted infections and behavioural studies also suggest an increase in risky sexual behaviour amongst gay men.

Using a retrospective review of public health records, investigators identified a total of 52 cases of primary or secondary syphilis in HIV-positive men in San Francisco between early 2001 and Spring 2003. The men had a median age of 36 years.

In total, 35 men (67%) had secondary syphilis, and 30 (58%) were taking highly active antiretroviral therapy (HAART) at the time of their syphilis diagnosis. The overwhelming majority of men received between one and three injections of benzathine penicillin to treat their syphilis and this therapy was successful in all but two patients.

A total of 36 men had their HIV viral load measured before and during their infection with syphilis. The investigators noted that during syphilis infection viral load in these men increased significantly by a mean 0.21 log₁₀ (p = 0.02). In men with secondary syphilis viral load increased by a mean of 0.33 log₁₀, and amongst men not taking HAART the mean increase in viral load was 0.25 log₁₀. Among the ten men who had secondary syphilis and were not taking HAART, viral load was increased by a mean of 0.34 log₁₀ during syphilis infection (p = 0.05).

Of the 26 men who had a detectable viral load before infection with syphilis, 13 experienced an increase in their viral load during infection with syphilis, and twelve of these men had an increase of 0.50 log₁₀. However, of the ten men taking HAART who had an undetectable viral load before syphilis was diagnosed, only two men had a detectable viral load during infection with syphilis.

For 35 men, data were available on viral load during and after infection with syphilis. Amongst these men, viral load did not decrease significantly after syphilis was treated (mean fall in viral load 0.10 log₁₀).

In further analysis, the investigators looked at viral load in men for whom data were available on viral load before, during, and after syphilis. The mean increase in viral load was 0.10 log₁₀ during infection with syphilis, and viral load fell by 0.3 log₁₀ after syphilis was treated. Viral load increased by a mean of 0.007 log₁₀ after syphilis compared to the level of viral load before syphilis infection. None of these changes in viral load was statistically significant.

The investigators also looked at the effect of syphilis infection on CD4 cell count. Amongst the 31 men who had before and during CD4 cell measurements available, the investigators noted a mean drop in CD4 cell count of 62 cells/mm³ (p = 0.04). The decreases in CD4 cell count were most notable in men who had secondary syphilis and were not taking HAART. Although CD4 cell count increased by a mean of 33 cells/mm³ after syphilis was treated, this change was not statistically significant (p = 0.23).

For 15 men, data on CD4 cell count before, during and after syphilis infection were available. No significant changes in CD4 cell count were seen in this subset of individuals.

“Syphilis infection was associated with a significant increase in the plasma viral load and a significant decrease in CD4 cell counts in HIV-infected men,” write the investigators. They point out that these changes were most notable in men with secondary infection, which could be the result of greater immune activation.

The investigators admit that their study has several limitations, not least that data on CD4 cell count and viral load before, during and after syphilis infection were available for only 19 men. The investigators call for larger confirmatory studies.

“Syphilis infection in HIV-infected men was associated with a significant increase in viral load and a significant decrease in CD4 cell count...Integrated public health efforts to prevent new syphilis infections [are required]…to reduce the spread of both diseases within affected communities,” conclude the investigators.