Background Concomitant syphilis and human immunodeficiency virus (HIV) infection is increasingly frequent in industrialised countries. We examined the effect of active syphilis on CD4 counts and plasma HIV-1 viral loads

Methods All patients with syphilis-HIV coinfection treated at the Sheba medical centre between 2007 and 2012 were included in the study. Patients were divided to those with early or secondary syphilis and to those with latent syphilis (early and late). CD4 cell counts and plasma HIV-1 viral loads were measured before, during and following syphilis treatment.

Results 17 patients were included: all were men having sex with men, 11 (65%) were treated with ART: 5 with primary syphilis, 4 patients with secondary syphilis, 2 patient with early latent and 6 patients with latent syphilis of unknown duration (rpr > 1:32), one of them with neurosyphilis. Median CD4 cell count significantly dropped during syphilis by 25%: from 575 (246–828) before syphilis diagnosis to 415 (195–630) cells/mm3 during syphilis (p = 0.0002). After penicillin treatment it rose back to 621 (250–1379) (p = 0.01).

Plasma VL in 6 of the patients that did not receive ART increased during syphilis, although this rise was not statistically significant.

Conclusions Syphilis was associated with a transient decrease in the CD4 cell count and with an increase in VL in HIV-infected men; This increase in VL, although statistically non sugnificant, may explain at least partially the increased risk of HIV transmission in HIV patients not treated by ART that are co infected with syphilis.