Treponema pallidum bacteraemia carries a risk of transmitting syphilis through transfusion. Australian Red Cross Lifeblood (Lifeblood) manages this risk using donor screening and serological testing in combination with modern blood processing practices. A recent risk model was based on an estimate that ~12% of seropositive donors, with potentially infectious syphilis, may be bacteraemic at time of donation. As it is difficult to culture T. pallidum, and serology test results do not necessarily equate to infectivity, nucleic acid detection could be used for detection of the pathogen in blood as an indication of potential infectivity. The aim of this study was to investigate whether T. pallidum nucleic acid was detectable in serologically syphilis reactive donations and pre-index donations.
Blood samples from 2016-2019 (n=58) were retrieved from the Lifeblood archive. These blood donations were selected from higher risk donors who serologically tested positive for syphilis and were considered potentially infectious. We also included any previous donations within 2 years prior to the index donation. DNA was extracted and tested by quantitative real time PCR for two conserved species-specific regions, pol-A and 16S. Additionally, amplicon sequencing for the 23S region was carried out.
T. pallidum pol-A DNA was amplified from 7 of 58 retrieved plasma samples, 2 of those 7 positive results were from the same donor. Four of those pol-A positive results were also positive for T. pallidum 16S DNA. As part of the extended genotyping investigation, T. pallidum-specific 23S regions were also detected in 3 of the pol-A and 16S positive samples.
This study demonstrated a T. pallidum nucleic acid prevalence in plasma from seropositive blood donors, with potentially infectious syphilis, was 12%. This number is concordant with the input data used in a previous risk model estimation for syphilis. As a result of donor screening in combination with serology, these donations were discarded. Current risk management protocols ensure that the risk of syphilis transfusion transmission is negligible.