When an STI doesn’t behave like an STI: the case of Trichomonas

The second conference issue to hit the headlines arises from a study led by Charlotte Gaydos of John Hopkins University investigating the prevalence in the US of the parasite trichomonas vaginalis. Not only does overall prevalence prove to be twice what might have been expected (8.7% as opposed to 4%); more surprisingly still, age-specific prevalence shows a progression with age – quite contrary, of course, to what we see in chlamydia or gonorrhea – reaching a peak of 13% in women of over 50 years. These results seem to corroborate the incidental findings arising from a study of the Gen-Probe Aptima, reported in this blog (see Andrea and Chapin).

The Gaydos study is as yet unpublished. It is reported to be based on the re-analysis for Trichomonas of left-over samples – urine, cervical or vaginal swab, or liquid pap smears – obtained, presumably for other purposes, from 7,593 women across 27 US states between July and December 2010, attending a range of health settings including private clinics, emergency departments, hospitals, jails and community health STD clinics. The results, as reported, conform to general expections of an STI in certain respects but not others. Prevalence, though remarkably high for black women (20%, or three times the level for whites) correlates, according to Gaydos, with ethnicity and relative affluence in the manner that earlier studies, relating to chlamydia or gonorrhoea, might lead us to expect. On the other hand, age-specific prevalence is quite at odds with such expectations. Compared with the overall prevalence of 8.7%, rates are lower in younger age groups – 8.5% for 18-19 year olds and 8.3% for those in their twenties – but rise to 11% for the 40s age-group, and 13% for those aged 50 and above.

These discrepancies are most surprising. One response already canvassed in an earlier blog would be to question whether what we are seeing here is an STI at all – or is it time dust down those older theories of transmission via the water-closet (see J.A. Burgess)? At all events, the figures raise questions that can only be answered by a fuller account of the epidemiological issues surrounding transmission – to start with, the issue of potential sampling bias in the trial itself. We look forward eagerly to a fuller discussion of these issues in the published paper.

ScienceDaily, “Sexually Transmitted Parasite Trichomonas Vaginalis Twice as Prevalent in Women Over 40, Study Finds”, 12th July 2011

BMJ Group Blogs: Sexually Transmitted Infections, 23rd March 2011

Just supposing Trichomonas Vaginalis proved not to be an exclusively sexually transmitted infection after all …… ?

Sarah B. Andrea & Kimberle C. Chapin, “Comparison of Aptima Trichomonas vaginalis Transcription-Mediated Amplification Assay and BD Affirm VPIII for Detection of T. Vaginalis in Symptomatic Women: Performance Parameters and Epidemiological Implications”, Journal of Clinical Microbiology, March 2011, p.866-869, Vol.49, No. 3

J.A. Burgess, “Trichomonas Vaginalis Infection from Splashing in Water Closets”, British Journal of Venereal Disease, 1963;39:248-250 doi:10.1136/sti.39.4.248

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