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7 approx.

*Data from Quinn (1994). "Data from WHO (1995). ND, no data included.

*Data from Quinn (1994). "Data from WHO (1995). ND, no data included.

Transmission

Forty years ago a review of the epidemiological evidence concluded that the vast majority of cases of trichomoniasis were acquired through sexual contact (Whittington, 1957) and nothing discovered since has seriously challenged that conclusion. Four lines of evidence support the classification of trichomoniasis as an almost exclusively sexually-acquired infection: (a) in most surveys, the prevalence is highest in the age groups corresponding to the period of maximum sexual activity and is much lower before puberty or after the menopause; (b) the prevalence is much higher in populations (such as STI clinic attenders) with a high prevalence of other STIs than in the general population (to the extent that such data are available); (c) although symptomatic trichomoniasis in males is uncommon, T. vaginalis can be recovered from the urethra and/or the prostate of a highly significant proportion of the male contacts of women with trichomoniasis; and (d) the parasite dies rapidly when dried or exposed to high or low temperatures (no resistant cyst is formed by T. vaginalis). Use of both male and female condoms reduces the risk of transmission. Mechanical transfer between sexual partners on fingers, vibrators or sex toys is possible and has been recorded. The number of cases of gonorrhoea and tricho-moniasis in women in England and Wales has declined in parallel over a 15 year period (Figure 11.3), suggesting that whatever has caused this drop is acting on similar epidemio-logical situations.

Most epidemiological studies conducted in the USA have shown a higher prevalence of tricho-moniasis in Black patients than in members of other racial groups (Cotch et al., 1991); this

Fig. 11.3 Number of cases of trichomoniasis and gonorrhoea in women, reported in England and Wales, 1979-1999

difference is usually attributed to socioeconomic and behavioural factors but an enhanced susceptibility due to a generally higher vaginal pH has also been suggested (Stevens Simon et al., 1994). For unknown reasons, the prevalence of trichomoniasis in women seems to decline more slowly with age than other STIs, such as gonorrhoea; it is possible that this phenomenon is caused by long-lasting asymptomatic infections that subsequently recrudesce. If such cases are common, they could have major epidemiological implications as reservoirs of infection.

Transmission of T. vaginalis without Deliberate Sexual Contact

Despite the undoubted overwhelming importance of sexual contact in the epidemiology of trichomoniasis, because T. vaginalis can survive for a surprisingly long time outside the body if kept moist (Table 11.2), the possibility of transmission via toilet seats, shared sponges or towels, communal bathing or living under poor and overcrowded conditions has been raised. Furthermore, Whittington (1957) showed that on four of 38 occasions T. vaginalis could be cultivated from toilet seats after use by women with trichomoniasis. Nevertheless, while admitting the theoretical possibility, she and most other authors regard transmission without sexual contact as very unlikely in practice.

Because trichomonads are found in both the mouth (T. tenax) and in the large bowel (P. hominis), it is natural to wonder if either organism could survive in the genitourinary tract if introduced there. There is very good evidence, however (summarized by Wenrich 1947), that each species is strictly site-specific.

Rosedale (1977), impressed by a number of infected women who were members of apparently strictly monogamous couples, felt that the label 'sexually transmitted' was unwarranted and caused unnecessary distress to his patients; in addition, he did not treat their male sexual partners and still obtained cure rates as good as other clinics which did. The author did not speculate as to how his female patients did become infected, but it has been postulated that chronic and asymptomatic infections could exist from birth (see below) and be provoked into clinical disease by the psychological and hormonal changes accompanying the start of regular sexual intercourse. Apart from any other objections, this interesting idea would seem to founder

Table 11.2 Survival of T. vaginalis outside the human body under various conditions

Material

Temperature (°C)

Variable T. vaginalis organisms still present after:

Reference

Vaginal exudate

10 approx.

Up to 48 hours (1/11 samples)

Whittington, 1951

Urine + vaginal secretion

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