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of treatment failure is due to genuinely resistant isolates. Careful measurement of sensitivity in vitro, using specified media and controlled oxygen tension, is necessary to obtain consistent results (Ackers, 1995) but a high level of resistance can be shown in some isolates. The mechanism of this resistance is, to some extent, understood (Edwards,
1993). Most such cases can be managed by increased and repeated doses but if that fails, no really effective alternative drugs are available, although occasional successes with a variety of preparations have been reported (Table 11.4) and discussed in a recent article (Lewis et al., 1997).
PREVENTION AND CONTROL
As with all the sexually transmitted infections, prevention requires changes in the way that people behave. Since the onset of the AIDS epidemic some 15 years ago, a growing barrage of advice and exhortation to practise ‘safe sex’ has filled the world’s media, and it seems more than a coincidence that the incidence of trichomoniasis, which had barely changed during the
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
1970s, has, since then, declined precipitately in many countries and populations.
Control of trichomoniasis requires accessible, affordable and high-quality health care, as well as health promotion; this is by no means easy or cheap to provide. Whilst most new resources in this field are, naturally, targeted at controlling the spread of HIV, there is now ample evidence that controlling other STIs is one of the most
cost-effective ways of doing this (Grosskurth et al., 1995). The availability of accurate and affordable diagnostic methods for trichomoniasis, whose symptoms are not specific enough to make syndromic management very useful, would be a major advance, since treatment is reasonably cheap and highly effective. Widespread implementation could have a very significant effect in slowing the spread of HIV.
OTHER HUMAN TRICHOMONADS
Both Trichomonas tenax and Pentatrichomonas hominis normally receive only brief mention in medical texts—they are of doubtful pathogenicity and therefore little studied, so there is not much to say. Dientamoeba fragilis is rather better known, as many people regard it as a genuine, if far from invariable, pathogen—but again, it attracts far less interest than, say, Entamoeba histolytica.
The same pattern will also be found true of this review. However, for the late Professor Honigberg, any trichomonad was of absorbing interest and any reader wanting to know much, much more about one of these three organisms can be confidently referred to the relevant chapters of the monograph which he edited (Honigberg, 1989; Ockert, 1989).
TRICHOMONAS TENAX (O. F. MULLER)
Dobell has stated that T. tenax was first seen in 1773 by Muller, who named it Cercomonas tenax. The name Trichomonas tenax is now universally employed, although the same organism has in the past been referred to as T. buccalis or T. elongata.
DESCRIPTION OF THE ORGANISM
T. tenax differs from T. vaginalis and P. hominis in being smaller—4-13 ^ (mean 7.1 ^) long x 29 ^ (mean 4.7 ^) wide—and from P. hominis in having only four anterior flagella and a recurrent flagellum that does not extend beyond the end of the undulating membrane (Figure 11.1). The axostyle is slender and appears to extend beyond the body. In humans it is found in the oral cavity, particularly in the periodontal crevices; it has also been recovered from the submaxillary glands, sinus cavities, the ear and the throat. Accounts of trichomonads, normally assumed to be T. tenax, causing respiratory tract pathology are controversial (see Ectopic Infections, below).
T. tenax may be cultivated in bacteria-containing media without undue difficulty, and such cultures are useful diagnostically, but axenization is more difficult than with the other human trichomo-nads. Originally, a chick embryo extract was regarded as essential but it was subsequently found that Diamond’s TPS-1 medium was suitable if modified to contain 0.01% agar. Ultrastructural studies show typical trichomonad features, including hydrogenosomes and ingested bacteria.
T. tenax is normally regarded as a non-pathogen that merely flourishes in the presence of already diseased tissue, but a number of workers in Eastern Europe and the Former Soviet Union regard it as causally linked to periodontal disease. Despite the detection of collagen-degrading secreted proteinases, neither adherence nor perceptible damage occurred when mammalian cell monolayers were exposed to the parasite in vitro.
One group of patients with gingivitis and T. tenax infection has been shown to produce specific serum antibodies, which faded away after successful treatment; this result clearly shows that parasite antigens can reach the immune system through damaged oral tissue but does not bear on the question of pathogenicity. Other immunological studies have concentrated on antigenic differences between isolates, including those recovered from the lung; the results have been admirably summarized by Honigberg (1978, 1989).