Diagnosic Dilemma No. 17 

Contributor: Dr Richard Bradbury, University of Tasmania

The following protozoa were noted in a modified iron-haematoxylin stained, sodium acetate formalin preserved faecal specimen submitted by an African refugee who had recently relocated to Tasmania. The trophozoites measured between 5-15 um long by 7-10 um wide. The patient was without symptoms.
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Fig. 17.1 - Torphozoites seen in faeces (modified iron-haematoxylin)

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The protozoa identified was Pentatrichomonas hominis (formerly Trichomonas hominis), a commensal protozoa of the human gastrointestinal tract. This protozoan may be found worldwide, and is common in parts of Northern and central Australia. Rates of infection vary depending on geographical region, though high rates of infection are found in central Australia. A zoonotic reservoir exists, the organism having been identified in dogs, cats, primates and guinea pigs (Wenrich 1944).

The organism has a distinctive jerky motility in saline preparations, often said to resemble a "man trapped inside a giant plastic bag". No cyst stage exists, and P. hominis trophozoites will quickly degenerate outside of the host, making identfication in stained slides difficult (Fig. 17.2). The organism may be visualised using standard parasite stains and Giemsa stain. Distinctive points for the identification of P. hominis and differentitation from other intestinal protozoa are the long, stylus shaped, axostyle and distinctive round nucleus (Bradbury, et al. 2010).

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Fig. 17.2 - Degenerate Pentatrichomonas hominis in faeces (May-Grunwald Giemsa stain)

Studies in the 1940's found that when P. hominis was implanted in the vagina, it did not survive beyond 48 hours (Stabler and Feo 1942), however, a recent study using PCR techniques identified two cases of P. hominis colonising the vagina in Tanzania (Crucitti, et al. 2004). This most probably represents faecal contamination of the vagina with P. hominis. The 1942 study was conducted on healthy volunteers, and it is possible that as yet unknown factors relating to hormone levels and vaginal flora may play a role in maintenance of vaginal P. hominis colonisation. These findings have raised concerns that P. hominis may be incorrectly identified as Trichomonas vaginalis in a child, leading to incorrect acccusations of sexual abuse. Points of morphological differentiation between P. hominis and T. vaginalis are that the undulating membrane of the former extends the entire length of the body of the trophozoite and the recurrent flagellum trails posteriorly. However, it must be emphasised that in any such case, identification should be confirmed by molecular means at a reference facility.

Although it is considered not to be an intestinal pathogen, P. hominis has been implicated as a cause of diarrhoea (Chunge et al. 1991, Okunsanya et al. 1994). Whilst the organism may be seen in large numbers in diarrhoeic stool, this may simply represent evacuation of protozoa from the small intestine with increased intestinal motility rather than aetiological cause of th diarrhoea. Rare ectopic infections have been observed in liver abscesses (Jakobsen et al. 1994) and in the pleural effusion of a patient with systemic lupus erythamatosis (Jongwutiwes et al. 2000).

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