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Speculative models of zoonoses caused by trichomonads.Trichomonads are listed on the right and colored according to primary hosts assigned historically in the literature. Unbroken lines represent known infections or transmission routes, and broken lines represent speculative infections or transmission routes for which data are lacking. The relationships are represented as follows: (blue box) trichomonads identified in wild bird species, in partially domesticated species (rock dove) and in fully domesticated species (chicken) circulate within these populations with variable host specificity (blue unbroken circle with arrow). Two of the four avian trichomonads listed (Tetratrichomonas sp. and Tetratrichomonas gallinarum) have been identified in human lungs, and Trichomonas gallinae and Trichomonas stableri are also included owing to their close relationship to Trichomonas tenax and Trichomonas vaginalis. (Red box) T. vaginalis and T. tenax are the two species considered human-specific, with known human-to-human infections (unbroken red circle). The close genetic relationship of the human and avian trichomonads suggests either independent zoonotic acquisitions from avian sources (broken blue arrow) or transfer of the parasites from humans to birds through environmental contamination (broken red arrow). (Green box) Tritrichomonas foetus has been isolated from a variety of pets and farm animals, with the same strain known to infect cattle and pigs (unbroken green arrow), but different genotypes infecting cattle and cats; the origins of dog infections remain unclear. Thus, there are at least two T. foetus genotypes capable of colonizing an extensive range of hosts, including humans [41] (broken green circle and arrow). The lack of precise epidemiological data is indicated by ‘?’. (Purple box) Pentatrichomonas hominis has been isolated from a variety of pets and farm animals [22], but little is known about its infection route and epidemiology; the same strain could be circulating between all identified hosts (broken purple circle). (Orange box) Dientamoeba fragilis has been isolated from farm animals (pigs) and non-human primates (gorillas), with the same strain known to infect pigs and humans (unbroken orange arrow). Recent evidence suggests that household pets do not play a role in transmission [82]; however, the origins remain unclear and multiple strains could be circulating in animal hosts (broken orange circle and arrow). Additionally, given recent prevalence and transmission data it seems unlikely that transmission from non-human hosts represents a significant proportion of infections. Contaminated surfaces and water, uncooked meat, or direct contact with pets and farm animals could lead to animal-to-human transmissions of trichomonads. Initial infections were presumably through the digestive tract (via oral ingestion) with further progression to the lungs for some (various) species or the urogenital tract (T. vaginalis).
