Morphological identification markers for distinguishing avian from mammalian Giardia species -: do they need reconsideration?

The intestinal, parasitic protozoa comprising the genus Giardia are major aetiological agents of diarrhoeal disease in numerous vertebrates, including humans, livestock and domestic pets. In previous investigations, morphological characteristics, in particular caudal flagellar length and asymmetry, have been used to distinguish avian from mammalian Giardia species, with scanning electron microscopy (SEM) often the technique of choice for observation. In this study, however, we observed considerable variation in caudal flagellar length and asymmetry, irrespective of avian or mammalian host origin. In addition, SEM sample preparation and the age of trophozoite cultures caused increases in the number of trophozoites with caudal flagellar length anomalies. Our findings, therefore, suggest that caudal flagellar characteristics are not reliable morphological markers for distinguishing Giardia species. In addition, the novel use of micromanipulation in establishing sub-clonal cultures of Giardia from single trophozoites was a highly accurate and successful microscopic technique.