Giardia is the first protozoan parasite of man discovered as early as 1681 by Antony van Leeuwenhoek and described later by Lambl in 1859. It is a member of order Diplomonadida belonging to class Zoomastigophorea. The taxonomic name of the organism has been subjected, during the past years, to many changes. As many as seven different binomial labels of the parasite can be traced in the past. Giardia intestinalis and G. lamblia used currently are synonymous names for the human parasite, which, together with parasites of wild and domestic mammals, belong to G. duodenalis morphological group (Filice 1952).

The recent research work provides data on Giardia being in the position of a 'missing link' between prokaryotes and eukaryotes and data indicating genetic heterogeneity of isolates suggesting that a human parasite is not a single species but may represent a species complex (Andrews et al. 1989; Kabnik and Peattie 1991).

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