The history of giardiasis.

This review paper outlines the history of giardiasis caused by Giardia duodenalis. Discovered in 1681, G. duodenalis is probably the first protozoan parasite ever observed by a human. Despite the early discovery, the taxonomic status of the protozoan remained uncertain for a long time. The reason for this is that G. duodenalis is a species complex comprising several phylogenetically distinct genotypes termed assemblages. Initially, it was thought that G. duodenalis is a primitive eukaryote because it lacks many subcellular organelles. However, recent research has shown that the protozoan has diverged from higher eukaryotes and that the lack of subcellular organelles is due to secondary loss and reduction. Based on paleoparasitological findings, G. duodenalis has parasitised humans since antiquity and has probably been spread globally by human migratory activity. Interestingly, it was not until 1987 that the pathogenicity of G. duodenalis was demonstrated for humans. Giardiasis is still a public health problem in the 21st century, particularly in young children living in areas with low hygiene standards.