Читать книгу Parasitology - Alan Gunn - Страница 60

The taxonomy of Giardia spp. is an interesting illustration of the insights provided by molecular phylogenetics. Despite the morphological similarities between Giardia spp. and other parasitic flagellates (e.g., Trichomonas spp., Trypanosoma spp.), the giardias represent much more primitive organisms. Within the genus Giardia, allocating isolates into species groups is very difficult. The first attempts were based on apparent host specificity since cysts are morphologically identical and attempts at in vitro culture to produce trophozoites was often unsuccessful. Subsequently, improvements in culture methods and microscope resolution enabled the examination of trophozoites. On this basis, there are six species of Giardia, but only one of them infects humans. For historical reasons, the species infecting humans is called Giardia duodenalis in Europe and Australia, the North Americans favour Giardia lamblia, while some authors refer to it as Giardia intestinalis. These species names are synonyms, and there is no ‘correct’ one; the only important issue is that locally, clinicians and scientists use the same name.

Numerous mammal species harbour Giardia spp. Organisation of the Giardia into groups and strains within those groups is based predominantly on genetic sequences (Thompson and Ash 2019). Unfortunately, many of the published studies involve characterisation and genetic sequencing of small numbers of isolates – in some cases only one parasite from one animal. This is a serious limitation, and the area of Giardia taxonomy clearly requires further work.