Causative agent of giardiosis, lambliosis
■ Giardia intestinalis (syn. Giardia lamblia, G. duodenalis), a parasite of worldwide distribution, occurs also in Europe with relatively high frequency. It is a parasite of the small intestine of humans that can cause enteritis. Infection occurs by peroral ingestion of Giardia cysts. Various species of mammalian animals are reservoir hosts. ■
Occurrence. G. intestinalis has a worldwide distribution with prevalence rates of 2-5% in industrialized countries and very high rates, up to 50%, in developing countries. Children up to the age of five are frequently infected.
Parasite and life cycle. Giardia exists in two morphological forms: a motile vegetative stage, the trophozoite, and a cyst stage. The trophozoites live on the small intestine mucosa (less frequently on the gallbladder mucosa as well). They resemble a pear split lengthwise, are 9-21 im long and 5-12 im wide and possess eight flagella, two nuclei—one on each side of the longitudinal axis—and two claw-shaped median bodies (Figs. 9.1 and 9.11a). Their dorsal side is convex, the anterior part of the ventral side forms a concave adhesive disk. Reproduction is by means of longitudinal binary fission of the trophozoites, which are able to produce variant specific surface proteins. G. intestinalis produces oval cysts (8-18 x 7-10 im) with four nuclei, flagella, and claw-shaped median bodies. The cysts (and, less frequently, trophozoites) are excreted in stool. Fig. 9.1 illustrates the life cycle of G. intestinalis.
Epidemiology. The genus Giardia includes several species (G. intestinalis, G. muris, G. agilis, etc.) that show morphological, biological, and genetic differences. Giardia isolates obtained from humans and various species of
— Giardia intestinalis: Life Cycle
domestic and wild mammals are morphologically uniform and correspond to G. intestinalis. However, this is a genetically heterogeneous species, i.e., it comprises a number of different genotypes that can be differentiated by means of isoenzymatic and DNA analysis. Several identical genotypes
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