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Principles and Practice of Clinical Parasitology. Edited by S. Gillespie & Richard D. Pearson Copyright © 2001 John Wiley & Sons Ltd Print ISBN 0-471-97729-2 Online ISBN 0-470-84250-4
David R. Hill
University of Connecticut School of Medicine, Farmington, CT, USA
Giardia lamblia, also known as G. duodenalis and G. intestinalis, is one of the most common intestinal protozoans throughout the world. In North America it is the most frequently isolated enteric parasite and in developing regions there is almost universal infection by the end of childhood. Humans infected with Giardia may have asymptomatic infections, acute diarrhea or chronic diarrhea with malabsorption, weight loss and failure to thrive.
Giardia was first described in 1681 by Antony van Leeuwenhoek, who wrote; ‘My excrement being so thin, I was... persuaded to examine it... wherein I have sometimes also seen animalcules a-moving very prettily; some of ’em bigger, others a bit less, than a blood-globule . . .; their bodies were somewhat longer than broad and their belly, which was flatlike, furnished with sundry little paws, wherewith they made such a stir in the clear medium...’ (Dobell, 1920). It was reported in more detail in 1859 by Lambl, after whom the human species was named (Lambl, 1859). However, it has only been in the last 30-40 years that its real role in diarrheal syndromes has been described. A large water-borne outbreak of giardiasis, which occurred during the winter season of 1965/1966 in Vail, Colorado, brought Giardia wide recognition (Moore et al., 1969) and the first major reviews of Giardia were published
in the 1970s (Petersen, 1972; Burke, 1975; Wolfe, 1975; Meyer and Radulescu, 1979). Since that time there has been extensive research describing the epidemiology and biology of Giardia. While much is known about this parasite, there is still much to learn, particularly about how Giardia causes diarrhea.
DESCRIPTION OF THE ORGANISM
Giardia is a flagellated, teardrop-shaped parasite which has only two life forms, the trophozoite and the cyst (Figures 10.1A,B). It belongs to the class Zoomastigophorea, the order Diplomona-dida and the family Hexamitidae (Meyer, 1990). It is one of the oldest eukaryotic organisms, based on the sequence analysis of its small 16S ribosomal RNA (Sogin et al., 1989). It also lacks many of the organelles typical of higher eukaryotes, such as mitochondria, peroxisomes and a typical Golgi apparatus (Adam, 1991; Gillin et al., 1996; Roger et al., 1998). Because of its early evolutionary status, the study of Giardia can yield valuable insight into eukaryotic development.
The trophozoite, which measures 9-21 pm in length x 5-15 pm in width (Figure 10.1A), contains four sets of posteriorly directed flagella, which aid in the parasite’s movement. The most prominent feature of the trophozoite is the ventral disk, which may help Giardia to attach
Principles and Practice of Clinical Parasitology
Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
Fig. 10.2 Scanning electron micrograph of the ventral surface of a Giardia lamblia trophozoite. The ventral adhesion disk and one pair of flagella are seen. Courtesy of S. L. Erlandsen, Department of Cell Biology and Neuroanatomy, University of Minnesota School of Medicine, Minneapolis, MN
to intestinal epithelial cells (Figure 10.2). This disk is composed of a tight, clockwise spiral of microtubules, bound together by microribbons (Feely et al., 1990; Adam, 1991; Thompson et al., 1993; Gillin et al., 1996; Upcroft and Upcroft,
1998). In the disk are the prominent antigens of tubulin within microtubules and giardins within the microribbons (Peattie, 1990; Marshall and Holberton, 1993). The microtubules are critical to the functioning of the disk, as well as to the
Fig. 10.1 (opposite) (A) A trophozoite and (B) a cyst are pictured in a trichrome stain of a stool sample. The teardrop shape and
two nuclei of the trophozoite with central karyosomes are readily apparent. The median body lies centrally. Trophozoites measure 5-15 pm wide and 9-21 pm long. In the cyst, the cytoplasm has separated from the smooth cyst wall. Centrally located axonemes, a transversely placed, claw-like median body, and two eccentrically located nuclei can be detected. Cysts measure 6-10 pm wide and 8-12 pm long