Giardia: overview and update.

Giardia is a protozoan flagellate that was first observed by chymotrypsin and trypsin, producing two trophozoites (vegetative stage) from each cyst [6, 7]. The trophozoites replicate in Van Leeuwenhoek in 1681 and more fully described by Lamb in 1859. It was initially thought to be a commensal in humans, the crypts of the duodenum and upper jejunum and reproduce asexually by binary fission. Some of the trophozoites then enbut it is now clearly recognized as a common cause of diarrhea and malabsorption. Giardia infects millions of people throughcyst in the ileum, possibly as a result of exposure to bile salts or from cholesterol starvation [8, 9] (figure 1). out the world in both epidemic and sporadic forms. It is transmitted through ingestion of contaminated water and food, The cysts can be round or oval and measure 11–14 1 7–10 mm (figure 2A). They each have four nuclei and contain person-to-person contact in child care centers, and male homosexual activity. axonemes and median bodies. The trophozoites measure 10–20 mm in length by 5–15 mm in width and have the shape of a teardrop when viewed from the dorsal or ventral aspects The Life Cycle (figure 2B). There is a ventral, concave sucking disk bearing four pairs of flagella, two axonemes, and two median bodies. Morphological classification schemes have placed Giardia in the Phylum Zoomastigophora, Class Zoomastigophorea, and The trophozoites have two nuclei that are identical by all criteria that have been studied and that are both transcriptionally Order Diplomonadida. More recently, Sogin et al. [1] placed Giardia as one of the most primitive eukaryotic organisms (by active. Giardia contain five chromosomes and are polyploid. Mitochondria, peroxisomes, smooth endoplasmic reticulum, means of molecular classification with use of small subunit rRNA). Three species of Giardia have been described on the and nucleoli have not been identified, consistent with the suggestion that Giardia is a primitive eukaryote [10]. The ventral basis of differences discernible by light microscopy; these species are G. agilis from amphibians; G. muris from rodents, birds disk acts as a suction cup, allowing mechanical attachment to the surface of the intestine (figure 3). and reptiles; and G. lamblia (also called G. intestinalis or G. duodenalis) from various mammals. Two additional species that are indistinguishable from G. lamblia by light microscopy, G. ardeae (from herons) and G. psittaci (from psittacine birds), Epidemiology have been identified on the basis of morphological differences Infections may result from the ingestion of 10 or fewer observed on electron microscopic examination [2, 3]. G. lamblia Giardia cysts [11]. Under favorable conditions of temperature is found in domestic animals such as cats and dogs, as well as and humidity, such as water at 4–107C, the cysts may remain a variety of wild animals including beavers, which have been viable for several months [12]. The cysts are relatively resistant implicated in waterborne outbreaks of giardiasis [4, 5]. to chlorination and to disinfection by ultraviolet light. Boiling The life cycle of Giardia is composed of two stages: the is very effective for inactivating Giardia cysts, but some cysts trophozoite and the cyst. The cyst is the infectious form of this may survive after freezing for a few days. Most human infecprotozoan and is relatively inert and environmentally resistant. tions result from ingestion of contaminated water or by direct After ingestion, excystation occurs in the duodenum as a result fecal-oral transmission, such as that occurring in child care of exposure to the acidic gastric pH and the pancreatic enzymes centers. Less commonly, transmission may occur via food contaminated by food handlers. Male homosexual sexual contact is another route of transmission [6]. Received 7 April 1997; revised 30 May 1997. G. lamblia is found worldwide and is especially common in Grant support: this work was supported in part by the National Institute areas where poor sanitary conditions and insufficient water of Allergy and Infectious Diseases—International Cooperation in Infectious treatment facilities prevail. Seasonality of giardiasis has been Diseases Research (AI35894). Reprints or correspondence: Dr. Ynes R. Ortega, Department of Veterinary reported, with a peak incidence during late summer in the Science and Microbiology, University of Arizona, Tucson, Arizona 85721. United Kingdom, the United States, and Mexico, but no seaClinical Infectious Diseases 1997;25:545–50 sonal pattern has been observed in day care situations. The q 1997 by The University of Chicago. All rights reserved. 1058–4838/97/2503–0001$03.00 prevalence of Giardia in stool specimens submitted for ova