Zoonotic intestinal parasites are prevalent around the world. One of the most common and studied is transmitted by the microscopic protozoan, Giardia intestinalis, also known as G. lamblia and G. duodenalis (Ivanov, 2010). This eukaryotic parasite is responsible for the transmission of the most common waterborne disease, giardiasis, which causes watery diarrhea, vomiting, and has even been linked to irritable bowl syndrome (Ankarklev et. al. 2010). Since its discovery in 1681, Giardia has been the focus of various research in humans and animals, focusing on its unique, yet simple characteristics, life cycle, and transmission pathways (Ali and Hill, 2003). This research has since identified tests and treatments for giardiasis (Rishniw et. al. 2010). Giardia is an important organism for studying the various pathogens which surround us and giving insights into new molecular mechanisms(Ankarklev et. al. 2010). History
In 1681, Antoine Van Leeuwenhock first discovered Giardia when examining his own stool sample under the newly discovered microscope (Thompson, 2008). It was not until 1859 when Lambl was able to describe the organisms morphological characteristics for which we identify as Giardia today (Ivanov, 2010). The name Giardia was first brought to light in 1882 by Kunstler for a flagellate found in tadpole intestine. Later in 1888, Blanchard suggested that the parasite should be named Lamblia, after Lambl whom described it. The names were later combined to form the genus, Giardia, and species, Lamblia (Thompson and Morris, 2011).
The genus, Giardia, belongs to the order Diplomonadida and family Hexamitidea (Thompson and Morris, 2011), which are binucleated flagellates found in anaerobic or microaerophilic environments (Ankarklev et. al. 2010). Within this genus are five species which are characterized based on host specificity and morphology: G. dueodenalis, G. agilis, G. muris, G. ardeae, and G. psittaci. The only species capable of parasitizing humans is G. dueodenalis (Ivanov, 2010; Thompson, 2008). Based on allozyme electrophoresis, scientists have been able to genotype Giardia and further differentiation of these genotypes has been established by polymerase chain reaction techniques (Thompson, 2008). G. duodenalis was determined to contain eight genotypes which are similar in morphology but differ with host specificity: A, B, C, D, E, F, G, and one group that is unnamed. Within these genotypes are subgroups, for example genotype A contains two subgroups, AⅠ and AII. AI is comprised of humans and animals that are closely related and has high zoonotic potential while BⅣ is human specific. There have been cases of a mix of subgroups B and C where close inhabitants, canines and humans, were infected (Ivanov, 2010).
Eukaryotic organisms are complex and must contain a nuclei and membrane bound organelles. With two nuclei, an endomembrane system, and a complex cytoskeleton, Giardia fits these requirements. However, by lacking typical higher eukaryotic organelles, mitochondria, peroxisomes, and the Golgi apparatus (Lujan and Touz, 2003), some refer to Giardia as the “link” between prokaryotes and eukaryotes (Ankarklev et. al. 2010). The organism’s parasitic lifestyle is thought to be the cause of the lack of complex organelles (Lujan and Touz, 2003). The simplicity of this parasite is an important characteristic for researchers when studying eukaryotic organisms by allowing the cell cycle to be studied outside of the host (Ankarklev et. al. 2010).
Giardia can be observed in two life forms, trophozoite and cyst. The trophozoite, which is the motile and never seen outside of the host, is characterized by two diploid nuclei, four pairs of flagella, and a unique organelle, the adhesive disc (Thompson and Morris, 2011). It is known for its teardrop shape and is approximately 15 μm long (Ali and Hill, 2003). The cyst, the infectious form, is non-motile, oval shaped,...
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