The Biology of Giardia Parasites

Abstract

Giardia intestinalis (syn. G. lamblia or G. duodenalis) is one of the ten major enteric parasites affecting humans worldwide. It is also considered the most common intestinal pathogenic protozoa of humans and is recognized as a recurrent parasite of other nonhuman species, including cattle, beavers, and domestic dogs. Even though Giardia was first observed by Van Leeuwenhoek in 1681, in the past it was debated whether Giardia was a pathogen. However, now it is accepted that Giardia can cause intestinal disease in humans and in a wide range of domestic and wild mammals. The clinical presentations of giardiasis range from an asymptomatic cyst excreting state to diarrhea, which can be acute, chronic, or intermittent (Karanis et al., 1996; Kirkpatrick and Benson, 1987; Monzingo and Hibler, 1987; Nizeyi et al., 1999; Olson et al., 1995; Pacha et al., 1985; Patton and Rabinowitz, 1994; Rickard et al., 1999; Sulaiman et al., 2003; Thompson et al., 2000; Wallis et al., 1984; Xiao, 1994). Recently, Giardia infections have been associated with growth faltering due to nutrient malabsorption (Berkman et al., 2002). Giardia intestinalis is a parasite of public health importance as it can be transmitted through several routes, including water (drinking as well as recreational) and fresh food products (Nichols, 2000). Since foodborne outbreaks occur more frequently on a smaller scale than waterborne outbreaks, they are identified less frequently. Nonetheless, there are well-documented outbreaks implicating G. intestinalis as the causative agent (Anonymous, 1989; Mintz et al., 1993). In developed countries, Giardia is currently referred as a reemerging infectious agent because of its increasing role in outbreaks of diarrhea in day-care centers, and water and foodborne outbreaks affecting the general population. However, in developing countries located in Asia, Africa, and Latin America, approximately 200 million people per year experience symptomatic giardiasis (Thompson et al., 2000). There are multiple recognized species of the genus Giardia, although only G. intestinalis is found to be pathogenic to humans. Recent genetic studies have identified distinct groups within this species (Andrews et al., 1989; Meloni et al., 1995), and several researchers now regard G. intestinalis as a species-complex (Andrews et al., 1989; Ey et al., 1997; Monis et al., 1998) (Table 2.1). Furthermore, the zoonotic potential of some animal isolates of G. intestinalis has recently been suggested (Sulaiman et al., 2003). These findings plus their impact on the public health emphasize the importance to understand the biology, epidemiology, transmission, control, and treatment of G. intestinalis parasites.