**2.** *Giardia* **generalities**

*Giardia* is one of the most primitive eukaryotic protozoa initially described by Leeuwenhoek in 1681; its taxonomy was confusing and complicated throughout the first half of the twentieth century. The name *Giardia lamblia* was well-known in the 1970s, but it was consolidated and changed to *Giardia duodenalis* or *Giardia intestinalis* in the 1990s. It is classified within the class Zoomastigophorea in the order Diplomaida and family Hexamitidae. Historically, 41 species have been described based on their hosts. To date, eight of these species have been detected in mammals: *duodenalis, enterica, canis, bovis, muris, cati, simondi*, and *microti* (**Table 1**) [6–9].

*Giardia* exists in two forms: an active form called a trophozoite and an inactive form called a cyst. The cyst measures 5 × 7 to 10 μm in diameter, containing four nuclei covered by a wall from


**Table 1.** Giardia species.

0.3 to 0.5 μm in thickness; it is composed of an exterior filament layer formed by glycoproteins and an internal membrane layer that makes it very resistant providing an environmentally stable life cycle; it helps withstanding long periods in water at temperatures less than 25°C, and it even makes it invulnerable to chlorination processes. Trophozoites are pear-shaped and are approximately 12–15 mm in length and 5–9 mm in width with a cytoskeleton that includes a medium size body, four pairs of flagella, and a ventral disk; they have two nuclei without nucleoli in its interior, which are found in front and symmetrically located [10, 11].

The vital cycle of *Giardia* starts with the ingestion of cysts by the host whether found in food or water contaminated with feces of the infection carriers; once consumed, the cyst enters in contact with the gastric acid destroying itself and excystation occurs and trophozoites are released, which pass through the upper small intestine infecting the duodenum and the upper part of the intestine where they are reproduced by binary fission adhering to the intestinal epithelium surface and triggering symptomatology. The adaptation mechanism of *Giardia* known as encystment is essential to the parasite for their survival once out of the intestine of the host since trophozoites are extremely sensitive to changes in temperature, humidity, and the presence of chemical agents. In this process, trophozoites descend through the host intestine, and when they find a cholesterol-poor environment, their differentiation to cysts is induced and eliminated with feces. It has been reported that infected persons excrete from 108 to 109 cysts in only one evacuation and can continue discarding them from 50 days and including months later after diarrhea has subsided [10, 12].

For the parasite to survive within the host and avoid the immune response, *Giardia* shows what is known as antigenic variation that allows it to elude the immunologic system and produce chronic and recurrent infection. Giardiasis symptoms in human beings show variable degrees. Acute giardiasis shows acute diarrhea and urticaria or it can show itself asymptomatically; the acute form is usually self-limiting lasting for 2–7 days; it is also possible to evolve to a subacute or chronic stage lasting from months to years; malabsorption results as the most frequent and harmful complication difficult to solve from a therapeutic point of view because it causes malnutrition and low weight [13–15].
