**2. Etiologic agent**

*G. lamblia* is a parasitic protozoan of the order *Retortomonadida* that alternates between trophozoites and cysts forms within its life cycle, stages responsible for the clinical illness, and the transmission of the disease, respectively. Under the light microscope, trophozoites appear actively swimming and with its characteristically teardrop (viewed dorsoventrally) or spoon (viewed from the side) shaped, measuring 10–20 μm by 5–15 μm by 2–4 μm, containing four pairs of flagella, two identical nuclei, with a convex dorsum and a ventral disc that acts as a suction cup to facilitate attachment of the organism to the small bowel villi (**Figure 1A**). On high-quality slides, the parasite movement shows "falling leave mobility" and resembles a human face because of the positions of the median bodies, nuclei, and axonemes. Uncharacteristically *Giardia* trophozoites lack definable Golgi, peroxisomes, and true mitochondria, but have a menant mitosome. Trophozoites divide by binary fission, and cyst develops as feces dehydrated in transiting to the large bowel.

Microscopically, *Giardia* cysts look oval shaped, measures about 11–14 by 7–10 μm, contains four nuclei (mature cyst)—usually situated at one end—and curved median bodies and linear axonemes (**Figure 1B** and **C**). During the process of encystment, which can be observed under the microscope, trophozoites initially become inactive, rounded, and increasingly refractile as encystment begins. Then, nuclear division (but not cytoplasmic) occurs to produce the quadrinucleate infectious cyst. *Giardia* cysts have a thick hyaline wall that protects them from environmental stressors such as the alkaline environment that characterize the small intestinal, water chlorination, high-altitude, or extreme temperatures such as in boiling water. Also, such strong protection allows cysts to survive in water up to 3 months [9]. Upon excystation in the small bowel, each cyst releases two trophozoites, which continue the life cycle.

**Figure 1.** *Giardia lamblia*. Stained trophozoite (A) with its characteristic teardrop shape, binucleate structure, and four pairs of flagella clearly visible. Stained (B) and unstained (C) cysts with its characteristic median (parabasal) bodies and four nuclei. Images are some of the best public-domain light micrographs of *Giardia*, published by the National Institute of Infectious Diseases of Tokyo, Japan. Images were originally published at http://www.nih.go.jp/niid/ja/ kansennohanashi/410-giardia.html.

*Giardia* species are currently classified in six species, which are distinguished based on its morphology and hosts: *Gracilinanus agilis* (amphibians), *G. lamblia* (mammals), *Giardia muris* (rodents), *Giardia psittaci*, and *Giardia ardeae* (both mainly in birds) (**Table 1**) [2]. Previously, many more species of *Giardia* were listed based only on microscopic and epidemiological criteria as well, but using molecular tests such as polymerase chain reaction (PCR), the list was shortened. Later on, genotyping studies confirmed that the species of *Giardia* could be classified in eight genetic groups (A–H): Groups A and B, which are found in humans and


**Table 1.** *Giardia* species.

The epidemiology of giardiasis still is a matter of great discussion. From the original debates around its pathogenicity to the later ones about its speciation and biology, *G. lamblia* has proven to be an enigmatic and interesting organism [2]. Although giardiasis is currently recognized as one of the main causes of diarrheal disease and a leading cause of death and illness among children under 5 years old in developing countries [3], the long-term impact of pediatric giardiasis remains unclear. Recent cohort studies have confirmed a high prevalence of persistent, subclinical giardiasis and its association with growth shortfalls [4], but such

Commonly, giardiasis prevalence among poor populations is reported as very high, and when the infection became chronic, it has been associated also with malnutrition and cognitive deficits [5]. In developed countries, giardiasis represents the leading cause of traveler's diarrhea and is frequently reported among citizens that traveled to developing countries and expose themselves to untreated water from lakes, streams, and swimming pools [6–8]. These and other epidemiologic characteristics of giardiasis will be discussed in detail in this chapter

*G. lamblia* is a parasitic protozoan of the order *Retortomonadida* that alternates between trophozoites and cysts forms within its life cycle, stages responsible for the clinical illness, and the transmission of the disease, respectively. Under the light microscope, trophozoites appear actively swimming and with its characteristically teardrop (viewed dorsoventrally) or spoon (viewed from the side) shaped, measuring 10–20 μm by 5–15 μm by 2–4 μm, containing four pairs of flagella, two identical nuclei, with a convex dorsum and a ventral disc that acts as a suction cup to facilitate attachment of the organism to the small bowel villi (**Figure 1A**). On high-quality slides, the parasite movement shows "falling leave mobility" and resembles a human face because of the positions of the median bodies, nuclei, and axonemes. Uncharacteristically *Giardia* trophozoites lack definable Golgi, peroxisomes, and true mitochondria, but have a menant mitosome. Trophozoites divide by binary fission, and cyst

Microscopically, *Giardia* cysts look oval shaped, measures about 11–14 by 7–10 μm, contains four nuclei (mature cyst)—usually situated at one end—and curved median bodies and linear axonemes (**Figure 1B** and **C**). During the process of encystment, which can be observed under the microscope, trophozoites initially become inactive, rounded, and increasingly refractile as encystment begins. Then, nuclear division (but not cytoplasmic) occurs to produce the quadrinucleate infectious cyst. *Giardia* cysts have a thick hyaline wall that protects them from environmental stressors such as the alkaline environment that characterize the small intestinal, water chlorination, high-altitude, or extreme temperatures such as in boiling water. Also, such strong protection allows cysts to survive in water up to 3 months [9]. Upon excystation in the small bowel, each cyst releases two trophozoites, which continue

evidence has not been consistently reported in the literature.

develops as feces dehydrated in transiting to the large bowel.

based on the classical and latest literature.

**2. Etiologic agent**

14 Current Topics in Giardiasis

the life cycle.

many domestic animals and wildlife [10], and the host-specific groups C–H. Among the latest ones, groups C and D infect dogs, cats, coyotes, and wolves [11]; group E infects cattle, sheep, goats, pigs, water buffaloes, and mouflons [12]; group F infects cats [12]; group G infects rats [12]; and group H infects marine animals [13]. This classification could be of great clinical value given that in addition to many genetic differences found, these genotypes exhibit several relevant biologic differences also. In example, genotype B seems to be more pathogenic than genotype A in humans, although they appear to grow slower [14]. Contradictory, the genetics of parasites of the genus *Giardia* is still poorly understood. Furthermore, recent evidence has cast doubt the classical view of an asexually replicating organism in favor of the occurrence of meiosis and genetic exchange. Such game breaker discovery demanded that the whole population genetics of *Giardia* should be reevaluated to take into account the effect of recombination on *G. lamblia* species [14].
