**4. Life cycle**

*Giardia intestinalis* is a noninvasive protozoan parasite with a simple life cycle and a simplified metabolism that depends on the host for nutrients such as purines, pyrimidines, cysteine, and cholesterol. The genus *Giardia* and its life cycle are relatively well recognized and clearly defined; members of the genus are flagellated protozoans belonging to the class Zoomastigophorea and order Diplomonadida, which lives and multiplies by asexual multiplication frequently on the luminal surface of the small intestine of its vertebrate host [14–16].

The trophozoite, which is 9–21 μm long, 5–15 μm wide, and 2–4 μm thick, lives in the small intestine and is responsible for many manifestations of the disease. The cytoskeleton composed of unique families of structural proteins and carbohydrates involves two nuclei, a ventral sucking adhesive disk by which it may adhere to intestinal epithelial cells, a median body, and four pairs of flagella that behave differently during motility. The dorsal surface is pear shaped and bilaterally symmetrical, with the two highly characteristic nuclei best visualized after staining. The newly emerged trophozoites infect the duodenum and jejunum where there is a favorable alkaline pH; they attach intimately to the intestinal epithelium by their ventral disk and begin to multiply by binary fission. Detection of soluble cyst wall proteins in the feces forms the basis of many stool antigen assays [1, 8, 16, 17].

**3. Epidemiology**

4 Current Topics in Giardiasis

2004 [4, 7, 9, 10].

measures [2, 6, 12].

**4. Life cycle**

ity, by oral-anal contact [1, 5, 11, 12].

Distributed worldwide, *Giardia* is probably the most frequent pathogenic intestinal protozoon in children and adults, and one of the most common nonviral causes of diarrhea, afflicting annually an approximate 280 million individuals. Due to its intensifying global burden and its developmental and socioeconomic impact on infected individuals, this parasitosis was incorporated in the Neglected Disease Initiative of the World Health Organization in

In developed countries, giardiasis is associated with social and climatic factors and is referred as a re-emerging infectious agent. Some epidemiological studies have shown that its prevalence varies between the population studied and the location, from 2 to 5% on industrialized countries to 20–30% in developing countries [1, 5, 11], and these changes in prevalence are associated, among others, with the hygiene infrastructure and the impact of the weather conditions, reason why environmental control efforts are necessary, which requires an integrated and systematic approach to decrease and mitigate the influence on the disease epidemiology; for this reason, it is linked to educational programs and other interventional

Although in Latin America, a restricted quantity of population-based studies has been performed, in Cuba, according to the last national intestinal parasites survey (n = 5850), the prevalence of *Giardia lamblia* infection was determined to be at 6.02% (95%CI 5.40–6.63) [13].

This parasitosis is highly infectious where *Giardia* cysts are habitually excreted in considerable population, especially in young children after ingestion of contaminated water or food and through person-to-person contact. Cysts can survive for months in cold water, and they are relatively resistant to chlorination, reason why between 10 and 100 cysts are sufficient to establish infection 100% of the time. Consequently, ingestion of water or food that contains small levels of contamination can result in the disease, which is more recurrent in summer and fall. Other usual ways to transmit them could be among day care center attendees and people who live in residential institutions, and it could also spread by means of sexual activ-

*Giardia intestinalis* is a noninvasive protozoan parasite with a simple life cycle and a simplified metabolism that depends on the host for nutrients such as purines, pyrimidines, cysteine, and cholesterol. The genus *Giardia* and its life cycle are relatively well recognized and clearly defined; members of the genus are flagellated protozoans belonging to the class Zoomastigophorea and order Diplomonadida, which lives and multiplies by asexual multiplication frequently on the luminal surface of the small intestine of its vertebrate host [14–16]. The trophozoite, which is 9–21 μm long, 5–15 μm wide, and 2–4 μm thick, lives in the small intestine and is responsible for many manifestations of the disease. The cytoskeleton composed *Giardia* takes advantage of host conditions at each step of its descent through the human gastrointestinal tract. After ingestion, *Giardia* infection is initiated by the acidic milieu in the stomach and the presence of bile and trypsin in the duodenum, and then, it reproduces in the small intestine, yielding two trophozoites from each cyst, which quickly divide again. Exposure of cysts to gastric acid triggers excystation, although the "excyzoites" do not emerge from the cyst until it passes into the small intestine. In vitro trophozoites double in number every 6 hours in the fastest growing isolates. The emerging parasites (excyzoites) quickly transform into trophozoites that attach to the intestinal epithelial cells using the adhesive disk, which is a major virulence factor; in this way, any excreted cysts are mature, highly infectious, and quite resistant to disinfectants routinely used for water treatment such as chlorine [8, 16, 18].

When the trophozoite senses a change in the environment as the cell is transported further down in the small intestine, it starts the encystation; in the earliest moment of this process, some specific vesicles are designed that allow its development and maturation, probably as a result of cholesterol starvation. At the same time with several proteins implicated in metabolic pathways, after that, several proteins also change their expression by important gene expression changes. *Giardia* has a considerable metabolic capacity as another parasite that lacks pathways for de novo biosynthesis of pyrimidines and purines for nucleotide salvage, but this process is conditioned by oxygen concentration, despite anaerobic metabolism and generating reactive oxygen species (ROS) by the host [1, 14–17].

Giardiasis as a multifactorial disease involves in its pathobiology complex interactions between host and parasite, differences in nutritional status, immune status, co-infections, and intestinal normal flora that could contribute to the differences in disease outcome observed among individuals from developing against developed countries. The roles of the host's intestinal normal flora and co-infections during *Giardia* infections are still largely unascertained [3, 12, 15, 19].

Some morphologically identical but genetically distinct *Giardia* infect humans and animals that now are divided into eight assemblages (A–H) what could bring a clarification about infection outcome. Thus, assemblages A and B are found in human and other animals, being considered zoonotic, whereas the other assemblages display host specificity and do not infect humans (C and B in dog, F in cat, E in hoofed animal, G in rodents, and H in sea mammals). Interestingly, this proportion is not altered when comparing data from developing and developed countries, but the prevalence of mixed infections is higher in the developing countries [8, 15–18].

The two assemblages (A and B) are composed of genetically distinguishable isolates, which may vary in infectivity, antigenicity, and virulence. In addition, human hosts vary in susceptibility to infection and disease and in the response to tolerance to infection. In this way, *Giardia* is an intraluminal parasite that adheres to the epithelium by way of an adhesive or sucking disk, although invasion of the epithelium either does not occur or is rare. Meanwhile, the number of trophozoites in the intestine can be so large that adherent organisms cover much of the epithelial surface. This could disrupt the epithelial brush border and contribute to lack of disaccharidase as could be seen in some patients [8, 14, 15].
