**3. Life cycle**

Life cycle begins with the infection by the ingestion of the cyst. Then the excystation continues, which starts at the stomach triggered by the exposure of the cyst to the gastric acid, the presence of bile and trypsin in the duodenum and/or the alkaline, protease-rich milieu, duodenum [2]. Excystation ends at the proximal small intestine where the emerging parasites (excyzoites) quickly transform into trophozoites that attach to the intestinal epithelial cells using the adhesive disc. The adhesive disc is essential for attachment and appears to play a major role in the virulence of *Giardia* [15]. Several disc-associated proteins have been identified using proteomics [16], and it is clear that the disc is an advanced cytoskeletal structure [17]. At the jejunum, the trophozoites start to encyst forming the wall that enables the parasite to survive outside the host for several weeks in cold water. This process is triggered by a particular composition of biliary secretions, possibly by a deprivation of cholesterol [18]. Regulatory factors are encystation-specific transcription factors, chromatin remodeling enzymes, and posttranslational modifications, which vary their expression in correlation with the variation of antigens on the parasite surface [19]. Finally, trophozoites and cysts are released with the stool, with cysts continuing the transmission of the disease when ingested by another host.

Reservoir hosts include humans, as well as a variety of animals, including cats, dogs, dairy cattle, beavers, and other farm, wild, and domesticated animals such as horses, pigs, cows, chinchillas, alpacas, lemurs, sheep, guinea pigs, monkeys, goats, and rats [20]. However, among all these animal hosts, only beavers, dogs, and humans have been implicated as a source of infection in different waterborne epidemics and outbreaks of giardiasis in humans. Additionally, it is important to highlight the key role of "reverse zoonotic transmission" (zooanthroponotic) in the epidemiology of *Giardia* infections, which means that humans have been identified as the source of infection for beavers, muskrats, and coyotes in the Unites States and Canada [21–23], muskoxen in the Canadian arctic [24], nonhuman primates (gorillas) and painted dogs in Africa [25], marsupials in Australia, house mice in remote islands [26], and marine mammals (seals) in various parts of the world [27].

Even in developed countries such as the United States, it is common to isolate *Giardia* cysts in the water reservoirs and unfiltered water supplies of major cities, the water supply that is not filtrated [28].
