**1. Introduction**

Among diarrheal diseases, Giardiasis induced by the protozoan parasite *Giardia duodenalis* plays a distinct role for a variety of reasons. Infections of humans and animals with the often

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overlooked protozoan parasite *Giardia duodenalis* have been reported worldwide. Giardiasis is mainly contracted with the consumption of food and drinking water contaminated with cysts—the environmentally resistant and dormant form of the parasite. *Giardia* cysts are excreted by livestock, wild, and companion animals. These cysts are equipped to survive in extreme and adverse conditions for a long time. The main route of infection is through spreading by water. Giardiasis often occurs as outbreaks, with devastating consequences on human health causing vast economic damage. And most importantly, prophylaxis in the form of drugs or vaccines is impossible. Unfortunately, the inability to involve improved, sensitive, and specific ways for rapid and reliable detection of *Giardia* using microscopic and molecular methods in different matrixes hamper the tracking of this parasite in the environment as well as in patients. For public health, frequent monitoring of water sources using LAMP as the preferred tool is quite effective in both accuracy and costs and can help to detect the parasite source at the earliest possible to avoid outbreaks.

#### **1.1.** *Giardia duodenalis:* **genetic assemblages and hosts**

*Giardia* is a Diplomonadida, flagellated protozoan with global distribution. The names *G. duodenalis*, *G. lamblia, and G. intestinalis* are the species names interchangeably used in current literature referring to the same organism. Based on genetic, structural, and biochemical data, they are systematically classified; *Giardia* belongs to phylum: Metamonada, subphylum: Trichozoa, superclass: Eopharyngia, class: Trepomonadea, subclass: Diplozoa, order: Giardiida, and family: Giardiidae [1, 2].

Anthony van Leeuwenhoek documented the genus in 1681 for the first time when he microscopically examined his own stool due to his continuous diarrheal sickness. In 1859, Lambl provided a detailed description of the trophozoite and the genus was named Lambl in honor of his work. Until 1879, the cyst stage of the life cycle was completely unknown awaiting Grassi to describe the robust parasitic stage that did not contain flagella (cysts) [3]. In order to give credit to the French zoologist Alfred Giard, Stiles changed the former name (genus and specific epithet) to *Giardia* [4] in 1915.

The *Giardia* genus can be divided into six different species: *G. duodenalis*, *G. agilis*, *G. muris*, *G. psittaci*, *G. ardae*, and *G. microti. G. duodenalis* can furthermore be divided into assemblages and subassemblages [5, 6]. Eight different *G. duodenalis* assemblages have been delineated (A–H) so far, of which assemblages A (subassemblages AI and AII) and B (subassemblages BIII and BIV) are mainly virulent for humans and are often referred to as "zoonotic" assemblages [6, 7]. Narrow host-adapted specificity has been found in assemblages C and D; dogs and canines with assemblage E and domestic livestock (cats) with assemblage F [5, 8]. Assemblage G is associated with rats and mice, whereas assemblage H infects gray seals and gulls [5].

#### **1.2. The** *Giardia* **life cycle**

The *Giardia* life cycle begins with the oral ingestion of a few cysts (ovoid, about 15 × 9 μm × 3 μm), which are resistant under environmental conditions retaining the infectious nature and are transmitted through contaminated water, food, or fecal-oral route (hands or fomites) [9, 10]. Acknowledging the resilience of these cysts, the parasite is highly virulent; only 1–10 cysts are capable of causing giardiasis [11]. Following an oral ingestion, the low pH of the stomach acid induces excystation (rupture of the cysts) and duplication (asexual replication) of the cell into two binucleated trophozoites. The process of excystation involves the activation of flagella pushing itself out through the cyst wall induced by the proteolytic activity in the duodenum. Simultaneously, the trophozoite undergoes an asexual duplication resulting in every single cyst producing two trophozoites. The trophozoites attach themselves to the duodenal epithelium with their ventral sucking disk and remain within the lumen of the host's proximal small intestine where they are nutritioned by phagocytosis on the dorsal side of the trophozoite. Freely moving or being attached to the mucosa by a ventral sucking disk, they multiply asexually by longitudinal binary fission resulting in manifold reproduction leading to an invasive growth of the trophozoites in the intestine. Trophozoites at the rectum form a robust cyst wall. The trigger for this encystment is still unclear. The process seems to be a result of exposure to and induction by bile salts, fatty acids, and a more alkaline environment. The trophozoite retracts the flagella and division of the nuclei follows before the cysts are excreted with feces.

#### **1.3. Symptoms of the disease**

overlooked protozoan parasite *Giardia duodenalis* have been reported worldwide. Giardiasis is mainly contracted with the consumption of food and drinking water contaminated with cysts—the environmentally resistant and dormant form of the parasite. *Giardia* cysts are excreted by livestock, wild, and companion animals. These cysts are equipped to survive in extreme and adverse conditions for a long time. The main route of infection is through spreading by water. Giardiasis often occurs as outbreaks, with devastating consequences on human health causing vast economic damage. And most importantly, prophylaxis in the form of drugs or vaccines is impossible. Unfortunately, the inability to involve improved, sensitive, and specific ways for rapid and reliable detection of *Giardia* using microscopic and molecular methods in different matrixes hamper the tracking of this parasite in the environment as well as in patients. For public health, frequent monitoring of water sources using LAMP as the preferred tool is quite effective in both accuracy and costs and can help to detect the parasite

*Giardia* is a Diplomonadida, flagellated protozoan with global distribution. The names *G. duodenalis*, *G. lamblia, and G. intestinalis* are the species names interchangeably used in current literature referring to the same organism. Based on genetic, structural, and biochemical data, they are systematically classified; *Giardia* belongs to phylum: Metamonada, subphylum: Trichozoa, superclass: Eopharyngia, class: Trepomonadea, subclass: Diplozoa,

Anthony van Leeuwenhoek documented the genus in 1681 for the first time when he microscopically examined his own stool due to his continuous diarrheal sickness. In 1859, Lambl provided a detailed description of the trophozoite and the genus was named Lambl in honor of his work. Until 1879, the cyst stage of the life cycle was completely unknown awaiting Grassi to describe the robust parasitic stage that did not contain flagella (cysts) [3]. In order to give credit to the French zoologist Alfred Giard, Stiles changed the former name (genus and

The *Giardia* genus can be divided into six different species: *G. duodenalis*, *G. agilis*, *G. muris*, *G. psittaci*, *G. ardae*, and *G. microti. G. duodenalis* can furthermore be divided into assemblages and subassemblages [5, 6]. Eight different *G. duodenalis* assemblages have been delineated (A–H) so far, of which assemblages A (subassemblages AI and AII) and B (subassemblages BIII and BIV) are mainly virulent for humans and are often referred to as "zoonotic" assemblages [6, 7]. Narrow host-adapted specificity has been found in assemblages C and D; dogs and canines with assemblage E and domestic livestock (cats) with assemblage F [5, 8]. Assemblage G is associated with rats and mice, whereas assemblage H infects gray seals and

The *Giardia* life cycle begins with the oral ingestion of a few cysts (ovoid, about 15 × 9 μm × 3 μm), which are resistant under environmental conditions retaining the infectious nature and are transmitted through contaminated water, food, or fecal-oral route (hands or fomites)

source at the earliest possible to avoid outbreaks.

110 Current Topics in Giardiasis

order: Giardiida, and family: Giardiidae [1, 2].

specific epithet) to *Giardia* [4] in 1915.

gulls [5].

**1.2. The** *Giardia* **life cycle**

**1.1.** *Giardia duodenalis:* **genetic assemblages and hosts**

*Giardia* has a global distribution and is a major contributor to the enormous burden of diarrheal diseases [5, 12, 13]. Giardiasis is a self-limiting disease in immunocompetent individuals with an incubation period of a few days up to 3 weeks. The clinical manifestation is between 1 and 12 days, rarely exceeding 2 weeks [14–16]. Clinically, asymptomatic giardiasis in immunocompetent individual is possible and is frequently associated with excretion of cysts, which however cannot be avoided. Apart from the assemblage, the symptomatic course of infection is confined to be more susceptible in children and elderly/aged people due to their immune incompetence and other host factors [17]. The main symptoms are diarrhea, bloating, weight loss, malabsorption, flatulence, abdominal cramps, nausea, vomiting, fatigue, anorexia, and chills [18–20]. Treatment with drugs is possible within the course of the disease or in chronic conditions. However, preventive vaccination is unavailable [21, 22].

#### **1.4. Transmission routes of giardiasis**

The transmission of cysts is possible by the fecal-oral route, through contaminated food or via water-based transmission. *Giardia* cysts are excreted by livestock, wild, and companion animals and are equipped to survive in extreme and adverse conditions for a long time. The likelihood of distribution and consequently the transmission of *Giardia* cysts shed through feces of wild and/or domestic animals that are evident especially after heavy rainfall or river flooding. Such environmental factors favor their transfer to aquifers, local privately farmed gardens, and open-air greenhouses. Waterborne distribution is estimated to be the main source of infection according to various studies of recent years [12, 13, 23]. The food-borne transmission to humans as well as through consumption of packed salads and/or green leaves by infected food-handlers has been reported [24, 25]. Person-to-person contact among schoolchildren attending day care centers and crosscontamination from the staff to their households are also possible and have a significant epidemiological impact [26–28].

## **1.5. Epidemiology**

The Robert Koch Institute (RKI) in Berlin is the only public health institute in Germany as well as a global health hub publishing weekly reports about illnesses in the German Epidemiological Bulletin. In 2009 and 2010, about 3500–4000 cases of giardiasis were reported [29]. In 2016, the reported *Giardia* cases were 522, whereas during the first 8 weeks of 2017, the reported cases were 415 equaling to about 50 giardiasis incidences per week [30]. In industrial countries, *G. duodenalis* often occurs after the holiday seasons through returning travelers from foreign countries. The post-travel, persistent diarrheal symptoms in patients are most likely related to giardiasis, which is why it is also called "traveler's disease" [31]. Especially, backpackers transmit the parasite to their homes after returning.

In sub-Saharan Africa (SSA), millions of people die of parasitic diseases annually. This includes neglected tropical diseases (NTDs). The geohelminths (soil-transmitted helminths [STHs]) and the intestinal *G. duodenalis* parasite alone infect hundreds of millions of people in SSA [32, 33].

According to the 2016 Statistical Yearbook of United Nation's High Commissioner for Refugees (UNHCR), forcibly displaced people exceeded the number of 65 million worldwide [34]. Until end of January 2016, more than 60,000 registered unaccompanied minor refugees (UMRs) were living in Germany, of which 1248 UMRs between January 2014 and December 2015 underwent an infectious disease screening. Interestingly, 29.2% (364 cases) were infected with more than one intestinal parasite and 7.6% of whom (95 cases) were diagnosed with *G. duodenalis* by immunofluorescence microscopy [35].
