Frontiers in Parasitology

Frontiers in Parasitology

Volume: 2

Water-borne Protozoa in Humans

Indexed in: EBSCO

Several parasites are able to spread diseases through contaminated water. While the spread of diseases through contaminated water appears to have a greater correlation with a lack of access to clean ...
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Giardia and Giardiasis

Pp. 150-220 (71)

Showgy Ma`ayeh and Staffan Svärd


Giardia intestinalis (syn. G. lamblia or G. duodenalis) is a unicellular protozoan parasite that infects the small intestines of humans and animals. The species G. intestinalis is composed of eight genotypes (called assemblages) designated from A to H. Only assemblages A and B infect humans, causing diarrhea and other associated symptoms. Giardiasis, the disease caused by the parasite, has a global distribution but is mainly endemic in developing countries where pronounced effects on children manifest in a failure to thrive condition. In adults, giardiasis might predispose for other gastrointestinal disorders such as Irritable Bowel Syndrome (IBS) after parasite clearance. The parasite exists in two forms: the cyst and the trophozoite. The cyst is the infectious form that is transmitted via the fecal-oral route. Infections can be asymptomatic, acute or chronic, which might be the result of interplay between parasite and host factors. The parasite is known to induce pathophysiological changes in the small intestines and trigger an immune response that result in parasite clearance. Host immune responses towards G. intestinalis, however, are not completely understood. Microscopy is the gold standard in clinical settings to diagnose giardiasis and treatment is usually with metronidazole or tinidazole but other drugs are also available. No human vaccines are available to date but some vaccine trials have shown promising results in animals. The spread of giardiasis is most common via water and is usually controlled through monitoring water treatment processes, by implementing a multibarrier approach, and by monitoring the quality of water in recreational venues. Giardiasis adds to the global health burden and increases the costs of health care in many countries and therefore, a better knowledge of disease transmission and control is required to mitigate the risks of infections.


Assemblages, Diarrhea, Diplomonad, Host-parasite interaction, Metronidazole, Protozoa, Van Leeuwenhoek, Variable surface proteins, Ventral disc, Virulence, Water-borne, Zoonosis.


Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden.