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.