Giardia (dʒiːˈɑrdiə or ˈdʒɑrdiə) is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between a swimming trophozoite and an infective, resistant cyst. Giardia were first described by the Dutch microscopist Antonie van Leeuwenhoek in 1681. The genus is named after French zoologist Alfred Mathieu Giard.
Like other diplomonads, Giardia have two nuclei, each with four associated flagella, and were thought to lack both mitochondria and Golgi apparatuses. However, they are now known to possess a complex endomembrane system as well as mitochondrial remnants, called mitosomes, through mitochondrial reduction.
The mitosomes are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur proteins. The synapomorphies of genus Giardia include cells with duplicate organelles, absence of cytostomes, and ventral adhesive disc.
About 40 species have been described from different animals, but many of them are probably synonyms. Currently, five to six morphologically distinct species are recognised. Giardia lamblia (=G. intestinalis, =G. duodenalis) infect humans and other mammals, G. muris is found from other mammals, G. ardeae and G. psittaci from birds, G. agilis from amphibians and G. microti from voles. Other described (but not certainly valid), species include:
Giardia ardeae
Giardia beckeri
Giardia beltrani
Giardia botauri
Giardia bovis
Giardia bradypi
Giardia canis
Giardia caprae
Giardia cati
Giardia caviae
Giardia chinchillae
Giardia dasi
Giardia equii
Giardia floridae
Giardia hegneri
Giardia herodiadis
Giardia hyderabadensis
Giardia irarae
Giardia marginalis
Giardia melospizae
Giardia nycticori
Giardia ondatrae
Giardia otomyis
Giardia pitymysi
Giardia pseudoardeae
Giardia psittaci
Giardia recurvirostrae
Giardia sanguinis
Giardia serpentis
Giardia simoni
Giardia sturnellae
Giardia suricatae
Giardia tucani
Giardia varani
Giardia viscaciae
Giardia wenyoni
Many different species of Giardia exist, so to differentiate between species, very specific PCR (Polymerase Chain Reactions) have been developed to detect specific Giardia spp.