# Giardia

Giardia (/dʒiːˈɑːrdiə/ or /ˈdʒɑːrdiə/) 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 seen by the Dutch microscopist Antonie van Leeuwenhoek in 1681. The genus is named after French zoologist Alfred Mathieu Giard.

## Characteristics

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.

## Systematics

About 40 species have been described, but most of them are probably synonyms. Currently, five to six morphologically distinct species are recognised. Giardia duodenalis (=G. intestinalis, =G. lamblia) infect humans and other mammals, G. microti infects voles, G. muris is found in other mammals, G. ardeae and G. psittaci in birds, and G. agilis in amphibians. 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
* Giardia dolus
Real-time polymerase chain reaction (PCR) tests have been developed to detect specific species of Giardia. Gene probe can also used to differentiate between species of Giardia. A more common and less time-consuming means of identifying different species of Giardia includes microscopy and immunofluorescence techniques.
Genetic and biochemical studies have revealed the heterogeneity of Giardia duodenalis, which contains probably at least eight lineages or cryptic species.

## Phylogeny

The phylogeny of Giardia is unclear, but two main theories exist. Firstly, Giardia may be extremely primitive eukaryotes that branched off early from other members of their group. This theory is supported by several features: their lack of complete mitochondria (see Characteristics) and other organelles, their primitive metabolic pathways, and their position on a phylogenetic tree. However, many of these differences have been refuted in recent years, and many researchers are supporting a second theory: that Giardia are highly evolved parasites, which have lost ancestral characteristics.

## Genome

A Giardia isolate (WB) was the first diplomonad to have its genome sequenced. Its 11.7 million basepair genome is compact in structure and content with simplified basic cellular machineries and metabolism. Currently the genomes of several other Giardia isolates and diplomonads (the fish pathogens Spironucleus vortens and S. salmonicida) are being sequenced.
A second isolate (the B assemblage) from humans has been sequenced along with a species from a pig (the E assemblage). There are ~5000 genes in the genome. The E assemblage is more closely related to the A assemblage than is the B. A number of chromosomal rearrangements are present.

## Infection

Giardia lives in the intestines of infected humans or other animals, individuals of which become infected by ingesting or coming into contact with contaminated foods, soil, or water tainted by the feces of an infected carrier.
The symptoms of Giardia, which may begin to appear 2 days after infection, can include mild to violent diarrhoea, excess gas, stomach or abdominal cramps, upset stomach, and nausea. Resulting dehydration and nutritional loss may need immediate treatment. A typical infection can be slight, resolve without treatment, and last between 2 and 6 weeks, although it can sometimes last longer and/or be more severe. Coexistence with the parasite is possible (symptoms fade), but an infected individual can remain a carrier and transmit it to others. Medication containing tinidazole or metronidazole decreases symptoms and time to resolution. Albendazole is also used, and has an anthelmintic (anti-worm) property as well, ideal for certain compounded issues when a general vermicidal agent is preferred. Giardia causes a disease called giardiasis, which causes the villi of the small intestine to atrophy and flatten, resulting in malabsorption in the intestine. Lactose intolerance can persist after the eradication of Giardia from the digestive tract.

## Prevalence

The prevalence of the infection depends on different factors; while the prevalence is estimated around 2% in some developed countries, in other countries from Asia, Africa or Latin America, the prevalence can be estimated between 20% and 40%. In some patients, giardiasis can be completely asymptomatic, so many more cases are estimated. The diagnostic method used can also infer in the identification and thus the count of cases. Due to their lack of knowledge and overall behavioral patterns, children aged under 5 years are the population with the most reported infections.