| Giardia | |
|---|---|
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| Giardiatrophozoite,SEM | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Metamonada |
| Phylum: | Fornicata |
| Class: | Eopharyngea |
| Order: | Diplomonadida |
| Family: | Hexamitidae |
| Subfamily: | Giardiinae |
| Genus: | Giardia Künstler, 1882[1] |
| Species | |
| Synonyms | |
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Giardia (/dʒiːˈɑːrdiə/ or/ˈdʒɑːrdiə/) is agenus ofanaerobicflagellatedprotozoanparasites of the phylumMetamonada that colonise and reproduce in the small intestines of severalvertebrates, causing the diseasegiardiasis. Their life cycle alternates between a binucleated motiletrophozoite and an infective, metabolically inert, environmentally resistant tetranucleatecyst. Cysts are transmitted between hosts through the fecal-oral route, contaminated water and/or food.Giardia were first seen by theDutchmicroscopistAntonie van Leeuwenhoek in 1681 under the light microscope.[3] The genus is named after FrenchzoologistAlfred Mathieu Giard.[4]
Giardia trophozoites are 12–15 μm long and 5–9 μm wide and have a shape of a pear bisected lengthwise. Like otherdiplomonads,Giardia has two transcriptionally operationalnuclei that contain an equal number of well-definedchromosomes and replicate synchronously with cell division. Thecytoskeleton ofGiarida consists of a median body, 4 pairs offlagella (anterior, ventral, posterior and caudal) and an adhesive disc.[5]Giardia lacks canonicalmitochondria andGolgi complexes; instead, it possesses an endomembrane-vesicle system as well as reduced mitochondria, calledmitosomes.[6][7][8][9] These mitosomes are involved in the maturation ofiron-sulfur proteins and do not participate inATP synthesis.[10] The adhesive disc, located on the ventral side, allows the parasite to attach itself to the host's intestinal epithelium.[11] Trophozoites multiply viabinary fission in the small intestine and encyst during the passage towards the large intestine.
Giardia cysts are immotile, oval-shaped, sturdy units about 8–12 μm by 7–10 μm. The cyst wall is ~0.4 μm thick and is composed of cyst wall proteins (CWP1, 2, 3) andN-acetylgalactosamine. The cyst bears four tetraploid nuclei inside as well as all the other organelles, some disassembled.[5] Nuclei in the cyst are in a close contact with each other and genetic material exchange (diplomixis) may occur between them,[12] at least in someGiardia species. Upon excystation, which takes place after cyst ingestion, one cysts releases four viable parasites.
G. duodenalis is a speciescomplex.[13] Eight morphologically similar but genetically distinctGiardia assemblages (species/clades) have been identified based on genome sequencing; each assemblage has been assigned a letter between A and H.[13] Every assemblage contains several subassemblages (subspecies or genotypes). Assemblages and sub-assemblages have different host specificity. Assemblages A and B occur in humans and many other vertebrates, assemblage C and D incanids, assemblage E inhoofed animals, assemblage F in cats, assemblage G inrodents, and assemblage H inpinnipeds.[14]
The assemblage A has been further divided into groups: AI , AII and AIII, based on genetic and biological differences. AI is a highly homogeneous group in which minimal sequence differences among isolates (about 300SNPs) and low allelicheterozygosity. AI is found primarily in animals and is mainlyzoonotic. AII, on the other hand, occurs mainly in humans, has a high of variability in sequence between isolates (about 30K SNPs), and a high allelic sequence heterozygosity.[15] Similar division is characteristic for assemblage B, where BIII and BIV isolates are distinguished by host range and genetic differences.[16] Systematics of other assemblages remains to be established.
Giardia is an early diverging eukaryote.[17] This is supported by several features: their lack of ATP-synthesizing mitochondria (seeCharacteristics) and other organelles, their primitivemetabolic pathways, and their position in aphylogenetic tree.[18]
AGiardia isolate (WB, AI genotype) was the firstdiplomonad to have itsgenome sequenced. Its almost 12 millionbasepair-long genome is compact in structure and content, with simplified basic cellular machineries and metabolism. There are about 5000 genes inGiardia genome.[19] Clinical isolates of B assemblage, along with a pig isolate of E assemblage are alsowhole-genome sequenced.[20] The E assemblage is more closely related to the A assemblage than is the B. A number ofchromosomal rearrangements are distinguishable between assemblages.
Currently, the genomes of otherGiardia isolates and otherdiplomonads (Spironucleus,Hexamita) are whole-genome sequenced.[21] Genome assemblies are available in several databases (e.g. giardiadb.org).
Giardia lives, in its motile trophozoite form, in theintestines of infected humans or other animals, individuals of which becomeinfected by ingesting or coming into contact with contaminated foods, soil, or water tainted by the feces of an infected carrier.[22] In order to spread to new hosts,Giardia forms environmentally resistant cysts, which can be spread via the fecal-oral route. The infectious dose is very low (1-10 cysts).[23]
The symptoms ofGiardia, which may begin to appear 3–25 days after infection, can include mild to profound fattydiarrhea, 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 host can remain a carrier and transmit it to others. Medication containingtinidazole ormetronidazole decreases symptoms and time to resolution.Albendazole is also used, and has ananthelmintic (anti-worm) property as well, ideal for certain compounded issues when a general vermicidal agent is preferred.Giardia infestation causes the microvilli of the small intestine to atrophy and flatten, resulting inmalabsorption in the intestine.Lactose intolerance can persist after the eradication ofGiardia from the digestive tract.[24]
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.[25]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.[26]
| General topics | ._Natuurkundige_te_Delft_Rijksmuseum_SK-A-957.jpeg) | |
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| Recognitions | ||
