Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Sarcomastigophora
that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis.
Their life cycle alternates between an actively 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.
Characteristics Like other diplomonads, Giardia have two nuclei,
each with four associated flagella, and lack both mitochondria and a Golgi apparatus. However
they are now known to possess a complex endomembrane system as well as mitochondrial relics, called
mitosomes. 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.
Infection and symptoms
Giardia lives inside the intestines of infected humans or other animals. Individuals become
infected through ingesting or coming into contact with contaminated food, soil, or water.
The Giardia parasite originates from contaminated items and surfaces that have been tainted
by the feces of an infected carrier. The symptoms of Giardia, which may begin to
appear 2 days after infection, include violent diarrhea, excess gas, stomach or abdominal
cramps, upset stomach, and nausea. Resulting dehydration and nutritional loss may need
immediate treatment. The typical infection within an individual can be slight, resolve
without treatment, and last between 2–6 weeks, although it can sometimes last longer
and/or be more severe. Coexistence with the parasite is possible, but one 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 anti-helmintic property
as well, ideal for certain compounded issues when a general vermicidal agent is preferred.
Giardia causes a disease called Giardisis 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.
Prevention Person-to-person transmission accounts for
the majority of Giardia infections and is usually associated with poor hygiene and sanitation.
Giardia is found on the surface of the ground, in the soil, in undercooked foods, and water
along with improper cleaning of fecal material from the hands after handling infected feces.
Water-borne transmission is associated with the ingestion of contaminated water. In the
U.S., outbreaks typically occur in small water systems using inadequately treated surface
water. Venereal transmission happens through fecal-oral contamination. Additionally, diaper
changing and inadequate hand washing are risk factors for transmission from infected children.
Lastly, food-borne epidemics of Giardia have developed through the contamination of food
by infected food-handlers. Systematics
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
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,
species include: Many different species of Giardia exist and to discriminate between
species very specific PCR have been developed to detect specific Giardia spp. Gene probe-based
detection is 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 lamblia, which contains probably at least eight lineages
or cryptic species. Genome
A Giardia isolate 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 are
being sequenced. A second isolate from humans has been sequenced
along with a species from a pig. 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. See also
List of parasites References
External links "Giardia". Parasites. Centers for Disease
Control and Prevention. March 2011. Mahbubani MH, Bej AK, Perlin MH, Schaefer
FW, Jakubowski W, Atlas RM. "Differentiation of Giardia duodenalis from other Giardia spp.
by using polymerase chain reaction and gene probes". J. Clin. Microbiol. 30: 74–8. PMC 264999.
PMID 1734070. LaCour, Michelle. "Who Is Giardia?". GIARDIA.
Stanford University. "Giardia". NCBI Taxonomy Browser. 5740.