Edit] Manifestation of infection

Giardia lamblia

Parasite life cycle.

Giardia lamblia (synonymous with Lamblia intestinalis and Giardia duodenalis) is a flagellated protozoan parasite that colonises and reproduces in the small intestine, causing giardiasis. The giardia parasite attaches to the epithelium by a ventral adhesive disc, and reproduces via binary fission^[1]. Giardiasis does not spread via the bloodstream, nor does it spread to other parts of the gastro-intestinal tract, but remains confined to the lumen of the small intestine^[2]. Giardia trophozoites absorb their nutrients from the lumen of the small intestine, and are anaerobes. If the organism is split and stained, it has a very characteristic pattern that resembles a smiley face.

Edit] Hosts

Giardia affects humans, but is also one of the most common parasites infecting cats, dogs and birds . Mammalian hosts also include cows, beavers, deer, and sheep.

Edit] Life cycle

Giardia infection can occur through ingestion of dormant cysts in contaminated water, food, or by the fecal-oral route (through poor hygiene practices). The Giardia cyst can survive for weeks to months in cold water^[3], and therefore can be present in contaminated wells and water systems, and even clean-looking mountain streams. They may also occur in city reservoirs and persist after water treatment, as the Giardia cysts are resistant to conventional water treatment methods such as chlorination and ozonolysis.^[3] Zoonotic transmission is also possible, and therefore Giardia infection is a concern for people camping in the wilderness or swimming in contaminated streams or lakes, especially the artificial lakes formed by beaver dams (hence the popular name for giardiasis, "Beaver Fever").

As well as waterborne sources, fecal-oral transmission can also occur, for example in day care centres, where children may have poor hygiene practices. Those who work with children are also at risk of being infected, as are family members of infected individuals. Not all Giardia infections are symptomatic, and many people can unknowingly serve as carriers of the parasite.

The life cycle begins with a noninfective cyst being excreted with the faeces of an infected individual. The cyst is hardy, providing protection from various degrees of heat and cold, desiccation, and infection from other organisms. A distinguishing characteristic of the cyst is four nuclei and a retracted cytoplasm. Once ingested by a host, the trophozoite emerges to an active state of feeding and motility. After the feeding stage, the trophozoite undergoes asexual replication through longitudinal binary fission. The resulting trophozoites and cysts then pass through the digestive system in the faeces. While the trophozoites may be found in the faeces, only the cysts are capable of surviving outside of the host.

Distinguishing features of the trophozoites are large karyosomes and lack of peripheral chromatin, giving the two nuclei a halo appearance. Cysts are distinguished by a retracted cytoplasm. This protozoan lacks mitochondria, although the discovery of the presence of mitochodrial remnants organelles in one recent study "indicate that Giardia is not primitively amitochondrial and that it has retained a functional organelle derived from the original mitochondrial endosymbiont"^[4]

edit] Manifestation of infection

Nomenclature for Giardia species are difficult, as humans and other animals appear to have morphologically identical parasites.

Colonization of the gut results in inflammation and villous atrophy, reducing the gut's absorptive capability. In humans, infection is symptomatic only about 50% of the time, and protocol for treating asymptomatic individuals is controversial.^[3] Symptoms of infection include (in order of frequency) diarrhea, malaise, excessive gas (often flatulence or a foul or sulphuric-tasting belch, which has been known to be so nauseating in taste that it can cause the infected person to vomit), steatorrhoea (pale, foul smelling, greasy stools), epigastric pain, bloating, nausea, diminished interest in food, possible (but rare) vomiting which is often violent, and weight loss.^[3] Pus, mucus and blood are not commonly present in the stool. It usually causes "explosive diarrhea" and while unpleasant, is not fatal. In healthy individuals, the condition is usually self-limiting, although the infection can be prolonged in patients who are immunocompromised, or who have decreased gastric acid secretion.^[3]

People with recurring Giardia infections, particularly those with a lack of IgA, may develop chronic disease.

Lactase deficiency may develop in an infection with Giardia, however this usually does not persist for more than a few weeks, and a full recovery is the norm^{[citation needed]}.

Some studies have shown that giardiasis should be considered as a cause of Vitamin B12 deficiency, this a result of the problems caused within the intestinal absorption system. ^[5]