Giardia lamblia

Giardia lamblia, also known as Giardia intestinalis, is a flagellated parasitic microorganism, that colonizes and reproduces in the small intestine, causing giardiasis. The parasite attaches to the epithelium by a ventral adhesive disc or sucker, and reproduces via binary fission. Giardiasis does not spread via the bloodstream, nor does it spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine. Giardia trophozoites absorb their nutrients from the lumen of the small intestine, and are anaerobes. If the organism is split and stained, its characteristic pattern resembles the familiar 'smiley face' symbol. Chief pathways of human infection include ingestion of untreated sewage, a phenomenon particularly common in many developing countries; contamination of natural waters also occurs in watersheds where intensive grazing occurs. Giardia infections occur worldwide, however Giardia lamblia is the most commonly identified intestinal parasite in the United States and Canada among children in day care centers, hikers, family members and immunocompromised adults. Approximately 20,000 cases per year in the United States are reported. G. lamblia takes on two morphologically distinct forms during its life cycle. The replicative form is a motile pear-shaped cell that survives only in host small intestines called a trophozoite. Trophozoites swim through the intestinal mucus until they eventually adhere to the host intestinal epithelium. Adhered trophozoites then divide by binary fission, forming either more trophozoites or the non-replicative cyst stage. Cysts pass through the host large intestine and are shed in the feces. G. lamblia cysts are resistant to environment stressors, and can survive in the environment for weeks to months if kept moist. Cysts remain dormant until ingested by a host animal. In the new host, environmental conditions trigger the cyst to produce two trophozoites, which then attach to epithelial cells, starting the cycle anew. The cyst can survive for weeks to months in cold water, so can be present in contaminated wells and water systems, especially stagnant water sources, such as naturally occurring ponds, storm water storage systems, and even clean-looking mountain streams. Can also be found on surfaces, soil, food, or water that has been contaminated with feces from infected humans or animals. They may also occur in city reservoirs and persist after water treatment, as the cysts are resistant to conventional water treatment methods, such as chlorination and ozonolysis. Zoonotic transmission is also possible, so 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'). In addition to waterborne sources, fecal–oral transmission can also occur, for example in day-care centers, 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. Giardia infects humans, but is also one of the most common parasites infecting cats, dogs and birds. Mammalian hosts also include dozens of species, including cattle, sheep, and goats. Cats can be cured easily and lambs usually simply lose weight, but in calves, the parasites can be fatal and often are not responsive to antibiotics or electrolytes. Carriers among calves can also be asymptomatic. This parasite is deadly for chinchillas, so extra care must be taken by providing them with safe water. Dogs have a high infection rate, as 30% of the population under one year old are known to be infected in kennels. The infection is more prevalent in puppies than in adult dogs. Infected dogs can be isolated and treated, or the entire pack at a kennel can be treated together regardless. Kennels should also be then cleaned with bleach or other cleaning disinfectants. The grass areas used for exercise should be considered contaminated for at least one month after dogs show signs of infection, as cysts can survive in the environment for long periods of time. Prevention can be achieved by quarantine of infected dogs for at least 20 days and careful management and maintenance of a clean water supply. G. lamblia trophozoites are pear-shaped cells, 10 to 20 micrometers long, 7 to 10 micrometers across, and 2 to 4 micrometers thick. They are motile by way of four pairs of flagella, which propel the trophozoites through the intestine. Notably, each G. lamblia cell has two nuclei, both of which actively transcribe genes. Adjacent to the nucleus, G. lamblia cells have an endoplasmic reticulum that extends through much of the cell. Trophozoites about to differentiate into cysts also contain prominent vesicles termed encystation-specific vesicles that disappear once cyst wall construction begins. Unlike most other eukaryotes, G. lamblia cells contain no visible mitochondria, but instead contains a substantially reduced metabolic organelle termed a mitosome. Additionally, cells appear to contain no Golgi bodies, and instead the secretory system consists entirely of the endoplasmic reticulum and numerous vesicles spread throughout the cell, termed peripheral vesicles. Peripheral vesicles are responsible both for taking up extracellular nutrients, and expelling waste outside the cell. Each cell also contains a pair of rigid structures called median bodies which make up part of the G. lamblia cytoskeleton. Trophozoites adhere to host epithelial cells via a specialized disk-shaped organelle called the ventral disk. Cysts are oval-shaped cells slightly smaller than trophozoites. They lack flagella, and are covered by a smooth, clear cyst wall. Each cyst contains the organelles for two trophzoites: four nuclei, two ventral disks, etc.