Giardia duodenalis, also known as Giardia intestinalis and Giardia lamblia, is a flagellated parasitic protozoan microorganism of the genus Giardia that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The parasite attaches to the intestinal epithelium by a ventral disc (syn. adhesive disc or sucker), and reproduces via binary fission. G. duodenalis is a non-invasive parasite, that does not spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine.Harrison's Internal Medicine, Harrison's Online Chapter 199 Protozoal intestinal infections and trochomoniasis The parasite exists in two forms; trophozoites and cysts. The microorganism can undergo encystation, transforming into a dormant cyst that enables it to survive outside of its host. Giardia trophozoites are anaerobic, and absorb their nutrients from the intestinal lumen. If the organism is stained, its characteristic pattern resembles the familiar "smiley face" symbol.
Chief pathways of human infection include ingestion of untreated drinking water (which is the most common method of transmission for this parasite), food, soil contaminated with human feces, and 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. It is the most commonly identified intestinal parasite among children in day-care centers, hikers and immunocompromised patients. About 20,000 cases per year in the United States are reported.
Almost half of those infected with giardiasis remain asymptomatic. For those who do experience symptoms, they usually appear 1 to 2 weeks after infection. Common symptoms include abdominal pain, nausea, and bloating, along with large, watery, foul-smelling, and greasy stools. Due to frequent loose stools, individuals with giardiasis often experience dehydration. It has also been shown that G. intestinalis damages the intestinal epithelium, which directly affects nutrient absorption. In severe cases, giardiasis can lead to chronic diarrhea, chronic fatigue syndrome and cognitive impairment in children.
Life cycle
G. duodenalis takes on two morphologically distinct forms during its lifecycle. Trophozoites are the replicative stage of the parasite, characterized by a pear-shaped, motile, flagellated cell that survives only in the small intestine of the host. The trophozoites do not penetrate host cells, but rather attaches to the intestinal epithelium cells to establish an infection. A cyst is the environmentally stable stage of the parasite, that facilitates transmission between hosts.
The infection process in the host begins with the ingestion of cysts, which pass through the stomach into the first part of small intestine, or the duodenum. Exposure to digestive enzymes and an acidic pH triggers excystation, where the cysts release trophozoites. Trophozoites swim through the intestinal mucus until they eventually adhere to the intestinal epithelium using their ventral disc. Adhered trophozoites can then feed, divide by binary fission and cause the disease. Trophozoites cause structural and functional damage to the host epithelial cells, impairing the intestine's ability to absorb nutrients effectively. Some trophozoites differentiate back into cysts under specific conditions, such as high organism density. As the trophozoites travel through the intestinal lumen to the large intestine, the alkaline environment and the presence of pancreatic proteases trigger the encystation. Trophozoites develop into cysts through encystation, which involves specialized vesicles (ESVs) that facilitate the formation of the cyst wall.
Cysts and trophozoites pass through the host's large intestine and are shed in the feces. Trophozoites cannot survive outside of the host, whereas cysts can remain viable in the external environment for several months. The cysts remain dormant until ingested by a host animal. When a new potential host ingests water or food contaminated with this feces, the cysts gain entry to the gastrointestinal tract of the new host, repeating the cycle.
Structure The trophozoite has an elaborate structure with two nuclei and four pairs of flagella which allow it to swim within the intestinal lumen of the host. It also has an adhesive disk on its ventral surface that is associated with the parasite's attachment to the intestinal epithelium. The adhesive disk is composed of microtubules, and is found only in Giardia. The parasite lacks Golgi apparatus and mitochondria but has mitosomes, which probably evolved from mitochondria. The mitosome is a double-membraned organelle, that lacks the enzymatic components required for classic mitochondrial functions, such as ATP synthesis and lipid metabolism. However, they do contain certain mitochondrial genes associated with iron-sulfur complex biosynthesis, which suggests that this organism likely lost its mitochondria during evolution.
The cyst contains four nuclei, axonemes, median bodies as well as the rigid cyst wall. Additionally, the cysts contain fragments of the adhesive disc within the cytoplasm. The two-layered cyst wall protects the parasite against different environmental conditions such as chemical treatments. The metabolic activity of the cyst is significantly lower compared to that of the trophozoites, which allows the parasite to survive for longer periods in harsh conditions, such as in cold environments.
Prevalence and epidemiology G. duodenalis causes an infection called giardiasis. This disease is the cause of both endemic and epidemic disease worldwide and is the most frequently identified intestinal parasite in the United States and Canada. An infected individual can excrete between 1 million and 1 billion cysts daily, and the infectious dose can be as low as 10 cysts. This makes Giardia extremely infectious.
It is estimated to infect over 280 million people world every year resulting in over 500,000 deaths. The most affected demographic is children 0 to 4 years of age. Globally G. duodenalis is the most commonly identified protozoal intestinal parasite. Giardia has common seasonal patterns in the distribution of infection rates with highest peaks in the late summer to early fall.
The cyst can survive for weeks to months in cold water, so the parasite can be present in contaminated wells and water systems, especially in stagnant water sources, such as naturally occurring ponds, storm-water storage systems, and even clean-looking mountain streams. Cysts can also be found on contaminated surfaces, soil and food. 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, but is less frequent. 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, Giardia infections are more commonly found in children compared to adults, this is believed to be due to fecal-oral transmission of the cysts. For example, in developed countries it affects approximately 2% of adults and 8% of children. In developing countries the prevalence rates reach 15% to 20% in children under 10 years old. Thus, there is a significant variation in infection rates based on geographical area.
Those who work with children are also at risk of being infected, as are family members of infected individuals. 7% of children aged 1 to 3 years and 11% of infants and toddlers tested for admission to day-care centers were found to be infected. Not all Giardia infections are symptomatic, and many people can unknowingly serve as carriers of the parasite. Re-infection and chronic infections of the parasite can occur.
Geographical prevalence G. duodenalis is the most widespread intestinal parasite affecting humans. The parasite Giardia duodenalis can be found all over the world, in both developing and industrialized nations. However, human infections are most common in tropical and subtropical climates. Giardia duodenalis is common around the world because the parasite resides in bodies of water; typically rivers, lakes, and recreational swimming pools. Giardiasis is more prevalent in developing countries, where the sanitation and overall hygiene is poorer compared to developed countries. In developed nations, giardiasis has a prevalence of 2–5%, whereas in developing nations it is significantly higher, ranging from 20% to 30%. In the United States, it has been discovered that a majority of whom are infected by the Giardia duodenalis parasite tend to reside in more urban areas, and, patients who are infected are more likely to live in the Southern United States.
Ecology 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 presumptively treated together. Kennels and 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.
Cell biology
G. duodenalis trophozoites are pear-shaped cells, 15 μm long, 8 μm across, and 3 μm thick. They are motile by way of four pairs of flagella, which propel the trophozoites through the intestine. Notably, each G. duodenalis cell has two nuclei, both of which actively transcribe genes. Adjacent to the nucleus, G. duodenalis 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. duodenalis cells contain no visible mitochondria, but instead contains a substantially reduced metabolic organelle known as a mitosome. Additionally, cells appear to contain no Golgi bodies, and instead the secretory system consists entirely of the endoplasmic reticulum and numerous vesicles dispersed 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 four nuclei and fragments of the ventral disc.
Metabolism