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Giardia duodenalis (Giardia lamblia or Giardia intestinalis)

Giardia duodenalis

  • Giardia duodenalis, also known as Giardia intestinalis and Giardia lamblia, is a flagellated parasitic microorganism of the genus Giardia that causes giardiasis, a diarrheal condition.
  • The parasite attaches to the epithelium through a ventral adhesive disc or sucker and reproduces through binary fission.
  • Giardiasis is confined to the lumen of the small intestine and does not spread through the bloodstream or other regions of the gastrointestinal tract.
  • Giardia has an outer membrane that allows it to retain life even outside of the host body and makes it chlorine-tolerant. Giardia trophozoites are anaerobes that obtain their nutrients from the lumen. If the organism is divided and stained, its distinctive pattern resembles the familiar “smiley face” symbol.
  • Ingestion of untreated drinking water (the most prevalent mode of transmission for this parasite), food, and soil contaminated with human excrement, as well as sewage, is a leading cause of human infection in many developing nations. There is also contamination of natural waters in watersheds with intensive grazing.
  • Globally, Giardia infections are prevalent. In the United States and Canada, daycare center children, hikers, family members, and immunocompromised individuals are the most likely to be infected with this parasite. Approximately 20,000 instances are reported annually in the United States.
Giardia duodenalis
Giardia duodenalis

Classification of Giardia duodenalis

Species:G. duodenalis

History of Giardia duodenalis

  • Giardia was first discovered in 1681 by Antonie van Leeuwenhoek, who observed it in his own stools.
  • In 1859, Alfred Mathieu Giard identified the parasite in a human patient and named it Giardia intestinalis.
  • Later, it was named after another scientist, Vilem Lambl, who independently discovered the organism in a patient in 1859.
  • Giardia duodenalis is now the most widely used name for the parasite.

Distribution of Giardia duodenalis

  • Giardia duodenalis is found worldwide, with higher prevalence in developing countries with poor sanitation.
  • It is a common cause of waterborne and foodborne illness outbreaks in both developed and developing countries.
  • In the United States, it is estimated that about 20,000 cases of giardiasis occur annually, with higher rates in children and travelers to endemic areas.
  • In developing countries, giardiasis is a major cause of diarrhea and malnutrition in children, particularly in areas with poor sanitation and limited access to clean water.
  • Giardia duodenalis can infect a wide range of mammals, including humans, livestock, pets, and wildlife, and is often transmitted through contaminated water sources or food.

Habitat of Giardia duodenalis

Giardia duodenalis is a flagellated protozoan parasite that can infect the gastrointestinal tract of humans and other animals. The habitat of Giardia duodenalis includes the following:

  1. Intestinal Tract: The primary habitat of Giardia duodenalis is the small intestine of the host, where it attaches to the intestinal wall using its adhesive disk.
  2. Water: Giardia duodenalis can also survive outside the host in water, particularly in fresh water sources such as lakes, rivers, and streams. It can form cysts that are resistant to environmental stressors and can survive for several weeks or months in water.
  3. Soil: In addition to water, Giardia duodenalis can also be found in soil, particularly in areas where there is significant animal fecal contamination.
  4. Food: Contaminated food can also serve as a habitat for Giardia duodenalis, particularly if it has been contaminated by infected feces.
  5. Hosts: Giardia duodenalis can infect a wide range of mammalian hosts, including humans, livestock, pets, and wildlife. It is primarily transmitted through the fecal-oral route, either through contaminated water or food, or through direct contact with infected individuals or animals.

In summary, the habitat of Giardia duodenalis includes the intestinal tract of the host, water, soil, contaminated food, and other mammalian hosts.

Morphology of Giardia duodenalis

G. lamblia exists in two morphological form- trophozoite and cyst


1. Trophozoite

  • Giardia duodenalis is a parasitic flagellated protozoan that can infect the gastrointestinal tracts of humans and other animals.
  • In its life cycle, there are two distinct stages: the trophozoite and the cyst. The trophozoite is the parasite’s active feeding stage and is responsible for intestinal colonization. It ranges in size from 9 to 21 µm in length and 4µm in width.
  • The trophozoite has a pear or tennis racket shape, with a broad, round anterior end and a decreasing posterior end.
  • The dorsal surface is convex, whereas the ventral surface is concave with a suction disc (adhesive disc) that serves as an attachment organ.
  • Giardia duodenalis-unique large curved and transverse median bodies are located behind the adhesive disc. The cytoplasm is uniform and delicately granulated, and motility is of the typical “falling leaf type”
  • The trophozoite has bilateral symmetry, and all of its organs are paired. It possesses four pairs of flagella, two median bodies, two axostyles, two nuclei, and two nuclei.
  • Each nucleus contains a massive central karyosome, giving the parasite a characteristic face-like appearance when stained.
  • It is convex dorsoventrally and has a concave suction disk ventrally, which aids in its attachment to the intestinal mucosa.
  • Consequently, it appears sickle-shaped when viewed from the side. The trophozoite, which resides in the duodenum, the first portion of the small intestine, is responsible for giardiasis symptoms.

2. Cyst

  • Giardia duodenalis has two distinct life cycle stages: trophozoite and cyst. The cyst is the infectious stage of the parasite and also serves as its diagnostic form.
  • A mature cyst measures 8-12µm in length and 7-10µm in width and is elliptical or ellipsoidal in shape. It is protected from the environment by a dense cyst wall.
  • Granulated cytoplasm is isolated from the cyst wall by a clear space. The axostyle forms a roughly diagonal dividing line within the cyst’s wall.
  • In some cases, a cyst contains four nuclei, while in others, it contains two nuclei, depending on its maturity stage. Inside the cytoplasm, the remaining flagella and the edges of the suction disc are visible.
  • The infective form of Giardia duodenalis is the cyst, which can be transmitted via the fecal-oral route. Cysts of the parasite are excreted in the feces of infected individuals, which can contaminate water and dietary sources.
  • Once ingested, the cysts release the trophozoites, which colonize in the small intestine and cause the diarrheal disease giardiasis. Cysts can persist in the environment for several weeks, making them a significant source of infection.
  • In order to prevent the spread of Giardia duodenalis, proper sanitation and hygiene practices, such as hand cleansing and consumption of safe food and water, are crucial.

Life cycle of Transmission of Giardia duodenalis

Life cycle of Transmission of Giardia duodenalis
Life cycle of Transmission of Giardia duodenalis
  1. Cysts, the infectious stage, are passed in the feces of infected people and can live for weeks in the environment.
  2. Cysts in tainted water and food are the primary route of infection for humans.
  3. A guy can become infected after ingesting as few as 10 cysts.
  4. The majority of victims are kids.
  5. Children, male homosexuals, and the mentally sick are also at risk for contracting the virus through direct interpersonal contact.
  6. Blood group A, achlorhydria, cannabis usage, chronic pancreatitis, malnutrition, and immunological abnormalities including 19A deficiency and hypogammaglobulinemia are all linked to an increased risk of contracting giardiasis.
  7. Two trophozoites are released from each cyst during the excystation process, which occurs in the small intestine within 30 minutes of ingestion.
  8. The trophozoites enter the jejunum and duodenum epithelial cells, where they develop and divide into two daughter trophozoites.
  9. The ventral disc of trophozoites and adhesins on the parasite surface mediate attachment, which is necessary to avoid being swept away by peristalsis.
  10. Trophozoites are sometimes passed in diarrheal stools during the infectious stages of these disorders.
  11. Colon encystment is common in poor situations.
  12. In order to process and transport the cyst wall protein antigens to the exterior of the plasma, Encystation-specific vesicles (ESV) form in the cytoplasm.
  13. The process of encystment begins with the flagellum retracting, then the cytoplasm condensing, and finally the cyst wall forming. When a cell matures, its nucleus splits into four halves. The adult cysts are then excreted in the feces, where they can persist for weeks in both soil and water and spread the disease to humans.
  14. Conjugated bile salts, an alkaline pH, and a lack of cholesterol all play a role in encouraging the encystation process.
  15. Per gram of stool, there could be as many as 200,000 cysts.
  16. A dose of 10–100 cysts is infectious.
Life cycle of Transmission of Giardia duodenalis
Life cycle of Transmission of Giardia duodenalis

Pathogenesis and pathology

Mode of transmission

  • Giardia is mostly spread from human to human.
  • Consumption of tainted foods and drinks is the primary route of infection.
  • Cross-contamination from unsanitary conditions at care facilities, hospitals, and institutions for the mentally ill.
  • Oral-genital and oral-anal sexual transmission
  • People with weakened immune systems, such as those with AIDS, X-linked gammaglobulinemia, or protein energy deficiency, are at greater risk of contracting giardiasis.

Virulence factors

  • Cytoskeleton: Motility, attachment, intracellular transport, cell division, and encystation/excystation are all dependent on the cytoskeleton, and Giardia’s cytoskeleton is comprised of microtubules (MT).
  • Cysts: Cysts are resilient and accountable for parasite transmission.

Pathogenesis of Giardia lamblia

  • Giardia is a non-invasive intestinal parasite.
  • Once excystation occurs, trophozoites are released and’swim’ to the microvilli-covered surface of the duodenum and jejunum, where they adhere to the enterocytes using their adhesive disc.
  • Lectins on the surface of Giardia attach to receptors on the surface of enterocytes. This attachment procedure damages microvilli, impeding their ability to absorb nutrients.
  • Rapid proliferation of trophozoites results in the formation of physical barriers between enterocytes and the intestinal lumen, further impeding nutrient absorption. This process causes enterocyte injury, villi atrophy, crypt hyperplasia, intestinal hyperpermeability, and damage to the brush border, resulting in a decrease in disaccharide enzyme secretion.
  • Lectins and other parasite-secreted cytopathic substances also cause indirect injury to the intestinal epithelium.
  • Trophozoites do not invade or penetrate adjacent tissue, nor do they infiltrate the bloodstream. Therefore, infections are typically limited to the intestinal lumen.
  • Giardiasis causes decreased jejunal electrolyte water and glucose absorption, and intestinal epithelium injury leads to electrolyte and fluid malabsorption, resulting in giardiasis-associated osmotic diarrhea.

Clinical manifestation of Giardia lamblia

Giardia lamblia infection, also known as giardiasis, is a common parasitic infection worldwide. The incubation period of the infection can range from 1-3 weeks, after which the clinical manifestation varies depending on the severity of the infection. It is important to note that a majority of the infections are asymptomatic, especially in adults.

  1. Acute giardiasis: However, in symptomatic infections, acute giardiasis is characterized by acute watery diarrhea, abdominal cramp, bloating, and flatulence, with occasional symptoms of nausea, vomiting, fever, rashes, or constipation. Pus, blood, and mucus are not seen in the stool. The condition usually lasts for 5-7 days.
  2. Chronic giardiasis: In chronic giardiasis, symptoms include chronic diarrhea with malabsorption of fat (steatorrhoea) and malabsorption of vitamin A, protein, and D-xylose, resulting in weight loss, malaise, and anorexia. Children may show signs of protuberance of the abdomen, spindly extremities, and stunted growth. Chronic giardiasis can last for several weeks, and extra-intestinal complications such as urticarial and reactive arthritis are rare.
  3. Complication: In adults, chronic giardiasis can lead to malabsorption syndrome and weight loss, while in children, it can cause growth retardation and delayed milestones achievements. However, it is important to note that giardiasis is a self-limited disease, and progression to the chronic state is only seen in 5% of infected people. Death is rare.

Laboratory diagnosis of Giardia duodenalis

There are several laboratory methods available for the diagnosis of Giardia duodenalis infection:

  • Microscopic examination of stool samples: This is the most common and widely used method. It involves the direct visualization of the parasite in stool samples using a light microscope. The trophozoites and cysts can be seen in fresh or preserved samples. However, this method is less sensitive and specific compared to other methods.
  • Concentration techniques: This method involves the concentration of the parasite from stool samples to increase the sensitivity of detection. Some of the commonly used concentration techniques include sedimentation, flotation, and centrifugation.
  • Immunodiagnostic tests: These tests detect specific antigens or antibodies against Giardia duodenalis in serum, feces, or saliva samples. Some of the commonly used immunodiagnostic tests include enzyme-linked immunosorbent assay (ELISA), immunofluorescence assay (IFA), and rapid diagnostic tests (RDTs).
  • Molecular methods: These methods detect the parasite’s DNA or RNA in stool samples using polymerase chain reaction (PCR) or other nucleic acid amplification techniques. This method is highly sensitive and specific, but it requires specialized equipment and expertise.
  • String test: This method involves swallowing a string with a gelatin capsule attached to it, allowing the capsule to reach the duodenum where it can collect the parasite. The string is then retrieved and examined for the presence of the parasite.

It is important to note that no single test is perfect, and a combination of methods may be necessary to diagnose Giardia duodenalis infection accurately.

Treatment of giardiasis of Giardia duodenalis

  • The treatment of giardiasis caused by Giardia duodenalis usually involves the use of specific medications such as metronidazole, tinidazole, or nitazoxanide. These drugs work by disrupting the metabolism of the parasite, leading to its death. Treatment is usually recommended for symptomatic patients, those with chronic infection, or those at risk of complications such as malabsorption syndrome.
  • Metronidazole is the most commonly used drug for giardiasis and is usually given in a dose of 250-750mg orally three times a day for 5-10 days. Tinidazole is an alternative medication that can be given as a single dose of 2g or 1g for children. Nitazoxanide is another option that can be given as a 3-day course of 500mg twice daily for adults or 7.5mg/kg twice daily for children.
  • In some cases, re-treatment may be necessary if symptoms persist or if the infection recurs. It is also important to avoid re-infection by practicing good hygiene, such as washing hands frequently, avoiding contaminated food and water, and avoiding sexual contact during treatment.
  • It is important to note that these medications may have side effects such as nausea, vomiting, headache, and metallic taste in the mouth. Additionally, they may interact with other medications, so it is important to inform the healthcare provider of any other medications being taken before starting treatment.

Prophylaxis of Giardia duodenalis

The following measures can help in the prevention of Giardia duodenalis infection:

  • Proper hygiene: Practice good personal hygiene, such as washing hands frequently with soap and water, especially after using the toilet or changing diapers, and before eating or handling food.
  • Safe drinking water: Drink safe and clean water from a reliable source. Boiling water for at least one minute, using a water filter, or adding water purification tablets can help to ensure safe drinking water.
  • Proper food handling: Properly wash fruits and vegetables before consuming them, and cook food thoroughly, especially meat.
  • Avoid swallowing contaminated water: Avoid swallowing water from rivers, lakes, and ponds while swimming or doing outdoor activities.
  • Avoid contact with infected individuals: Avoid contact with infected individuals, including their feces or any material that may be contaminated with feces.
  • Avoid travel to high-risk areas: Avoid travel to areas with poor sanitation and hygiene, particularly in developing countries.
  • Use protective measures: Use protective measures, such as wearing gloves and a mask, when working with soil or handling animals.

Overall, maintaining proper hygiene, consuming safe drinking water, and avoiding contact with contaminated material are the key preventive measures to avoid Giardia duodenalis infection.


What is Giardia duodenalis?

Giardia duodenalis is a unicellular protozoan parasite that causes giardiasis, an intestinal infection that can lead to diarrhea, abdominal pain, and other gastrointestinal symptoms.

How is Giardia duodenalis transmitted?

The parasite is transmitted through the ingestion of cysts in contaminated water, food, or surfaces. Person-to-person transmission can also occur through the fecal-oral route.

What are the symptoms of Giardia duodenalis infection?

The symptoms of Giardia duodenalis infection can vary from mild to severe and include diarrhea, abdominal cramps, bloating, gas, nausea, and weight loss.

How is Giardia duodenalis diagnosed?

Giardia duodenalis can be diagnosed through a stool examination for the presence of cysts or trophozoites. Alternatively, antigen detection tests and nucleic acid amplification tests can also be used.

What is the treatment for Giardia duodenalis infection?

The treatment for Giardia duodenalis infection usually involves a course of antibiotics, such as metronidazole or tinidazole. Anti-parasitic drugs like nitazoxanide, furazolidone, and paromomycin are also effective in treating the infection.

Can Giardia duodenalis infection be prevented?

Giardia duodenalis infection can be prevented by practicing good hygiene, such as washing hands with soap and water before eating, avoiding contaminated food and water sources, and treating water with chlorine or other disinfectants.

Is Giardia duodenalis infection contagious?

Yes, Giardia duodenalis infection can be contagious and is spread through the fecal-oral route. Person-to-person transmission can occur through close contact with infected individuals or through contact with contaminated surfaces.

How long does it take for Giardia duodenalis symptoms to appear after infection?

Symptoms of Giardia duodenalis infection usually appear 1 to 3 weeks after exposure to the parasite.

Can Giardia duodenalis infection recur after treatment?

Yes, Giardia duodenalis infection can recur after treatment, particularly in individuals who have weakened immune systems or who are exposed to contaminated sources of water or food.

Can animals transmit Giardia duodenalis to humans?

Yes, animals can transmit Giardia duodenalis to humans, particularly through contact with contaminated fecal matter. Common sources of infection include dogs and cats, as well as livestock and wildlife.


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  • Robertson, L. J. (2014). Giardia duodenalis. Microbiology of Waterborne Diseases, 375–405. doi:10.1016/b978-0-12-415846-7.00019-6
  • Lalle, M., & Fiorillo, A. (2019). The protein 14-3-3: A functionally versatile molecule in Giardia duodenalis. Giardia and Giardiasis, Part A, 51–103. doi:10.1016/bs.apar.2019.08.002
  • Jex, A. R., Svärd, S., Hagen, K. D., Starcevich, H., Emery, S. J., Balan, B., … Dawson, S. C. (2020). Recent advances in functional research in Giardia intestinalis. Advances in Parasitology. doi:10.1016/bs.apar.2019.12.002
  • Robertson, L. J. (2014). Giardia duodenalis. Encyclopedia of Food Microbiology, 94–98. doi:10.1016/b978-0-12-384730-0.00147-6

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