Giardia lamblia

Giardia lamblia




Giardia lamblia (=G. intestinalis, =G. duodenalis) also called Giardia duodenalis, Giardia intestinalis and pear-shaped flagellate is a common and well-known anaerobic flagellated protozoan parasites colonize in human (or in canine) small intestines and cause diarrhea, stomach pain etc. Its character of parasitic zoonoses make them also infecting other mammalian such as mice, rabbits, birds, reptiles and amphibians. Giardia lamblia prefer to live in cold water like rivers in mountains, cold springs and contaminated pools, which became the major cause of diarrhea in the US. Traveling in the developing world, dinning food without proper cooking, changing diapers etc. would be major risk factors of Giardiasis. The stool tests is the major diagnosis tool.



In 1681, the Dutch scientist Antonie van Leeuwenhoek first found giardia through microscope. In 1915, Giardia lamblia was officially named after scientist Alfred Mathieu Giard who further studied it. In 2010, Andersson et al. has sequenced Giardia’s genome and discovered its ~5000 genes with 11.7 million base pair building blocks.


Cellular and physiological properties

Giardia is a diplomonad with two nuclei and duplicate organelles follow by four associated flagella and without cytostomes, Golgi apparatus and mitochondria. However, they have a mitochondrial remnant, mitosomes, which take part in the maturation of iron-sulfur proteins rather than ATP synthesis. The life cycle of Giardia lamblia consist of reproductive phase and resting phase that present two different forms: a swimming trophozoite and an infective cyst (Figure 1).

Genotyping of Giardia lamblia sub-classified eight genetic assemblages (from A to H). Among eight assemblages, A and B infect are the most dominate assemblages that infect wide range of species including human being. Various species of Giardia exist in different kind of hosts were identified by PCR or genetic tools include G. ardeae and G. psittaci from birds, G. agilis from amphibians, G. microti from voles and G. muris from other vertebrates.


In 2013, WHO estimated that there were about 280 million people diagnosed with the Giardia infection around the world. The prevalence rate of Giardia in developed countries was 5% and over 20% among developing countries. In 2018, it present that 15,584 reported cases in 3–7% prevalence of the population in the US, especially high incidence in California, New York, Florida, and Wisconsin. Besides, the highest incidence months of giardiasis would be July, August, and September in the America. Furthermore, 23 of the 31 countries in the Europe had reported total 17,278 confirmed giardiasis cases in 2014. Thus, Giardiasis become one of the top 10 parasite diseases in human beings.


Risk factors

  1. Travelers to countries where giardiasis is common.
  2. Contacting contaminated drinking water, lakes or rivers, animals who have the disease.
  3. Giardiasis often happened in the summer because of the higher activities rate in the wilderness.



Giardia exist in rivers, streams, wells and pools. It infect humans or animals by their contacting of contaminated foods, water, contaminated feces and sniffing unclean ground. It is reported that people (or pets) infected with Giardia may have no symptoms, but still spread the disease. Animals such as canines, cows, rodents, beavers, and sheep are also infecting targets of Giardia lamblia.


Figure1. The life cycle of Giardia lamblia )


Giardia could transform between cyst form and trophozoite form. It spread and infect host by the cyst form that can remain contagious in cold water up to 3 months. After arriving intestines, Giardia stick to intestinal wall and colonize in the gut by altering its appearance into trophoziote form. Furthermore, they would recover back to cyst form when excreting out with stools. Giardia is spreading through fecal-oral route by animals’ contacting of contaminated water and food. It is one of the most common waterborne outbreaks of diarrhea in the America. Giardia spread between people and animals   (Figure 1).


When sojourning in the gastrointestinal tract with the trophoziote form, Giardia would engage in inhibiting of brush border enzymes that involved assimilation of disaccharide sugars, altering microvillus’ morphology that cause poor absorption of nutrients and water, triggering apoptosis of intestinal epithelial cells and enhancing intestinal permeability. Eventually, they would cause several clinical symptoms include diarrhea and intestinal malabsorption with or without histological changes.


For increasing intestinal permeability, Giardia lamblia would take several dedicated process such as assisting proliferation of crypt cells and using enzymes to degrade proteins on the villi of the brush border. The degraded proteins would likely lead to immunological recruitment and activation of host T lymphocytes on endothelial cells for removing injured cells. In addition, Giardia triggered cell apoptosis by downregulating of the anti-apoptotic Bcl-2 and upregulating of the proapoptotic Bax would facilitate breaking down intestinal barrier and enhancing permeability. They could also execute tactics of consuming all local arginine for decreasing the formation of the gas nitric oxide and protecting its own development.


Signs and symptoms

The parasitic disease caused from the infection of Giardia, called Giardiasis, also known as beaver fever. A 10% of Giardiasis can be temperate and showed no sign, which recovery by host’s immune function without extra treatments. Nevertheless, 90% infected hosts show apparent symptoms 2 days after infection and last 1 to 6 weeks. Giardia would make atrophy in the small intestinal villi that would lead to foul- smelling diarrhea and cause dehydration accompanied with malnutrition.


Diarrhea is the most common and dominant sign (in both humans and animals) accompanied by other symptoms include excess gas, abdominal cramps, abdominal pain, weight loss, nausea, vomiting, bloody feces and fever. However, only about 15% of infected hosts would occur fever. Most canines would be less active instead of exhibiting fever. Lesser common cases shows signs beyond intestinal system as itchy skin, hives and swelling of the eyes and joints. Medication should be administrated as soon as possible. Chronic diarrhea could last for weeks or months if untreated. Symptoms such as post-infectious irritable bowel syndrome, lactose intolerance and food allergies may occur and remain even if Giardia were dealt with medication.



  1. Detecting antigens in stool specimens, which is a sensitive and common choice for diagnosing giardiasis
  2. Trichrome stain of fecal specimens. It generally stained fecal films for examining intestinal protozoa including Giardia.
  3. The routine fecal flotation test with additional special zinc sulfate solution could reveal parasites on the specimen slides of the feces.
  4. The enzyme-linked immunosorbent assay (ELISA) testing are about 90% accuracy of identifying G. lamblia.
  5. Microscopic examination may present inconsistent result in examining trophozoites and cysts in infected hosts
  6. The Entero-Test show low sensitivity in diagnosing G. lamblia. Besides, Serological tests are not suitable in detecting G. lamblia.
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  1. Canine:


Fenbendazole and metronidazole are the most common medication for eliminating Giardia in dogs. Medication are usually performed 3 to 10 days. If dogs occur refractory diarrhea, combined treatment of fenbendazole and metronidazole may be necessary.



Once dehydration or severe diarrhea occurred, supportive treatment with highly digestible diet could take place alongside existing drugs.



Furthermore, owners should bath infected canines frequently to wash out Giardia cysts on their hairs.


  1. Human:

Natural immunity:

People without symptoms do not always necessary need treatment as it usually resolves on its own.


Metronidazole is recommended as the first line drug for curing Giardiasis. Besides, nitroimidazole medication such as metronidazole, tinidazole, secnidazole or ornidazole will be used when symptoms is acute, go worse or persist; similarly, the anthelmintic drug albendazole would be used for killing Giardia sometimes. Nonetheless, albendazole may exist some side effects, including neurologic or gastrointestinal problems. Moreover, paromomycin and metronidazole are recommended during pregnancy.

Both nitroimidazoles and albendazole (but not nitazoxanide) show resistance issue on treating patients. The mechanism of causing resistance remain unclear. Other drugs such as quinacrine, nitazoxanide, bacitracin zinc, furazolidone, paromomycin and mepacrine are available when nitroimidazole-resistant occur.



People might suffer lactose intolerant by giardia infection. Hence, milk is forbidden for a few weeks. In 2018, Lalle et al. suggest that taking probiotics in combination with conventional treatment would benefit on curing Giardiasis



Most of the outbreaks of giardiasis are water-borne transmission, and the majority of infection cases are associated with poor sanitation. Thus improving hygiene is the prior consideration for preventing giardiasis.



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