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Giardia Life Cycle

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MVSAT 2024

Giardiasis is a chronic, intestinal, protozoal infection seen around the world in most mammals, many birds, and people too. As such it is a Zoonosis, infection is common in dogs, cats, ruminants, pigs, and the highest rates of infection are seen in young animals. Giardia species are found in mice, amphibians, and warm-blooded vertebrates. The species complexes infecting vertebrates include G Ardea and G Psitta from birds. G MicrotiI from muskrats and voles and G Duodenalis also known as G Intestinalis and G Lamblia which has a wide mammalian host range. As flageolet protozoan trophozoites Giardia attached to the brush border of the intestine, usually the proximal small intestine. Upon encystation in the smaller large intestine cysts then pass in the faces squared as infective and can survive for weeks in the environment.

The pre-Payton period is generally 2 to 10 days, sis shedding may be continued over several days over a week, but is often intermittent, especially in the chronic phase of infection. Within this scheme obviously Giardia via injunction of materials contaminated by feces. Only smaller numbers of cysts are needed to initiate infection. High humidity facilitates the survival of cyst in the environment and animal overcrowding facilitates transmission. A not uncommon means by which humans become infected is through drinking untreated or unfiltered water, while hiking, camping, or even accidentally swallowing a bit of water while swimming. Giardia is present in many waterways and it should be assumed that all water is contaminated and needs to be treated. Coldwater from the mountain stream looks enticing but don’t fall for it. Symptoms may take 1 to 2 weeks to develop but may continue to 6 weeks leading to diarrhea, stomach cramps, nausea, or vomiting, flatulence, and greasy stools. Ensure that water is free of Giardia parasites by using one of these 3 methods

  1. Filter the water with less than one-micron pore size filter.

  2. Boiling water for at least 1 minute at a full boil.

  3. Treating it with chlorine or iodine according to directions.

Gross intestinal lesions are seldom evident, Giardia causes an increase in the epithelial permeability and an increase in the number and activation of T lymphocytes in the gut. This leads to a shortening of the brush border microvilli and causes reduced absorption of water, electrolytes, and nutrients. Together with decreased activity of the small intestinal brush border enzyme, the host suffers from malabsorptive diarrhea and malnutrition. Reduced activity of lipase and increased production of mucin leads to steatorrhea and mucus diarrhea. Clinical signs are often normal, however, Giardia infections in small animals may manifest as chronic weight loss and chronic or intermittent diarrhea particularly in the young. Feces appear soft, pale, malodorous and contain mucus and fat. Watery diarrhea is unusual and fecal blood is usually not observed, however, an animal may vomit. Differential diagnosis includes virtually all other causes of malabsorption and maldigestion such as exocrine pancreatic insufficiency.

Young remanence giardiasis can result in diarrhea, unresponsive to antibiotics, or coccidia stats. It should be suspected when feces are mucoid and pasty and animals less than 6 months of age show reduced weight gain. Giardia diagnosis involves a saline smear of feces that may show model oval trophozoites, unlike yeast or trichomoniasis they have a double nucleus. Fecal cysts 9 to 15 by 7 to 10 microns can be concentrated by the centrifugation floatation technique using zinc sulfate. Identification can be facilitated by iodine staining, because of intermittent excretion 3 fecal examinations should be performed over a period of 3 to 5 days. Specific detection can be enhanced immunofluorescence and commercially licensed techniques optimized for the species of interest. There are no FDA approved drugs for the treatment of giardiasis for dogs and cats. However, fenbendazole has been approved for use in Europe and is used off-label in the US. It has been shown to be safe in pregnant and lactating animals. A praziquantel parental and for battle, the combination has been used for 3 days. Extra label use of metronidazole for 5 days is effective in eliminating Giardia species from about 2 out of 3 infected dogs, but may be associated with the violation and neurological side effects.

Combination or sequential strategies, be sure to bathe the animals to remove cysts have also been tried. Currently, no drug is FDA approved for the treatment of Giardia in ruminants. So off label users would consult with the food animal residue avoidance database for guidance on withdrawal time fenbendazole and albendazole significantly reduce cysts excretion and clinical improvement in calves. Paromomycin for 5 days is also found to be highly efficacious in calves. For production animals and group house domestic animal management is the key to the control of Giardia. Cysts can cause infection and reinfection particularly if housing is crowded. Giardia cysts get infected immediately when they are passed in the feces and survive in the environment. Cysts are said to be a source of infection and also reinfection in the case of animals particularly those in crowded conditions. Feces should be removed from the environment daily, dogs and cats should bathe to remove cysts from the hair coat.

Disinfection with quaternary ammonium and steam or boiling water can inactivate cysts in the environment. To facilitate efficacy solutions should be left greater than 5 minutes before rinsing. Yards and animal runs cannot be disinfected and should be considered contaminated for at least a month after the removal of the last infected animal. Cysts may be inactivated by allowing the surface to dry thoroughly after cleaning. In summary, Giardia is a protozoan seen worldwide that causes chronic intestinal infection in most mammals, many birds, and people. Infection is common in dogs, cats, ruminants, and pigs. The highest rates of infection are seen in young animals. Affected domestic animals can be treated with antiprotozoal agents often in combination, but proper hygiene constitutes the best approach to animals in group housing.   

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Giardia Life Cycle

The cysts are said to be more resistant forms and are responsible for the transmission of giardiasis. Both cysts and trophozoites can be found in the feces at the diagnostic stages. The giardia life cycle is as follows 

  1. Giardia cysts are said to be the infective stage of G. intestinalis. Only as few as 10 cysts will be able to cause infection. These types of cysts are typically ingested by the consumption of contaminated food or water or fecal-orally. They are capable of surviving outside of the body for a period of several months, and they are also relatively resistant to chlorination, UV exposure, and even freezing. The cysts are resistant forms and can survive for several months even in the cold water. Infection can occur by the ingestion of cysts which is present in the contaminated water, the food, or by the fecal-oral route through the hands or fomites.

  2. The time when cysts are ingested, the low pH of the stomach acid will be producing excystation, in which we see the activated flagella will break through the cyst wall. This will usually occur in the small intestine, specifically at the duodenum. Excystation will release the trophozoites, with each cyst producing at least two trophozoites. 

  3. Now within the small intestine, the trophozoites will reproduce asexually and will be multiplied by the longitudinal binary fission, now remaining in the lumen of proximal small bowel where either they will float free or will become attached to the mucosa of the lumen. 

  4. Some of the trophozoites will then encyst in the small intestine. Encystation tends to occur most likely as a result of exposure to bile salts and fatty acids, and also a more alkaline environment. Encystation which usually occurs as the parasites transit toward the colon. The cyst then which is found at is the stage found most commonly in non-diarrheal feces 

  5. Both the cysts and trophozoites will be then passed in the feces, and are very infectious immediately or it will be shortly afterward. There is also the possibility of person-to-person transmission. Giardia can also infect the animals, and the beavers have been widely associated with the giardia outbreaks, although it is not definitively and their importance as a reservoir is unclear.

Clinical Features

The clinical features of Giardia include the below

  1. The incubation period of Giardia infection varies from 1 week to 3 weeks.

  2. The majority of the infections there go unnoticed, that is they remain asymptomatic.

  3. Children are the ones who are most commonly affected, their ratio is about 40 to 80 %.

  4. They suffer from acute Giardia infection or chronic Giardia infection.

  5. Acute infections are associated with watery diarrhea, abdominal cramps, bloating, and flatulence.

  6. An acute episode of Giardia may last for 5 to 7 days, it can resolve on its own or it can go to a chronic stage.

  7. The chronic Giardia is associated with chronic diarrhea with malabsorption of the fat, vitamin A, proteins, and carbohydrates.

  8. There will be weight loss, malaise, nausea, and anorexia.

  9. The complication of chronic Giardia is the growth retardation and weight loss.

  10. There can be delayed milestones in younger children.

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FAQs on Giardia Life Cycle

1. How is Giardia Affecting Our Countries?

Giardia is a particular issue in developing countries and is the main cause of diarrhea like symptoms and abdominal pain. It is a major issue in developing countries because of overcrowding, the unhygienic environment, and poor quality of the water quality control in these areas. In the case of developing countries, the prevalence is said to be the highest in children, we can imagine how it will be a major issue.

2. What are the Different Forms of Giardia?

Giardia intestinalis or lamblia protozoa has 2 forms, there is the Giardia cyst which is the infective form which is about 10 micrometers in diameter and is ovoid cell shape, it contains 4 nuclei in the cytoplasm. The Giardia cyst can become a giardia trophozoite, which is the mobile form that moves around. It is pear-shaped with 8 flagellants and thus it is known as protozoa flagellate. It contains 2 anterior suction disks that allow it to attach itself to the mucosal surface of the intestine and it contains 2 nuclei in the cytoplasm.

3. What are The Different Giardia Species and How is it Systematically Classified?

The parasite belonging to Giardia inhabits the intestinal tract of the vertebrates. There are various Giardia species which affect various animals including human beings, they are as follows

  • Giardia intestinalis, which is also called Giardia lamblia or Giardia duodenalis infect humans and some of the rodents. 

  • Giardia muris, infect the mammals.

  • Giardia agilis, are the ones who infect the amphibians.

  • Giardia psittaci is the one that infects the birds.

  • Giardia ardee, there are the ones that are infecting the rodents.   

The systematic classification of Giardia is as given below

  • Kingdom - Protista

  • Sub-kingdom - Protozoa

  • Phylum - Sarcomastigophora

  • Class - Kinetoplastida

  • Order - Diplomonadida

4. What are Trophozoites and Cysts and What are their Features?

Giardia has 2 morphological forms trophozoites and cyst


  • The trophozoite is a pear-shaped disk with a broad rounded anterior end and tapering posterior end. 

  • The parasite measures 10 to 20 micrometers in length and 5 to 15 micrometers in breadth.

  • The dorsal surface, if you see the parasite from the lateral angle is convex. While the ventral surface is concave. Hence it appears in sickle shape in the lateral view.

  • The ventral surface is occupied by a bilobed adhesive disk. It is through this disk the parasite is going to attach itself to the intestinal mucus.

  • The parasite is bilaterally symmetrical with 2 medium bodies, 2 axonemes, and 4 pairs of flagella.

  • It has 2 nuclei with a central karyosome, giving a character face like appearance to the parasite.

  • 2 axostyles, they run diagonally across the cytoplasm.

  • The presence of 4 pairs of flagella will give characteristic falling leaf motility to the parasite.    


  • This is the infective stage of the parasite 

  • This is oval or ellipsoidal in shape measuring 8 to 12 micrometers in length and 7 to 10 micrometers in breadth.

  • The thick cyst wall that surrounds the cyst will give a clear hallow like structure when we do the permanent staining of the cyst.

  • Cyst consists of a finely granular cytoplasm that is separated from the cyst wall by a clear space.

  • Cyst consists of 4 nuclei which remain in pairs on either side of the axostyle and median bodies. 

  • When we do the iodine mount of the cyst, it will appear brown in color. 

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