Giardia: Introduction, Morphology, Life- Cycle, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes

Introduction of Giardia

Giardia is a genus of parasitic protozoa that can infect the small intestine of mammals, including humans. The species that infects humans is called Giardia lamblia or Giardia intestinalis. It is one of the most common causes of waterborne disease in the world and is responsible for causing giardiasis, a diarrheal illness that can last for weeks or months if left untreated.

Giardia is found in the feces of infected animals and humans, and it can be transmitted through the consumption of contaminated food or water, or through direct contact with an infected individual. The parasite is resistant to chlorine and other common disinfectants, which makes it difficult to eliminate from water sources.

Symptoms of giardiasis include diarrhea, abdominal pain, nausea, and vomiting. In severe cases, dehydration and weight loss can occur. Treatment typically involves a course of antibiotics to eliminate the parasite, along with measures to address symptoms and prevent dehydration.

Prevention of giardiasis involves practicing good hygiene, including washing hands frequently and avoiding the consumption of contaminated water or food. Water sources can be treated with filters or other methods to remove the parasite, and in some cases, boiling water can be effective in killing the parasite.

Morphology of Giardia

Giardia is a unicellular organism and has a characteristic teardrop or pear-shaped morphology. It is about 10-20 micrometers long and 5-15 micrometers wide, making it one of the largest protozoan parasites to infect humans.

The organism has two nuclei, which are visible under a microscope, and eight flagella that it uses for locomotion. The flagella are arranged in pairs along the length of the organism and give it a distinctive “falling leaf” motion as it moves.

Giardia also has a unique ventral adhesive disk that it uses to attach to the lining of the small intestine. This disk is composed of microtubules and serves to anchor the organism in place while it feeds and reproduces.

Under electron microscopy, the organism appears to have a “double-walled” appearance, with an outer plasma membrane and an inner membranous sac, called the axoneme. The axoneme contains the flagella and is responsible for the organism’s motility.

Overall, the morphology of Giardia is well-adapted for its parasitic lifestyle, allowing it to attach to the host’s intestinal lining, move through the intestine, and feed on nutrients in the host’s digestive system.

Life Cycle of Giardia

The life cycle of Giardia involves two stages: a trophozoite stage, in which the organism is actively feeding and reproducing in the host’s intestine, and a cyst stage, which allows the organism to survive outside the host and be transmitted to new hosts.

During the trophozoite stage, the organism is motile and attaches to the lining of the small intestine using its ventral adhesive disk. The organism reproduces by binary fission, with one trophozoite dividing into two identical daughter cells. The trophozoites feed on the host’s intestinal contents, including nutrients and mucus.

As the trophozoites near the end of the small intestine, they begin to encyst or form protective cysts around themselves. The cysts are resistant to environmental stresses, such as drying or exposure to disinfectants, and can survive outside the host for weeks to months.

When a host ingests the cysts through contaminated food or water, the cysts pass through the stomach and small intestine and into the large intestine, where they break open and release the trophozoites. The trophozoites then attach to the intestinal lining and begin feeding and reproducing, completing the life cycle.

It is important to note that not all Giardia infections result in symptomatic disease, and some individuals may be asymptomatic carriers of the parasite. Additionally, cyst shedding can continue for several weeks after symptoms have resolved, which can contribute to the spread of the parasite.

Pathogenicity of Giardia

Giardia can cause a diarrheal illness known as giardiasis, which is one of the most common waterborne diseases in the world. The severity of the disease can vary from asymptomatic infection to chronic, debilitating diarrhea.

The pathogenicity of Giardia is thought to be due to a combination of factors, including its ability to attach to the host’s intestinal lining, its production of toxins that damage the intestinal cells, and its disruption of the normal digestive processes in the intestine.

The attachment of Giardia to the intestinal lining can cause inflammation and damage to the cells, leading to diarrhea, abdominal pain, and cramping. The toxins produced by the organism can further damage the intestinal cells and contribute to the symptoms of the disease.

In addition, Giardia can interfere with the absorption of nutrients in the intestine, leading to malnutrition and weight loss, especially in children and immunocompromised individuals.

The severity of giardiasis can be influenced by factors such as the age and immune status of the host, the strain of the parasite, and the dose of the organism ingested.

Overall, the pathogenicity of Giardia is complex and multifactorial, and further research is needed to fully understand the mechanisms underlying the disease.

Lab Diagnosis of Giardia

The laboratory diagnosis of Giardia typically involves the identification of the organism in fecal samples using microscopy or antigen detection assays.


  • Microscopic examination of stool samples using special stains, such as iodine or trichrome stains, can reveal the characteristic teardrop shape of the Giardia trophozoites or cysts.
  • Trophozoites are usually seen in loose or watery stools, while cysts are more common in formed stools.

Antigen Detection:

  • Antigen detection assays, such as enzyme immunoassays (EIAs) or immunochromatographic tests, can detect Giardia antigens in stool samples.
  • These tests are often more sensitive than microscopy and can detect the antigen even in small numbers of parasites.
  • They are also easier to perform and can provide results more quickly than microscopy.

Nucleic Acid Amplification:

  • Polymerase chain reaction (PCR) is a highly sensitive and specific method for detecting Giardia DNA in stool samples.
  • PCR can be used to identify the species and genotype of the parasite, which can be useful for epidemiological studies and tracking outbreaks.

It is important to collect multiple stool samples over several days to increase the chances of detecting the parasite, as the shedding of Giardia cysts can be intermittent.

In addition to laboratory tests, the diagnosis of giardiasis also involves consideration of the patient’s symptoms, travel history, and potential exposures to contaminated food or water.

Treatment of Giardia

The treatment of giardiasis typically involves the use of antimicrobial medications, which can be highly effective in clearing the infection. Commonly used medications include:

  1. Metronidazole: This is the most commonly used drug for the treatment of giardiasis. It is usually given for 5-7 days and can be given to both adults and children.
  2. Tinidazole: This medication is similar to metronidazole and is also highly effective against Giardia. It is usually given as a single dose.
  3. Nitazoxanide: This medication is approved for the treatment of giardiasis in both adults and children. It is usually given for 3 days.
  4. Paromomycin: This medication is an alternative treatment for giardiasis, especially in pregnant women and children. It is usually given for 7 days.

It is important to note that some strains of Giardia may be resistant to certain medications, so treatment should be guided by laboratory testing whenever possible.

In addition to medication, rehydration therapy may be necessary to replace fluids and electrolytes lost through diarrhea. Probiotics may also be helpful in restoring the normal intestinal microbiota after treatment.

It is important to treat giardiasis promptly to prevent complications such as malnutrition and chronic diarrhea, especially in young children and immunocompromised individuals.

Prevention of Giardia

The prevention of giardiasis involves several measures to reduce the risk of infection. These include:

  1. Practice good hygiene: Frequent hand washing with soap and water is important, especially after using the toilet, changing diapers, and before preparing or eating food.
  2. Avoid contaminated water: Drinking water should be from a safe and reliable source. If the safety of drinking water is uncertain, it should be boiled, filtered, or treated with chemical disinfectants before consumption. Avoid swimming in or drinking from untreated water sources such as rivers, lakes, or ponds.
  3. Practice safe food handling: Raw and undercooked foods, especially meat and seafood, should be cooked thoroughly. Fruits and vegetables should be washed thoroughly before consumption.
  4. Avoid close contact with infected individuals: Avoiding close contact with individuals who have diarrhea, especially in childcare settings or communal living situations, can reduce the risk of infection.
  5. Practice safe sex: Condom use during sexual activity can reduce the risk of transmission of Giardia.
  6. Properly disposing of fecal waste: Proper sanitation and waste disposal practices, including the use of toilets and appropriate disposal of diapers, can reduce environmental contamination with Giardia.

Overall, prevention of giardiasis involves maintaining good hygiene practices, avoiding contaminated food and water, and reducing exposure to infected individuals.

Keynotes on Giardia

Here are some keynotes on Giardia:

  • Giardia is a microscopic parasite that can cause diarrheal illness in humans and animals.
  • The organism has two distinct forms: the motile trophozoite and the infectious cyst.
  • Giardiasis is a common cause of waterborne illness and is often associated with contaminated drinking water.
  • Symptoms of giardiasis can include diarrhea, abdominal pain, bloating, nausea, and weight loss.
  • The diagnosis of giardiasis involves laboratory tests such as microscopy, antigen detection, or nucleic acid amplification, as well as consideration of the patient’s symptoms and potential exposures.
  • Treatment of giardiasis typically involves the use of antimicrobial medications such as metronidazole, tinidazole, nitazoxanide, or paromomycin.
  • Prevention of giardiasis involves several measures such as practicing good hygiene, avoiding contaminated water and food, and reducing exposure to infected individuals.
  • Giardiasis can be particularly serious in young children, pregnant women, and immunocompromised individuals, and prompt treatment is important to prevent complications.

Further Readings

  1. Isenberg clinical microbiology procedures Handbook
    2nd edition. Vol. 2
  2. Merkel and Voge’s medical parasitology
    9th edition.
  3. Parasitology: 12th edition
    By K. D. Chatterjee
  4. District laboratory practice in Tropical countries –Part-I.
    By Monica Chesbrough.
  5. Atlas of Medical Helminthology and protozoology -4th edn  -P.L.  Chiodini, A.H. Moody, D.W. Manser
  6. Medical Parasitology by Abhay R. Satoskar, Gary L. Simon, Peter J. Hotez and Moriya Tsuji

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