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Giardia in the Intestine: Parasite Facts and Giardiasis Disease

Linda Crampton is an experienced teacher with a first-class honors degree in biology. She writes about the scientific basis of disease.

An Intriguing Organism

Giardia is an interesting and unusual parasite that infects humans and animals. It causes a very unpleasant disease called giardiasis. Symptoms often include diarrhea, abdominal cramps, and nausea. The worst symptoms last for about a week, but an infected person may not feel completely well for weeks or even months. Occasionally, the condition may be a long-term problem.

The parasite is microscopic but can have a major effect on us when it's present in sufficient numbers. Researchers have made some intriguing discoveries about the organism's structure and about its behavior in our small intestine, where it lives. These discoveries may lead to better treatments for giardiasis.

Giardia can infect the small intestine.

Giardia can infect the small intestine.

The stomach leads to the small intestine, which consists of the duodenum, jejunum, and ileum. The ileum is continuous with the large intestine. The large intestine leads to the anus, where feces or stool is released. Giardia lives in the small intestine and passes out of the body through the large one.

Features of Giardia

Giardia is a single-celled organism. Giardia lamblia (also known as G. intestinalis and G. duodenalis) infects humans. It has an interesting appearance under a scanning electron microscope, as shown in the first photo in this article. The parasite has a pear-shaped body. The eight thread-like structures extending from the body are called flagella. They enable Giardia to move. The parasite has a concave disk on the lower surface of its body that enables it to attach to cells in the intestine.

Giardia is anaerobic, which means it doesn't require oxygen in order to survive. Unlike some anaerobes, it's also aerotolerant. If oxygen is present in its environment, it isn't harmed by the chemical.

The flagellated form of the organism is called a trophozoite. The trophozoite seems to feed mainly on glucose, which it obtains from the lumen (central cavity) in the intestine. The glucose ultimately comes from the food that we eat. The food is digested in the small intestine. Glucose is one of the products of digestion. It's absorbed through the lining of the intestine, enters the bloodstream, and travels to our cells. The cells break the glucose down to produce energy. Glucose may not be the only nutrient absorbed by the parasite.

Giardia muris is found in mice. The concave disk is also found in other members of the genus.

Giardia muris is found in mice. The concave disk is also found in other members of the genus.

The name "Giardia" honours Alfred Mathieu Giard, a French zoologist who lived from 1846 to 1908. He studied parasites, including the organism now known as Giardia lamblia.

Prokaryotes, Eukaryotes, and Giardia Cells

Eukaryotic cells contain structures surrounded by membrane; prokaryotic cells lack internal membranes. Bacteria and archaea (which were once classified as bacteria) are prokaryotes. Other organisms, including humans and Giardia, are eukaryotes. Despite their so-called primitive structure compared to eukaryotes, prokaryotes can perform all of the activities necessary for a successful life.

Until quite recently, Giardia was thought to lack some of the structures found in other eukaryotes. As a result, it was believed to be an intermediate organism linking prokaryotes and eukaryotes. Researchers have now discovered that the parasite has some of the "missing" structures, though they are in a simplified form. The parasite may have developed its unusual features as an adaptation to its lifestyle and may not be as primitive as was once thought.

Giardia parasites in our body are most often attached to the lining of the small intestine. They are also found in the liquid in the lumen of the intestine, however. They swim through liquids in a process known as falling leaf motility, which is shown in the video above. The parasite looks as though it's tumbling through the liquid.

Unusual Cell Structure

A Giardia cell contains two nuclei. The nucleus is the organelle that contains the genetic material of an organism. (Organelles are structures with specific functions and are surrounded by membrane.) Most eukaryotic cells contain one nucleus. The reason why Giardia has two and the similarities and differences between them are not yet clear.

For a while, it was thought that Giardia lacked mitochondria. Mitochondria are organelles that produce most of the energy needed by a eukaryotic cell. Researchers now know that the parasite contains simplified mitochondria, which are called mitosomes.

Like mitochondria, mitosomes are enclosed by a double membrane. They don't make ATP (adenosine triphosphate), the energy-storing molecule produced by fully developed mitochondria, however. While it might be tempting to suspect that mitosomes produce energy for the parasite as mitochondria do, at the moment, their only confirmed function is to make complexes containing iron and sulfur. Unlike mitochondria, mitosomes have no genes.

Giardia lacks the typical Golgi complex found in eukaryotic cells. The complex is also known as the Golgi body and the Golgi apparatus. It receives, processes, and packages materials. The processed materials are sent to their destination in vesicles. Researchers have discovered that a system resembling that of the Golgi complex exists during the encystation process in Giardia.

The hallmark of encystation is the biogenesis of encystation-specific vesicles (ESVs), which transport cyst-wall materials that later merge with the plasma membrane and lay down the cyst wall.

— USDA (United States Department of Agriculture)

Giardia sp. attached to the intestinal lining of a gerbil (left); a Giardia cyst (right)

Giardia sp. attached to the intestinal lining of a gerbil (left); a Giardia cyst (right)

Trophozoites photo by Dr. Stan Erlandsen, CDC Public Image Health Library, public domain license; cyst photo by Joel Mills, Wikimedia Commons, CC BY-SA 3.0 license

Life Cycle of the Parasite

While the parasite is inside our body, some individuals move towards the large intestine and become encysted. A cyst consists of an inactive form of an organism surrounded by a protective covering. It's activated in a suitable environment. The cysts and some of the active parasites (the trophozoites) leave the body in the feces.

The trophozoites don't live for long once they are outside the body, but the cysts can survive for several months under the right conditions. Uncooked food, water, or objects that are contaminated with feces from an infected person can transfer the cysts into another person's body via contact with the mouth. Non-living objects that can transmit an infectious organism are known as fomites.

Once the cysts are swallowed, the acidic conditions in the stomach start to weaken their protective wall. Each cyst releases two trophozoites in the intestine. The trophozoites eventually reproduce by a process called binary fission. In this process, a single trophozoite divides to make two trophozoites. The life cycle of Giardia is summarized below.

People sometimes see what appears to be a face in a picture of a Giardia cell. The cell is shown in the illustration below. The two "eyes" are the nuclei. The hair and whiskers are the flagella.

Possible Symptoms of Giardiasis

Giardiasis can be found around the world and in other mammals besides humans. It's more common in developing countries but isn't limited to them. Not everyone with Giardia in their intestine exhibits symptoms. Those who do get sick may feel very uncomfortable, however. Possible symptoms of giardiasis include:

  • abdominal cramps
  • bloating
  • flatulence
  • nausea
  • watery diarrhea, which may have a strong odor
  • soft, greasy feces that may float
  • fatigue

Someone with these or any other unexplained symptoms should visit a doctor for a diagnosis and treatment recommendations. Only some of the symptoms listed above may be present in a particular patient with giardiasis. In addition, Giardia may not be responsible for the symptoms that are present. If they are due to a Giardia infection, a doctor may prescribe an antibiotic. Symptoms that last for a long time may cause weight loss, malnutrition, or lactose intolerance.

Giardia can be found in every region of the U.S. and around the world.

— CDC (Centers for Disease Control and Prevention)

Why Can Giardia Cause Lactose Intolerance?

Researchers know that Giardia can cause damage to the intestinal lining. Villi are folds on the lining that increase the surface area for nutrient absorption. A Giardia infection can sometimes cause the villi to atrophy.

Lactase is an enzyme that digests a carbohydrate called lactose found in dairy products. The enzyme is secreted by cells in the intestinal lining. Lactose intolerance is a condition in which lactase is unavailable. As a result, lactose from food enters the large intestine without being digested. Here certain bacteria break the lactose down, producing gas.

Giardia may cause lactose intolerance by damaging the cells that make lactase. Fortunately, the intestinal lining has the ability to regenerate, so removing the parasites may eventually cause the villi to regrow and lactose intolerance to disappear.

Preventing Giardiasis

According to health experts, the steps listed below are important for preventing a Giardia infection. They are also good steps to prevent some other infectious diseases and should really be a regular part of our lives.

  • Wash your hands after coming into contact with feces from a human or an animal.
  • Wear gloves or other protection in situations where contact with feces may occur accidentally, such as during gardening.
  • Make sure that you wash your hands after using a toilet or changing a diaper.
  • Always wash your hands before preparing food and before eating.
  • Don't drink raw or undercooked food in high-risk areas. Deciding which areas are high-risk requires some research, especially for travellers.
  • Don't drink untreated water or eat ice cubes made from this water. Tap water may not be safe in some countries. Research is necessary.
  • Avoid drinking untreated water during activities such as hiking.

People who have been diagnosed with giardiasis or who have diarrhea should avoid entering water that surrounds other people, such as the water in swimming pools and hot tubs. An infected person may accidentally release feces and parasites into the water. Even small quantities of feces can be dangerous. According to the CDC, Giardia cysts can survive for up to forty-five minutes in swimming pool water, even if the water has been properly chlorinated. If a healthy person swallows infected water, they may get sick.

Swallowing even a small amount of pool water that has been contaminated with the Giardia germ can make you sick.

— Centers for Disease Control and Prevention

A Zoonotic Disease and Beaver Fever

A zoonotic disease is one that can be passed from an animal to a human. Giardia lamblia is said to be zoonotic since it infects dogs, cats, and other mammals, as well as humans. The risk of us being infected by a sick pet is considered to be small. There are multiple strains of the parasite within the species. Strains that infect our pets are generally different from the ones that infect us. Health experts say it's possible that some strains can be transferred from pets to humans, however. It's, therefore, a good idea to use techniques to prevent infection when helping a sick pet, as it is when helping a sick human.

Some strains of the parasite may be transmitted to humans via feces from wildlife. Giardiasis is sometimes known as beaver fever. The name arose from an outbreak of giardiasis after hikers in Banff National Park in Alberta drank water contaminated with beaver feces. The name "fever" isn't really appropriate, however. A low-grade fever is a possible symptom of giardiasis, but it isn't very common.

A Potential Effect of Giardia on the Gut Lining

Understanding Giardia's actions in the intestine may not only be interesting biologically but may also be useful in the search for better treatments for giardiasis. It would be helpful to know exactly how the parasite causes symptoms of ill health.

A team consisting of researchers from multiple universities has made a discovery that may be significant. They added Giardia to lab cultures containing cells from the intestinal lining. They found that the parasite made chemicals from different families of proteins. The scientists say that the chemicals from one of these families resemble human proteins known as tenascins. Tenascins in our body help to control cell adhesion and the separation of cells.

The scientists found that the tenascin-like proteins from Giardia caused the separation of intestinal cells that were joined. This led to the release of chemicals from inside the cells, which bacteria in the intestine might be able to use as nutrients. If these bacteria are harmful, giving them food might allow them to multiply and cause symptoms of giardiasis to be worse.

It may eventually be possible to give patients with giardiasis a treatment that neutralizes the parasite's proteins, helping people to recover from the illness. It should be noted that the research was done in lab equipment and not in the human body. The experiment may or may not reflect real-life conditions.

A stained preparation of Giardia lamblia from a patient

A stained preparation of Giardia lamblia from a patient

The Importance of Dealing With the Parasite

The recent discoveries about Giardia's structure and activity are fascinating from a biological point of view. Dealing with the effects of the parasite is probably more important than understanding its biology for people with an infection. Understanding biology is important for researchers and medical practitioners, though.

The parasite infection can sometimes be severe or long-lasting. Although complications are rare, symptoms sometimes appear outside the intestine. These symptoms include urticaria (hives), spasms of the airways, and reactive arthritis. Many details about Giardia's structure and activity are still unknown. Learning more about the parasite and its behavior and creating improved giardiasis treatments are worthy goals.


  • Information about the Giardia parasite and its effects from the CDC (Centers for Disease Control and Prevention)
  • Facts about Giardia from Stanford University
  • Intestinal protozoa (including Giardia) from Tulane University
  • Mitosomes in Giardia intestinalis from Springer
  • Information about encystation in the parasite from the American Society for Microbiology
  • Facts about Giardia in swimming pools from the CDC (Centers for Disease Control and Prevention)
  • The parasite secretes tenascins from Oxford Academic
  • New discoveries about how the parasite makes us ill from the BBC (British Broadcasting Corporation)

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

Questions & Answers

Question: Can Giardia grow on facial skin?

Answer: The parasite lives in the intestine and causes intestinal problems. According to the CDC (Centers for Disease Control and Prevention), it may occasionally cause itchy skin, hives, or eye and joint swelling. This isn’t because the parasite has spread to other parts of the body. Giardia doesn’t live in the skin. From its habitat in the intestine, however, it may produce chemicals or trigger reactions that affect other areas.

Question: Can Giardia survive on clothes?

Answer: Yes, it is possible if the clothing is contaminated with infected faeces. It’s important to handle the clothing with protective gloves and to carefully dispose of the gloves afterwards.

The website of the CDC (Centers for Disease Control and Prevention) says that clothing should be washed daily while a person or pet in the home has a Giardia infection. They also say the clothing should be washed in a washing machine and then dried in a clothes dryer at the highest heat setting for thirty minutes. They say that if a clothes dryer isn’t available, the clothes should be thoroughly dried under direct sunlight.

© 2018 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on June 01, 2018:

I appreciate your visit and comment, Peggy. The parasite is definitely worth studying.

Peggy Woods from Houston, Texas on June 01, 2018:

The fact that the Giardia parasite can cause long lasting effects is good reason to keep studying this organism. This was fascinating to read. Your tips on preventing this disease is well worth heeding.

Linda Crampton (author) from British Columbia, Canada on March 26, 2018:

Thanks, Peg. I know what you mean about the parasites being creepy!

Peg Cole from North Dallas, Texas on March 26, 2018:

You always present a fascinating study of life's organisms. Creepy little things, these Giardia.

Linda Crampton (author) from British Columbia, Canada on February 13, 2018:

Thanks for the comment, Nikki. I hope you never experience giardiasis. I hope I never do, too!

Nikki Khan from London on February 13, 2018:

Very interesting hub on Giardia,,have read about it first time.

Will keep in mind about this.

Thanks for sharing Linda.

Linda Crampton (author) from British Columbia, Canada on February 12, 2018:

Hi, Natalie. I'm going to continue being careful as well. The thought of experiencing giardiasis is not pleasant!

Natalie Frank from Chicago, IL on February 12, 2018:

Very informative. I've never heard of this before but from now on will be extra careful. Thanks for the article.

Linda Crampton (author) from British Columbia, Canada on February 11, 2018:

Thank you very much, Thelma.

Thelma Alberts from Germany on February 11, 2018:

Very interesting hub. I have not heard about this parasite before, not even the name of it. You have done a lot of research about this and it is very informative. Well done. Thanks for sharing.

Linda Crampton (author) from British Columbia, Canada on February 10, 2018:

Thanks for the comment, Bede. The face is hard to forget!

Bede from Minnesota on February 10, 2018:

Thank you for your research and for raising greater awareness to these little monsters. It brings home the importance of good education and better facilities for good hygiene. The smiley faces made me laugh.

Linda Crampton (author) from British Columbia, Canada on February 09, 2018:

Hi, Larry. The parasite does have a strange appearance! I think it's a very interesting creature.

Larry Rankin from Oklahoma on February 09, 2018:

It even looks scary, lol.

Great hub!

Linda Crampton (author) from British Columbia, Canada on February 09, 2018:

Hi, Devika. I hope researchers discover more facts about the parasite soon. It can be a big problem.

Devika Primić from Dubrovnik, Croatia on February 09, 2018:

An interesting insight to this research. A way of life that most people ignore. Facts are the proof of the underlining problem and you shared with a lot of thought and time.

Linda Crampton (author) from British Columbia, Canada on February 08, 2018:

Thank you for the visit and the comment, Alan. I think that education is very important, especially in today's world.

jonnycomelately on February 08, 2018:

Really well written and most informative, Linda, thank you.

I am always inspired by the research undertaken by people around the world. It takes good basic primary education; enlightened teachers encouraging secondary students through to their university years; students willing to put discipline followed by effort, sense of wonder and curiosity into their studies and work.

A breath of fresh air in this climate of doubt and mistrust.

Linda Crampton (author) from British Columbia, Canada on February 08, 2018:

Thank you very much, Audrey!

Linda Crampton (author) from British Columbia, Canada on February 08, 2018:

Hi, Dora. Thanks for the visit. It is important to avoid the parasite whenever possible. It would be great if nobody had to suffer from the effects of an infection.

Audrey Howitt from California on February 08, 2018:

What a great article on a nasty little critter!

Dora Weithers from The Caribbean on February 08, 2018:

Thank you for this lesson on giardia. Some of it is over my head, but I understand the danger of having this parasite in the body, and the importance of avoiding it. You do much to keep us in good health.

Linda Crampton (author) from British Columbia, Canada on February 08, 2018:

Thanks for the comment, Bill. I always appreciate your visits and kindness.

Linda Crampton (author) from British Columbia, Canada on February 08, 2018:

Hi, Mary. Going from one country to another must be very interesting. The precautions that you follow sound great.

Bill Holland from Olympia, WA on February 08, 2018:

The only word in the title I understood was intestine, but you managed to explain it all in a way even my mind could grasp. Very interesting, Linda!

Mary Norton from Ontario, Canada on February 08, 2018:

This is really informative especially because we work in many countries. On rule I follow is cook everything and just stay away from fresh things. The first thing we do is get a kettle and boil the water. However, I'll be even more careful now.

Linda Crampton (author) from British Columbia, Canada on February 07, 2018:

That definitely does sound scary, Melody! I'm glad your mother got treatment. Thanks for sharing the information.

Melody Lassalle from California on February 07, 2018:

This is so informative! I can speak to the health dangers of Giardia. A year ago, my mom had some mild digestive issues. Then one day, she felt very ill as if she had a stomach virus. She ended up in ER at the end of the day because of shortness of breath.

After several tests, they diagnosed her with Giardia. Since she hadn't felt well for a couple of weeks, it was hard to pinpoint when and where she might have contracted it.

I've gone camping many times in my life and always thought of Giardia in terms of mountain water. I never dreamed someone might catch it outside that setting. Live and learn, as they say.

It took almost a month on a bland diet before her digestive tract was back to normal. It was a very scary episode!

Linda Crampton (author) from British Columbia, Canada on February 07, 2018:

Hi, Flourish. Sushi can be dangerous if it's not prepared by an expert. I've read that expert sushi chefs know how to freeze the fish in order to kill parasites. Specific conditions are required to ensure that the fish is safe to eat.

FlourishAnyway from USA on February 07, 2018:

This type of article makes me want to hose down in bleach water just to self-sanitize and never touch another person’s hands. Why is it safe to eat sushi that’s raw? Certainly there has to be contact with fish poop.

Linda Crampton (author) from British Columbia, Canada on February 07, 2018:

Thank you very much for such a kind comment, Jackie! I always appreciate your visits. I agree with your comment related to hygiene. I love my dog very much and always let him lick me when he wants to—except on my face (although he does reach it occasionally). I'm very mindful that a dog's understanding of hygiene isn't the same as a human's! Avoiding contact with the face is probably even more important in the case of young children with immature immune systems.

Jackie Lynnley from the beautiful south on February 07, 2018:

Wow, that is a lot of information!

Proves my point though I know many will not agree with but I hate to see parents let dogs lick their babies (and young children) in the face and mouth. Surely there is feces all in their mouths and it could be very serious.

I guess there are more chances they won't catch something like this but who knows?

Great stuff, Linda. You are a masterful writer!