Giardia in the Intestine: The Parasite and Giardiasis
An Intriguing Organism
Giardia is an interesting and unusual parasite that infects humans and animals. It causes a very unpleasant disease called giardiasis. Symptoms 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 big effect when present in sufficient numbers. Researchers have made some intriguing discoveries about the parasite's structure and about its behaviour in the small intestine, where it lives. These discoveries may lead to better treatments for giardiasis.
The stomach leads to the small intestine, which consists of the duodenum, jejunum, and ileum. 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.
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. The parasite has a pear-shaped body. The eight thread-like structures extending from the body are called flagella. The flagella 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, however.
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.
Living and Moving Giardia lamblia (No Sound)
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. Instead, they carry out reactions involving iron and sulphur. 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)
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 often notice what looks like the face in a magnified picture of a Giardia cell. The two "eyes" in the face 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
- watery diarrhea, which may have a strong odour
- soft, greasy feces that may float
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. If the symptoms 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 within 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.
Giardiasis in Children (or Adults)
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. 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.
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, however. 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 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.
Researchers at the University of East Anglia have 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 two families of proteins. The scientists say that one of these families "mimics" human proteins known as the 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 university's research was done in lab equipment and not in the human body, however. The experiment may or may not reflect real-life conditions.
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. The understanding may lead to better treatments or prevention methods for giardiasis, however.
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 behaviour could be very important with respect to giardiasis treatment.
Information about the Giardia parasite and its effects from the CDC
Facts about Giardia from Stanford University
Intestinal protozoa (including Giardia) from Tulane University
Mitosomes in Giardia intestinalis (abstract) from Springer
Information about encystation in the parasite from the American Society for Microbiology
A project for investigating cyst production from the USDA
Facts about Giardia in swimming pools from the CDC
Parasite mimics human proteins from the University of East Anglia
New discoveries about how the parasite makes us ill from the BBC (British Broadcasting Corporation)
Questions & Answers
© 2018 Linda Crampton