The fluids. These cysts are excreted in the

The anaerobic parasite Giardia lamblia is one of the most common intestinal protozoans in the United States. In this lesson, we examine the major characteristics of Giardia and discuss treatment and prevention options.

Leeuwenhoek and Giardia

One of the first things students of cell biology learn is that Anton van Leeuwenhoek ground glass into magnifying lenses and observed tiny organisms in water droplets. He called the organisms animalcules, and for this discovery, he gained the title of Father of Microbiology.

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But there is another story about Leeuwenhoek that you might not be familiar with. In 1681, he suffered from an acute bout of severe diarrhea. You can probably guess what happened next. Leeuwenhoek looked at it through one of his magnifying lenses and discovered that his fecal material was teeming with tiny, swimming organisms. Using his crude microscope, Leeuwenhoek was most likely the first person to see the protozoan Giardia.

Giardia lamblia

Giardia lamblia is an anaerobic protozoan parasite commonly found in water sources. There are many distinct species of Giardia, usually specific to different species of vertebrates. For this lesson, we will focus on G. lamblia because it is the major species affecting humans and associated mammals.Giardia transitions through two life stages: trophozoite and cyst.

The trophozoite is the actively dividing, vegetative stage of the protozoan. If you look at a Giardia trophozoite through a microscope, several interesting things are visible. First, the trophozoite appears to be looking back at you. Giardia trophozoites are diplomonads, meaning they have two nuclei.

The cell is shaped like a teardrop with the two nuclei resembling eyes. At the narrow point of the teardrop, Giardia has several pairs of flagella, which provide motility. On the bottom surface of the cell is a disc-shaped sucker used for attachment. But we’ll come back to that in a few minutes.The Giardia cyst is an oval-shaped resistant structure with a very thick wall. These cysts are dormant structures that Giardia uses to survive the harsh aerobic conditions encountered while outside of the host. The cysts are highly resistant to desiccation and chemical disinfection.

They are almost completely immune to the effects of chlorine, making them difficult to eradicate from water supplies that use solely chlorine for sanitation.


Let’s examine the lifecycle of a typical Giardia cell. We’ll start with a beaver swimming around underwater while damming a creek. The beaver invariably swallows some creek water contaminated with Giardia cysts.

The cysts are able to pass through the stomach intact, but the low pH initiates excystation, or the process of reactivating from the cyst form. In the duodenum of the small intestine, the trophozoites emerge and multiply very rapidly by binary fission. Those ventral discs come in handy now for attachment to the intestinal walls. Here the trophozoites take in nutrients and divide, releasing many new infectious cysts into the intestinal fluids. These cysts are excreted in the feces of the beaver and can immediately infect a new host.For this example, I used the beaver for a very good reason. The beaver is considered the primary reservoir host of Giardia.

A reservoir host is a nonhuman host that serves as a means of sustaining an infectious organism as a potential source of human infection. It makes sense that the beaver, along with the muskrat, would be effective reservoir hosts, as they are both primarily aquatic. These species can be continuously reinfected while shedding cysts. In fact, Giardiasis is often called ‘Beaver Fever.

‘ But in actuality, any bird or mammal can play the role of the beaver in the Giardia infection lifecycle.

Infection and Prevention

So how do humans acquire Giardia and the associated disease Giardiasis? Drinking contaminated water and the fecal-oral route are the most common pathways of entry. An estimated 97% of surface waters contain cysts, including rivers, streams, lakes, ponds, and even swimming pools.

Any waterway that is home to animals or contaminated with feces probably contains Giardia cysts.Campers are usually considered high-risk because they drink from natural water sources. Children in day care are also considered high-risk because of the high probability of fecal-oral contamination and transmission. There have also been cases of individuals acquiring infections through sexual contact.What is the best way to prevent Giardiasis? The number one preventative is to filter, then boil your drinking water. The addition of chemical treatments with iodine and chlorine will further improve sterilization. Also, proper hygiene such as thorough hand-washing can eliminate many of the risks associated with fecal-oral transmission.


Suppose a couple weeks ago, you were out for a hike, got really thirsty, and drank water from a stream without boiling it. The water contained Giardia cysts that traveled through your digestive tract. When the cysts reached your stomach, the trophozoites began to emerge from the cysts.

Once they reached the small intestine, the trophozoites used their ventral discs to attach to the intestinal lining. Now you don’t feel so great.Acute illness develops after about a seven-day incubation in the small intestine. At that point, things get pretty severe. For several days, you experience explosive and foul-smelling, watery diarrhea accompanied by excessive, foul-smelling gas.

Giardiasis is usually not accompanied by blood or mucous in the stool, helping to differentiate it from viral or bacterial gastrointestinal illnesses. There is the possibility of dehydration and malnutrition, so some patients require IV fluids and nutrients. This acute stage can last anywhere from one to three weeks.After the acute stage, there are some prolonged symptoms. Abdominal pain, nausea, and occasional diarrhea can last for another two to ten weeks. During both phases, you are constantly shedding infectious cysts and can continue to shed cysts as a symptom-free carrier.Exposure to Giardia doesn’t guarantee disease.

You usually have to ingest over 25 cysts to produce an active infection, and most people never develop symptoms even if they do consume that many. But I still wouldn’t recommend drinking stream water.

Diagnosis and Treatment

How can we determine if these symptoms are Giardia? It used to be common to mimic Leeuwenhoek and simply look for Giardia in a fecal sample. Now it is much easier and more pleasant to the lab technician to check a patient’s blood for antibodies to Giardia using a simple lab test.Let’s assume you test positive for Giardia.

Now what? Well, your prognosis is excellent. In most cases the infection will run its course without medical intervention. In more severe cases, the antibiotic Metronidazole has been effective at reducing symptoms and shortening duration. Quinacrine is also effective but can cause side effects, so its use is reserved for cases that are resistant to Metronidazole. Whether the illness is treated or not, patients can still act as carriers and shed cysts for years.Scientists estimate that 2% of adults and 6 to 8% of children in developed countries, like the United States, have been infected with Giardia. In developing countries, it is even higher, with estimates that 33% of people have had Giardiasis.

This makes Giardia the most common intestinal protozoal infection in humans in the United States.

Lesson Summary

It’s time to review. In this lesson, we looked at the protozoal parasite Giardia lamblia. This parasite is found in natural waterways and can cause severe gastrointestinal disease in infected organisms. The trophozoite stage is vegetative, living and reproducing in the small intestine of hosts. The cyst stage is dormant, infectious, and excreted in the host’s feces. The cyst is also the form found in natural waterways, where it can lay in wait to be consumed by a new host.

The beaver acts as a major reservoir host, maintaining Giardia in waterways.Most people acquire Giardia by drinking water containing cysts. The disease Giardiasis, characterized by severe diarrhea, is usually allowed to run its course but occasionally requires antibiotics and IV fluids. Proper boiling and filtration of drinking water is enough to kill or eliminate cysts and is the most effective method of prevention.

Learning Outcomes

When you have finished this lesson, you should be able to recognize the symptoms of Giardiasis, explain how Giardia lamblia causes the disease, and identify how to prevent infection.


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