Giardia A Common Intestinal Parasite

Giardia A Common Intestinal Parasite


A 36-year-old single male schoolteacher was seen in the office with a complaint of diarrhea over the past 4 weeks and a 12-pound weight loss. His normal pattern of bowel movements was once a day. Now he has to leave the classroom several times a day because of crampy abdominal pain and loose urgent stools. He was having 4 to 6 foul-smelling yellow bowel movements a day, sometimes at night. There was no blood in the stools. He did notice some early morning nausea. He lived alone and enjoyed camping in the mountains with his girlfriend. On occasion he did fill his canteen with mountain spring water. A flexible sigmoidoscopy "scope" test done in the office was normal. He was asked to submit a fresh morning stool specimen to the hospital laboratory for analysis. The lab called a few days later and reported that he had a heavy infection with the parasite, Giardia. Anti-parasitic treatment was begun and he returned to normal in about 10 days. He was advised to avoid drinking from streams and rivers. He has had no further symptoms.


What is Giardia?


This recent patient is an excellent example of an infection with Giardia (gee AR dee ah), a microscopic parasite common in Western Pennsylvania. It is a simple single celled organism about the size of a red blood cell. This parasite lives in the intestinal tract of mammals such as humans, dogs, cats, bears, and beaver. (This explains the common name, "beaver fever.") Although Giardia was first described in the 1800's, it was not recognized to be a cause of disease until 1981. It is now recognized as the most common cause of non-bacterial infectious diarrhea in North America.

Where does Giardia come from?

Studies have also shown that, unlike many other pathogens, Giardia is not host specific. This means that Giardia cysts excreted by animals can infect and cause illness in humans. Giardia is found in water which has been infected with the stool or feces of infected animals or humans. Outside of the body, Giardia is protected by an outer shell called a cyst. In this form, Giardia can survive for long periods of time. To become infected, a person must eat or drink food or water which contains the Giardia cysts. A common source of infection is water from springs or rivers which is contaminated with the feces of animals. Giardia cysts are far harder to kill with chlorine or iodine than bacteria or viruses. When viable cysts enter the digestive tract, they form active parasites, feed, and multiply. More cysts are passed in the individual's stool making it contagious if bathroom hygiene is lax.

What are the symptoms of Giardia infection?

A few people may harbor the infection and have no symptoms. In the U.S., it is estimated that 2% of the population are carriers. Even thought these individuals have no symptoms, they can still be contagious and pass the infection onto others. However, most individuals with Giardia have symptoms. These may include yellow foul-smelling frothy diarrhea, fever, cramps, gas, and weight loss. Usually the symptoms begin about a week after exposure, last a few weeks and then subside spontaneously. Sometimes, however, the infection becomes persistent with the parasite setting up permanent residence in the upper small intestine.

Who can develop Giardia infection?

Anyone is at risk, but children, the elderly, and those with compromised immune systems seem most vulnerable. Anyone exposed to animal or human feces is at increased risk. Drinking contaminated water from rivers and lakes during summer-time camping trips is a common source. Caregivers and children in daycare centers are at increased risk of infection. Children are especially susceptible because they put so many things into their mouths. It doesn't take much. Unlike bacterial infection which requires exposure to tens of thousands of bacteria to cause disease, studies have shown that as little as one or two Giardia cysts can transmit the disease.


Until recently, the diagnosis depended on finding Giardia cysts excreted in the stool, or feces. This was somewhat difficult since searching stool specimens for the tiny cysts was not always successful. Now we have a Giardia antigen assay that can detect the parasite with much more sensitivity and accuracy. If your doctor suspects Giardia as a possible cause of your symptoms, he will request that a fresh stool specimen be brought to the hospital laboratory for such an analysis.


In most otherwise healthy individuals, Giardia is usually cleared without specific treatment in less than a month. For those with persistent symptoms or carriers, medical treatment is usually effective. In adults, Flagyl (metronidazole) is most effective in treating Giardia infections. The normal dose is 250mg four times a day for 7 days with an initial success rate of about 85%. You cannot drink any alcohol during Flagyl treatment. There are other antibiotics available.

Lactose Intolerance

Sometimes, the lining of the small intestine is damaged by the infection. This may lead to digestive difficulties even after the infection is gone. In fact, about 40% of those who are infected with Giardia develop problems with lactose (milk sugar) intolerance. This may last up to 6 months after the infection is treated. Symptoms of lactose intolerance may include diarrhea, gas, cramps, and bloating after the ingestion of milk products.

Preventing infection

Simple common sense precautions are helpful in preventing infection. Always wash hands carefully before handling food or dishes, especially after using the toilet, gardening, changing diapers, or handling pets. Teach your children to wash their hands before eating and after using the bathroom. Avoid unpasteurized milk and dairy products. Clean surfaces where diapers are changed after every use. Don't swallow water from lakes, rivers, pools or Jacuzzis. Water swallowed accidentally while swimming may contain the organism. Never cook for other people if you have diarrhea. Take care when traveling in developing countries. Foods and drinks, in particular raw fruits and vegetables, tap water or ice made from tap water, unpasteurized milk or dairy products, and items from street vendors may be contaminated. When a community outbreak occurs through a contaminated water supply, the local health department issues a Boil Water Order. Although the Giardia cyst is resistant to chlorine, it is very sensitive to heat. Bringing water to a rolling boil for three minutes is an excellent way to ensure that it is safe for drinking, cooking, brushing teeth, etc. After it cools, put it in clean containers with a lid in the refrigerator. Coffee makers do not get hot enough to kill Giardia. Bottled water may be used during such an alert, although there are not universal standards as to bottled water purity. Chemical disinfectants and microfine filters are also available to purify contaminated water. Portable water filters can be used at home or by campers to remove Giardia from the water. Read the label and be sure that it is rated effective for removing Giardia. Food that is well cooked does not pose a risk of infection.

Text & Images Courtesy of Three Rivers Endoscopy Center
© Dr. Robert Fusco, Three Rivers Endoscopy Center, All Rights Reserved

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