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Scientists Sequence Genome Of Intestinal Parasite That Afflicts Hikers And Kids In Daycare

Armen Hareyan's picture

Giardia lamblia is a strange-looking parasite that swims in the gut, spreads through stool, persists in contaminated water, and is responsible for more than 20,000 reported infections a year in the United States. Now it has finally spilled its genetic secrets.

In the September 28, 2007, issue of the journal Science, an international team of researchers led by scientists at the Marine Biological Laboratory (MBL) in Woods Hole, MA, and funded by the National Institute of Allergy and Infectious Diseases (NIAID), one of the National Institutes of Health (NIH), describes the complete genetic sequence of the parasite.

The bane of hikers drinking from mountain streams and of small children in daycare, G. lamblia is the most common intestinal parasite identified by public health laboratories in the United States, according to a 2005 report by the Centers for Disease Control and Prevention. Analysis of its genome has already shed light on the organism's evolution and revealed opportunities for future research.

"Existing drugs can effectively treat people with Giardia infections, but as with many pathogens, the concern is that the parasite will develop resistance to these medications," says NIAID Director Anthony S. Fauci, M.D. "The Giardia lamblia genome shows us that the parasite has a large complement of unusual proteins that are potential targets for new drugs or vaccines."

Giardia spends one phase of its lifecycle in the environment and the other in the gut of an infected human or wild animal. To maintain this dual existence, the parasite has two radically different microscopic forms.

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In water, Giardia exists as a hardy, highly infectious cyst, which can survive for months, even in fresh water devoid of all nutrients. In the gut, Giardia exists in a swimming and feeding form known as a trophozoite.

The awakening of the dormant cyst happens quickly after someone swallows contaminated water or food. After the cysts encounter the warm acidic juices in the stomach, they change into trophozoites. Within about two hours, these trophozoites will be swimming in the intestines.

Unlike many other parasites, trophozoites do not invade tissues or cells. Instead they simply attach to cells, drink in nutrients and multiply. The parasite evades the immune system and persists in the intestine by shifting the proteins it displays on its surfaces.

Giardia performs this molecular chicanery so well that half of all people who are infected are unaware that they even harbor the parasite. Symptoms for the unfortunate other half include nausea, diarrhea, bloating and abdominal cramping. Because trophozoites cling to intestinal cells that absorb fats and nutrients, such infections can lead to severe complications such as poor nutrient absorption and weight loss.

Existing drugs can effectively treat people with Giardia infections, the disease known as giardiasis, but most infections resolve on their own. When trophozoites detach from the intestinal wall, they may swim and reattach to new intestinal cells, or they may pass down the digestive tract and into the bowels, transform back into cysts and be passed through the stools.

The completed genome is a publicly available resource that should help advance research on new ways to treat or prevent Giardia infections because it provides scientists with a comprehensive dossier of the parasite's genes