How can Giardia lamblia spread through drinking water and affect digestion?

Giardia lamblia is a single-celled parasite that causes the diarrheal illness known as giardiasis. It spreads through drinking water because the parasite can exist in a hardy, nonreplicative cyst stage, which is enveloped in a tough outer shell that allows it to survive in the environment for weeks or months. When a person ingests water contaminated with these cysts—often from the feces of infected humans or animals—the parasite enters the digestive system.1

Once ingested, the cysts release motile trophozoites that adhere to the epithelium of the small intestine. By attaching to the intestinal lining and dividing, the parasite disrupts normal digestive function. This leads to clinical symptoms such as bloating, stomach cramps, and large, foul-smelling, fatty stools. In some cases, particularly in children, the infection can cause chronic malabsorption of nutrients. Treatment typically involves antimicrobial drugs and the management of dehydration.1