Giardiasis is caused by an infection with a microscopic parasite called *Giardia duodenalis* (also known as *Giardia lamblia* or *Giardia intestinalis*). This parasite is a protozoan, meaning it is a single-celled organism that can live and multiply inside the intestines of humans and animals.
The primary cause of giardiasis is the ingestion of *Giardia* cysts, which are the infectious form of the parasite. These cysts are hardy and can survive for long periods outside the body, especially in water or on surfaces contaminated with feces from an infected person or animal. When a person swallows these cysts, usually through contaminated water, food, or direct contact with an infected individual, the cysts travel to the small intestine. There, the acidic environment of the stomach triggers the cysts to transform into their active form called trophozoites, which then attach to the lining of the intestines and multiply, causing infection.
The main routes through which giardiasis spreads include:
– **Contaminated water:** Drinking or swallowing water from lakes, rivers, streams, or poorly treated municipal water supplies that contain *Giardia* cysts is a common way people become infected. Because the cysts are resistant to chlorine and other disinfectants used in water treatment, outbreaks can occur if water is not properly filtered or boiled.
– **Contaminated food:** Food can become contaminated if it is washed or prepared with water containing *Giardia* cysts or if handled by someone who is infected and has not washed their hands properly.
– **Person-to-person contact:** This is especially common in settings like daycare centers, nursing homes, or households where close contact occurs. If an infected person does not wash their hands thoroughly after using the bathroom, they can spread cysts to others by touching shared objects, food, or surfaces.
– **Animal contact:** Pets and wild animals can carry *Giardia* and contaminate the environment with cysts through their feces, which can then infect humans.
Once inside the intestines, the trophozoites interfere with the normal function of the intestinal lining. They disrupt the tight junctions between intestinal cells, damage the mucus layer, and trigger inflammation. This leads to symptoms such as diarrhea, abdominal cramps, bloating, nausea, and malabsorption of nutrients. The parasite’s secreted enzymes degrade proteins that maintain the intestinal barrier, increasing intestinal permeability and allowing harmful substances to penetrate deeper layers of the gut wall, worsening inflammation and symptoms.
Giardiasis can present in different forms: some people may have acute symptoms that last a few weeks, others may develop chronic infections with ongoing digestive problems, and some may carry the parasite without any symptoms at all, unknowingly spreading it to others.
The risk of giardiasis is higher in areas with poor sanitation, inadequate water treatment, and crowded living conditions. Travelers to such regions, children in daycare, and individuals with weakened immune systems are particularly vulnerable.
In summary, giardiasis is caused by ingesting *Giardia* cysts through contaminated water, food, or direct contact with infected individuals or animals. The parasite’s ability to survive harsh environmental conditions and its impact on the intestinal lining make it a common and sometimes persistent cause of gastrointestinal illness worldwide.
