I'm a 25-year-old woman who's been skinny all my life, seriously wanting to gain weight. Recently, when the doctor did a test on my fecal matter, he discovered amoeba parasites and I had to take medication for 10 days. Could this be one of the reasons I couldn't gain weight all these years? What should I do now?

M.J.C.

Amoeba (specifically Entamoeba histolytica) is an intestinal parasite that is most commonly found among people who live in tropical countries. In those areas, up to 80 percent of the population may be carriers of this parasite.

People who travel to tropical regions and have contact with contaminated water or food may acquire amoeba infection. Certain groups in the United States are also at higher risk of infection: institutionalized persons, homosexual men with multiple sexual partners, and children in daycare centers.

In people who are carriers, often only cysts of the parasite are shed in the stool, while the patient develops no symptoms. These patients may never be diagnosed because they have no reason to see a doctor. However, in other people, the amoebas can invade the colon. Common symptoms in these patients include diarrhea (which may be bloody), crampy abdominal pain, weight loss and fatigue. The infection may last a long time, and symptoms may wax and wane before a diagnosis is made.

In some cases, the amoebas will invade through the colon and into the liver, causing an abscess to form. Fever and right-sided abdominal pain are typical symptoms of this complication. The most severe complications of amoebic infection include perforation or obstruction of the colon. Patients who develop these complications will be acutely ill with severe abdominal pain and fever. They will require surgery, often on an emergency basis.

The diagnosis of amoeba infection is made by examination of the stools under a microscope. In some cases, a colonoscopy is done, which can detect the damage (ulcers, bleeding) caused by the parasite. Because the infection is not very common, your doctor may not order such tests unless you let him know that you have recently traveled to a tropical area or belong to a high-risk group.

Treatment for the invasive form of amoeba infection includes metronidazole (Flagyl). This antibiotic is less effective for the cyst form of the infection in the colon, and another agent may be needed.

In your case, it's difficult to determine if the amoeba was responsible for your inability to gain weight, although it certainly is possible. It may be that your infection was insignificant and that you were only passing cysts. If a colonoscopy was done and findings were consistent with invasive infection, this would support your theory. Either way, you should continue to follow up with your doctor to monitor your symptoms and confirm that the amoeba infection was eradicated.