Medical Edge Newspaper Column

Parasites May Not Be The Problem

July 23, 2007
Dear Mayo Clinic:
I was diagnosed with Blastocystis hominis, a microscopic parasite. I was put on the antibiotic metronidazole (Flagyl) but still have the parasite. What else could I try to get rid of it? -- Cape Cod, Mass.

Answer:
While mention of parasites can make anyone squeamish, Blastocystis hominis (B. hominis) is common. In fact, it's the most common parasite in human stools. In the United States, an estimated 1.5 to 10 percent of people have this parasite in their stools. In developing countries, 30 to 50 percent of people carry this parasite.

B. hominis is one of many types of single cell organisms that dwell in the gastrointestinal tract. Some are harmless, and others do cause disease. It's not clear that B. hominis causes illness. Many people who carry B. hominis don't have any symptoms, and treatment is not needed.

Some studies have suggested an association between intestinal symptoms, B. hominis, and response to treatment with medication, but the connection is far from clear. While some people with B. hominis do suffer from intestinal symptoms, there could be other disorders causing their symptoms. B. hominis might be an innocent bystander.

There are many effective medications for patients with diarrhea, cramps or other intestinal symptoms. For possible symptomatic B. hominis infestations these include: metronidazole (Flagyl), sulfamethoxazole and trimethoprim (Bactrim), iodoquinol and nitazoxanide.

Bottom line: If you don't have any symptoms, you don't need treatment. If you do have persistent symptoms, see your doctor for an examination and diagnosis. Any persistent intestinal symptoms are likely not due to B. hominis. Another possible diagnosis could be irritable bowel syndrome -- a much more common disease characterized by cramps and diarrhea without any connection to parasites.

-- Rodney Thompson, M.D., Infectious Diseases, Mayo Clinic, Rochester, Minn.