Lettuce bug is still causing us misery


Entamoeba histolytica (aka lettuce bug) is an intestinal parasite, and if I remember correctly, the first mobile organism we ever visualized under the microscope in high school or college biology. For many decades, most cases seen in the U.S. occur in recent immigrants and travelers returning from endemic areas. With this in mind, you can understand my surprise at finding that most of the prescription refill requests I received on my on-call weekends in my first job following my GI fellowship were for anti-amebic medications. This became so common that I requested our in-house lab notify me the next time they saw ameba in a stool specimen. My examination under the microscope revealed E. coli (a harmless bug) and not E. histolytica (the bad dude) – problem solved!

Worldwide each year, an estimated 50 million cases of amebiasis occur, resulting in up to 100,000 deaths. Most of these are in Central America, Africa, and the Indian subcontinent. In the U.S. the overall prevalence is four percent, but with only 10 percent of ameba infections causing invasive disease, only one percent of persons with E. histolytica found microscopically develop symptomatic amebiasis. Since the disease is transmitted by the fecal-oral route, most cases are seen where there is inadequate sanitation and water treatment. In addition to those cases most commonly seen in the U.S. mentioned above, there is an increased prevalence in the institutionalized (especially in the mentally retarded and in male homosexuals). Patients on steroids (cortisone-type drugs) are more susceptible.

Ameba is transmitted via ingestion of the cystic form of the parasite, which is viable in the environment for weeks to months. The cysts can be found in fecally contaminated soil, fertilizer, water, or on the contaminated hands of food handlers. In our intestinal tract, excystation occurs resulting in the invasive form of the organism (trophozoites) which can invade the colon wall leading to tissue destruction, bloody diarrhea, colitis, and extraintestinal conditions, the most common of which is liver abscess. Amebic colitis is gradual in onset, with symptoms of diarrhea (which may be bloody), cramping abdominal pain, weight loss and fever.


Diagnosis is usually made via examination of stool or blood. Identification in stool (cysts in solid stools; trophozoites in liquid stools) may require microscopic examination of up to six specimens. The sensitivity is said to be 85-95 percent. The WHO (World Health Organization) recommends an E. histolytica-specific serological (immunologic) test. The antigen detection tests yield a 71-100 percent sensitivity and the antibody tests are 70-90 percent sensitive. The antibody tests cannot distinguish between new from past infection, for the antibodies may persist for years after acute infection. In symptomatic patients, sigmoidoscopy may reveal typical amebic ulcers which may be aspirated and the specimen examined microscopically and serologically.


Most folks with amebic colitis can be treated at home with oral anti-amebic drugs and hydration. Exceptions would be severe colitis with dehydration requiring IV fluids, liver abscess, and suspected colonic or liver rupture. Follow-up stool exam after completion of therapy is recommended to ensure bug eradication.

Of course, as with all infectious diseases, successful prevention surely beats treatment, and some preventive measures are:

l Uncooked foods, including vegetables and salads should be avoided in developing areas.

l Potentially contaminated water and ice should be likewise avoided. Boil water for over one minute. Use bottled water if possible. Portable filters provide various degrees of protection.

l Remember, in humans, ameba infections does not confer long-term immunity.

l Development of a vaccine is still in its infancy.

Dr. William McKell is a Northsider. His email is bmmckell2012@comcast.net.

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