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Six Commonly Missed Diagnoses: Parasites

If internet scare tactics from companies selling herbal supplements for parasites weren’t enough, the cable TV show “Monsters Inside Me” with its toe-curling film clips has cinched it. We’re in a new “Alien versus Predator” mode, though you might ask which one is us and which them. Those really large parasites you’ll see wriggling across your flat-screen Sony as you scream “I can’t watch this!” (but do) are all helminths, better known as worms. Worldwide, intestinal worm infestation is pretty common, but mainly in underdeveloped countries where people have a bit more contact with raw sewage than we do here in Chicago (unless you swim in the river).

The helminths are the stars of “Monsters Inside Me” because they’re big enough to see without a microscope. When patients ask me to check them for parasites or report “something wriggling in my toilet,” it’s the helminths they have in mind. Obsessing about parasites, despite your best efforts your mind keeps rewinding to John Hurt’s untimely and utterly galvanizing death-by-parasite in the original “Alien.”

Despite studying parasites of all stripes in medical school and testing thousands of patients for them, I think I’ve seen fewer than a dozen cases of intestinal helminth infestation in my entire career. Most of these were in children who’d picked up a pinworm infection (E. vermicularis), little ones being very democratic about their anal hygiene. Pinworm intestinal infection is generally harmless, its main symptom itching around the anus. Treatment using a worm killer, called a vermifuge, wipes out the pinworms after one or two doses.

A second infection is hookworm, once prevalent in the southern US and caused by a combination of inadequate plumbing and a preference for barefootedness. Probably most of the characters in Erskine Caldwell’s novels like God’s Little Acre and Tobacco Road had hookworm. Through a very circuitous route, the worm enters through skin and ends up in the intestines. Once diagnosed it’s easy to treat. I’ve seen exactly one case of hookworm, in a returning Peace Corps worker who should have known better.

Other less common helminths are roundworm (Ascaris lumbricoides) and the scary looking tapeworm (several species), once common when we were sloppy with our meat, poultry, and fish preparation. Jewish grandmothers and great-grandmothers were frequently victims of fish tapeworm (Diphyllobothrium), infected by sampling their uncooked haddock while making gefilte fish. These days it’s our sushi/sashimi addiction that renders us susceptible to fish tapeworm. Given the amount of sushi we eat, though, the actual incidence of tapeworm is extremely rare.

What’s living inside me?
It’s these worms (and that TV show) that cause people to lie awake at night wondering what’s living inside them. Well stop wondering and go back to sleep. I want to emphasize that all of these are (a) rare, (b) easy to diagnose, and (c) fairly easy to treat.

You’ll not see the single-celled protozoa infestation stories much on TV. Characters visible only with a microscope just lack the panache, the stage presence, to terrify. However, it’s infections with these little buggers that we’re discussing today.

Doctors are in full agreement that two specific intestinal protozoa–ameba (E. histolytica) and giardia (G. Lamblia)–are troublemakers. Both are acquired by the unappetizing “fecal-oral” route of transmission, which is exactly what it sounds like (now go wash your hands). Most people don’t recall just where they might have acquired amebiasis or giardiasis, but as a general rule you get it from someone who didn’t wash hands after using the toilet. Campers occasionally get giardiasis from drinking water downstream from a herd of cattle. Both infections are manifested by a sudden onset of diarrhea. In the case of giardia, it can be quite explosive and lead to dehydration. Just about anyone who has sudden diarrhea after travelling that doesn’t respond to the routine antibiotics for traveler’s diarrhea (Cipro, Xifaxan, Flagyl) should be tested for these protozoa.

Less agreement among physicians occurs with three other parasites: Endolimax nana, Dientamoeba fragilis, and Blastocystis hominis. For years, these three were considered harmless. Unfortunately, most lab technicians assigned to poop examination (a part-time job I myself held during medical school) are not well-trained in checking for them. It’s very likely a reasonable percentage of patients written off as irritable bowel syndrome/diarrhea dominant actually have an infection with one of these three. The main clues that their IBS is protozoan in origin are that the diarrhea emerges as a new event in their lives, unrelated to stress, and isn’t triggered by any particular foods. “I got traveler’s diarrhea while in Mexico and despite antibiotics, it never went away,” sends me a clear signal to check for parasites.

An easy fix
The combination of lab techs missing these parasites and physician disbelief that the three are troublemakers qualifies parasites for inclusion on our Commonly Missed Diagnosis list. Most of the really good parasite labs–Genova (formerly Great Smokies), Meta Metrix, Doctor’s Data, Meridian Valley–are virtually never used by physicians affiliated with large US medical centers, these doctors preferring to blindly trust their hospital labs. I remember one patient telling me how she handed her Genova parasite test result (showing heavy counts of dientamoeba and blastocystis) to her Northwestern gastroenterologist. He saw the lab and tossed it back without reading it, saying “This is garbage. Never heard of this lab. Bet you got it from a chiropractor.”  This, of course, can potentially lead to years of unnecessary diarrhea.

Nutritional guru Alan Gaby, MD, once remarked that if a patient has no access to one of the parasitology labs, she should take her specimen to a veterinarian and palm off her poop as belonging to her beloved pet. Veterinarians are much better than conventional physicians at diagnosing parasites.

Once a diagnosis of any of the three parasites is made, treatment is generally straightforward with an anti-parasite med like Flagyl (metronidazole), an antibiotic like the sulfa drug Bactrim, or a combination of these. After treatment for any parasite, whether it’s a helminth (worm) or a protozoan, a follow-up stool collection is mandatory to ensure it’s been completely eradicated.

Sleep well…

Be well,

David Edelberg, MD

Leave a Comment


  1. Pam Quicksall says:

    I have had the parasite Blasto in me since the late 70s and have cysts throughout my body in or near all major organs. I’ve had several surgeries where the cysts have been removed thinking they were tumors !! Many doctors still do not believe !!

  2. Dr E says:

    Hi Pam
    If the doctors saw the blasto under a microscope on the removed tumors, I don’t understand their disbelief!

  3. Dana says:

    Dr. E, are there alternative treatments for parasites such as blastocystis hominis? I did a Genova lab and discovered I had this in my system, but am nursing and also try to avoid meds as much as I can..thanks for your input!

  4. Dr E says:

    Hi Dana
    During your nursing period, I really wouldn’t bother with treatment as B Hominis overall is not harmful to your (or your baby’s) health. Once you’ve finished then most of the time, conventional meds or herbal combinations will clear it

  5. Dana says:

    Thank you for taking the time to respond, I appreciate your feedback!

  6. Pam Quicksall says:

    Is it unhealthy to drink only distilled water? I distill my own water and drink it exclusively ! A relative has told me that I will not get enough electrolytes and dehydrate. I have a problem with parasites and I’m taking herbs for that. A few times my speech has been incoherent for a minute or two and she thinks it is because of the lack of the right kind of water I think it is because of the buildup of Blastocysts in my brain. I have had the parasites for 38 years after a visit to Mexico and they keep coming back ! I have been treated with Flagl three times since 1881. In my case is it safe to drink only distilled water ?

  7. Dr E says:

    Hi Pam
    Your friend is correct. Drinking only distilled water deprives your body of valuable minerals and I suspect was the cause of your garbled speech. Blastocystis usually responds to triple therapy. See this link
    https://aac.asm.org/content/59/8/4417

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