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Intestinal parasites

Posted Jan 24 2011 9:29am

Intestinal parasites are a big cause of morbidity and mortality worldwide. They don’t just occur in underprivileged parts of the world though – there are several that are pretty common here in the US.

Intestinal parasites fall into two big categories: protozoa and helminths. Protozoa are little one-celled creatures that reproduce by binary fission. They can be intracellular (like plasmodium, the organism that causes malaria) or extracellualar (like the GI protozoa we’ll talk about in a minute). Helminths are basically regular old animals – worms – that reproduce sexually and have real organ systems.

The most common GI parasites in the US are: Entamoeba histolytica, Giardia lamblia, Enterobius vermicularis, Necator americanus and Ancylostoma duodenale. Let’s take a quick look at these guys.

Entamoeba histolytica is a protozoan. It’s an amoeba, like the name says, and it’s passed through the fecal-oral route (yuck). It gets into the gut, invades the mucosa, and attracts neutrophils, causing ulceration. Most infections are actually asymptomatic – but the symptomatic ones have a pretty high mortality (it’s the #2 cause of protozoan death, after malaria). Presenting symptoms include fever (sometimes the only symptom) and bloody diarrhea.

Giardia lamblia is a cute little protozoan that looks like a bearded man (see the electron micrograph above). It loves to live in water – even water that looks nice and pristine. Giardia infection s sometimes called “backpacker’s diarrhea” or “beaver fever” in reference to the organism’s occurrence in lakes and streams, and its use of the beaver as a host. It’s the second most common parasite in the US (after pinworm). Symptoms usually start about a week or two after exposure. The presentation is variable, but can include “foul-smelling” diarrhea (what diarrhea isn’t?), malabsorption, weight loss, and fever.

Enterobius vermicularis is a tiny helminth – so tiny that it has the nickname “pinworm.” Pinworm infection is the most common helminth infection in the US; it’s particularly common in kids. The story is not a pretty one. The little worms live in the intestine, and reproduce there. The female crawls out at night (really) and lays eggs – up to 15,000 of them – on the perianal skin. The eggs hatch, and the babies crawl back in. This causes intense itching, and you know how kids love to put their hands into their mouths…so you get the picture.

Anyway – this is generally not a dangerous infection; the worst thing about it is that it keeps kids up at night. The diagnosis of pinworm is different than that of most parasitic infections (which are usually found on examination of stool for ova and parasites). It is made using the “scotch tape” test: you press a piece of scotch tape to the perianal area at night (or upon awakening in the morning), usually for three days in a row, then look at the tape under the microscope.

Ancylostoma duodenale and Necator americanus are both hookworms. They like to live in the soil, and they were endemic (especially Necator) in the southeastern US before the 1900s – when night soil (ugh, look it up) and bare feet were common. It’s less common now – but it holds the title of #2 helminth infection (after pinworm).

The life cycle is weird: the baby worms get into the host through intact skin (usually on the feet). They migrate through the blood to the lungs, where they are coughed up and swallowed, winding up in the intestine. The adult worms attach to the mucosa of the intestine and actually drink blood (about 0.25 mL a day). Most infections are asymptomatic, but some – understandably – cause an iron-deficiency anemia.

The diagnosis of most of these infections involves sending off a stool sample for ova and parasites (with the exception of pinworm, which, as mentioned above, is diagnosed using the scotch tape test). But the most difficult part of the diagnosis is to actually think of sending off a sample! These infections often present with pretty vague symptoms. So keep these parasites in the back of your mind when a patient comes in with diarrhea, fever, or even unexplained iron-deficiency anemia.

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