5 Rare Parasites Found in the Human Body

Many amoebas are utterly harmless, but their microscopic size makes it easy for both harmful and benign ones to invade the human body without detection.
ŠiStockphoto.com/Nancy Nehring

A parasite is an organism that depends on another organism -- a host -- to live. While your mind may wander to a relative who always needs a favor (or a loan), we're talking about the non-human kind. Worms, amoebas and the like.

There are a few types of parasites that cause disease in humans. Some make a home inside you, such as trichinella spiralis, the roundworm normally contracted from eating raw or undercooked pork that causes trichinosis infections. Others are exterior pests, such as a flea sucking your blood. Whether inside or out, parasites are separated into three primary groups: protozoa, helminths and arthropods.


Parasitic protozoa are single-celled organisms so small you need a microscope to see them. They like to eat bacteria and other microbes, and often they live harmoniously with us in our intestinal tract. Some types cause disease, though, such as the protozoa Plasmodium, which causes malaria. Helminths are the second type. These are worm parasites, such as tapeworms and pinworms, and are a bit bigger than protozoa. Arthropods, the third, aren't parasites themselves, but these insects and spiders are common hosts of parasitic diseases.

Most parasitic infections happen in warm climates; for example, Southeast Asia or other tropic or sub-tropic parts of the world. One of the most common parasitic infections is malaria, which kills an estimated 1 million people every year worldwide, mostly in sub-Saharan Africa. Usually, the parasites' goal is not to kill the host, although that can happen, but a parasite relies on that host for its survival, after all, so why intentionally ruin a good thing?

In the United States, the picture looks a little different from the ones in tropical areas, although parasitic infections are still a concern. The most common parasitic infection in the U.S. is trichomoniasis, a sexually-transmitted disease caused by the protozoan trichomonas vaginalis, with about 7.4 million cases occurring every year. Giardia, a protozoan that causes intestinal problems, follows, causing an estimated 2 million infections every year. And cryptosporidium, a protozoan most often picked up during recreational water sports and activities, rounds out the top three list by causing 300,000 infections per year [source: CDC].

The parasites we're interested in, however, are not these common ones, but their rare counterparts. These are the ones that might not have treatment. The ones that just might make you think twice about how you cook your food or where you swim. Let's kick this parasitic party off by introducing spargana, the parasite that causes sparganosis infection.

5: Spargana

What's hungry for your brain (and other tissues), can grow up to 11.8 inches (30 centimeters) in length and can live up to 20 years in your body? Charming spargana.

Sparganosis is an intestinal infection caused by the spargana parasite, a tapeworm, and is a food-borne illness that mainly affects animals other than humans. You're only at risk of becoming infected if you eat undercooked meat from an infected bird, reptile (such as snake meat), amphibian (such as frog meat) or mammal, or by drinking contaminated water.


Symptoms of sparganosis vary wildly, depending on what part of your body the parasite makes its home in. Some locations may be symptom-free but if it decides your brain looks cozy, you may experience headaches or seizures.

Most sparganosis infections occur in China, Japan, Korea, Thailand and other Southeast Asian countries. Sparganosis is rare. In China, approximately 1,000 sparganosis infections were reported across 22 provinces between 1927 and 2007 [source: Li]. And in the 67 years between 1943 and 2010 there were only 52 cases reported in Thailand [source: Anantaphruti]. In the U.S., the infection rate among humans is still much lower than in Thailand.

4: Gnathostoma Spinigerum

Gnathostomiasis infections are rare worldwide. They're reported mostly in Southeast Asia, but are now also seen in Central and South America. This type of infection is considered by the Centers for Disease Control and Prevention (CDC) to be an emerging imported disease. This means that it's spreading around the world just as we do as travelers.

Gnathostomiasis is caused by gnathostoma spinigerum, a helminth-type parasite, which you are at risk for ingesting when you eat undercooked or raw infected foods, including freshwater fish, crustaceans (shrimp, crab, crayfish), frog meat, pork and chicken.


Humans aren't actually very good hosts for this parasite, which can live for as long as a decade in your body after you've been infected. We're unintended, accidental hosts for this organism -- they aren't able to reproduce inside a human body as they do inside other hosts. Good for you, but bad for the parasite. Instead, they live out their 10 to 12 year life cycle migrating throughout your body, causing swelling under your skin.

3: Japanese Lung Fluke

Paragonimiasis is an infection caused by a fluke, a type of parasitic worm, most often paragonimus westermani and paragonimus kellicotti in humans. It usually affects the lungs, although it can infect other parts of the body, such as the brain. The good news? Only crayfish and crabs carry the parasites responsible for this infection.

The risk of being infected with a lung fluke is highest in Japan, Korea, China, the Philippines and Indonesia, as well as some regions of Africa and South America, but the P. kellicotti parasite is also found in crayfish in the American South and Midwest.


After you consume a lung fluke, the first symptoms to appear are usually abdominal pain, diarrhea, fever, coughing and chest pain while the parasite travels from your stomach through your body and into your lungs. Symptoms may often look a lot like bronchitis, meningitis or even tuberculosis.

Lung flukes grow to be anywhere from about 0.3 to 0.5 inches (7.5 to 12 millimeters) in length, and just like the parasites we've already talked about, lung flukes can live inside a human host for as long as 20 years [source: CDC].

2: Naegleria Fowleri

Naegleria infection is caused by a parasitic amoeba found in freshwater lakes, ponds, rivers and hot springs. Only the Naegleria fowleri species infects humans, and does so by entering the body through the nose, where the amoeba then causes an infection called primary amebic meningoencephalitis (PAM).

The symptoms of PAM begin anywhere from one day to one week after the amoeba gets into your body, and often resemble the symptoms of a bacterial meningitis infection: headache, fever, nausea, vomiting and a stiff neck.


Contracting PAM is very rare in the United States. Only 32 cases were reported in the U.S. between 2001 and 2010 [source: [url='http://www.cdc.gov/parasites/naegleria/']CDC]. Unfortunately, surviving the infection is also rare. It's estimated that out of about 200 reported cases worldwide, no more than 12 or so patients have survived [source: Parija]. Once symptoms begin, the infection moves quickly, causing additional problems including seizures, confusion and hallucinations, and patients with the infection often die in less than two weeks (and sometimes less than one week).

You can minimize your risk of being infected with Naegleria fowleri by avoiding swimming or taking part in any activities where you'll be in warm, untreated or inadequately treated freshwater.

1: Candiru

There are more than 2,000 types of catfish, but there's no need to fear most of them. Except the candiru. It's perfectly fine to fear the candiru. This tiny catfish -- it's only about 0.5 to 3 inches (1.3 to 7.6 centimeters) in length, though they can grow as long as 6 inches (15 centimeters) -- is the stuff of urban legend, except in this particular case, the tale is true [source: Animal Planet]. Candirus are parasitic freshwater catfish that live in the Amazon river, and they just might want to get intimate with your orifices. All of them. Yes, including that one.

These fish like to set up shop inside a host and feed on blood, and they find those hosts because of their acute sense for finding nitrogen. Nitrogen is normally excreted through the gills of a fish, but humans also excrete nitrogen -- in our urine. And since candiru don't know the difference between a gill and a urethra, remember to use the bathroom before you go swimming in the Amazon. It will help reduce your risk of a small catfish anchoring itself inside your urethra.


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More Great Links

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  • Animal Planet - River Monsters. "Candiru Catfish." (June 1, 2012) http://animal.discovery.com/fish/river-monsters/candiru-catfish/
  • Aubrey, Allison. "What Does It Take to Clean Fresh Food?" NPR. 2007. (June 1, 2012) http://www.npr.org/templates/story/story.php?storyId=14540742
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