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Millions of Americans Have a Parasite and Don't Realize It

Very few people have even heard of Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, or trichomoniasis.
Cat surrounded by floating parasites
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Americans often like to joke about the threat of Montezuma's Revenge—otherwise known as traveler's diarrhea—when they visit developing countries, but a significant number of people in the US actually already have parasites living inside them right now. Chances are you know one, or it's you.

Pinworms, giardia, head lice, and cyclospora hardly ever kill people in industrialized countries. You'd be correct, too, in thinking that most parasites—like intestinal worms—are more of a developing-country problem than a developed-country problem. But there are five in the US that you probably haven't heard of, which the Centers for Disease Control calls the neglected parasitic infections: Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, and trichomoniasis. They aren't household names, they're much more serious than an itch, and according to the CDC, there are more than 100 million cases of these five in Americans right now.


“The CDC targeted these five diseases for public health action based on the number of people infected, the severity of illnesses, and the ability to prevent and treat them,” says Monica Parise, director of the CDC's Division of Parasitic Diseases and Malaria. “We know much less than we would like to about the health and economic burden of these infections in the US.”

With the perennial story of someone pooping out a five-foot-long tapeworm appearing in your feed, you may wonder how 100 million people could not know they've become human hotels to creatures wiggling through their bodies. One reason is that monster parasites—like those five-footers—are outliers. Most are sneaky and very small. “Some of these parasites have evolved along with us for millennia,” says Bobbi Pritt, director of the Mayo Clinic's Clinical Parasitology Laboratory. “They don't cause symptoms or they cause very minimal symptoms, so they're not likely to come to medical attention until it's too late.”

Toxocariasis is one such parasite you can catch in the US. It's a roundworm that lives in the intestines of dogs and cats, and it thrives when owners don't get their pets de-wormed on a regular basis, Pritt says. “To put it bluntly, the parasite doesn't really know what to do in our bodies,” she says. It knows it's not inside a dog or a cat, where it'd rather be, so it spends its life in a continual larval phase roaming through the body, boring holes in any organ it runs into, including eyeballs and brain matter, as it looks for a place to settle down.


Once it cuts tunnels through your eyeballs, the damage is irreparable and you can lose your vision, Pritt says. If it goes to the brain, it can be fatal. Forty-six million people—14 percent of the US—have toxocariasis, although the CDC says true numbers are higher because people rarely connect eventual blindness with roundworms slithering undetected through the body.

If you remember the “brain controlling” cat parasite that had a media publicity run a few years ago, you've heard of toxoplasmosis, which is common not just in cats, but also in many other animals, such as mice. Like any social media frenzy, the exotic part gets talked about the most. People should worry less about whether their minds are under control, however, and more about the definitive dangers of contracting toxoplasmosis. “The two big deals are if a woman who is pregnant [gets infected], or if you [get infected] and later become immuno-compromised,” Pritt says.

Pregnant women who catch toxoplasmosis will likely be fine, she says, but the baby doesn't have the same immune responses and defenses. For a baby, it could be fatal. Once you're infected with toxoplasmosis, she adds, you're infected for life. Your body keeps it in check, but if you ever come down with an immunity problem, the parasite can reactivate and travel to the brain, and that too can be fatal. About 800,000 Americans catch toxoplasmosis every year, Pritt says, and more than 60 million have it currently, according to the CDC. That's 18 percent of the US population.


Less common, but harder to avoid, is trichomoniasis. You could avoid mice and outdoor cats to guard against toxoplasmosis, but are you willing to be celibate to avoid trich? “Here's a fun fact,” Pritt says cheerfully. “ Trichomoniasis, [a sexually transmitted parasite], is more common than chlamydia, syphilis, or gonorrhea.” Every year, 1.1 million Americans contract trich, but hardly anyone knows because 70 percent of the people with it show no symptoms, according to the CDC.

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Most men don't know they're infected because it rarely presents symptoms. “It used to be considered a nuisance parasite, and people didn't think it was a big deal,” Pritt says, “but there actually are some bad side effects.” Women who contract it are more susceptible to contracting other viruses, such as HIV. If she becomes pregnant, she's more likely to give premature delivery to an underweight baby.

Unlike toxocariasis, toxoplasmosis, and trichomoniasis, Chagas disease is not often caught in the US. Rather, it's mostly found in immigrants from Latin American countries who lived in poor quality housing. It comes from the kissing bug, named because it's attracted to the carbon dioxide you exhale, Pritt says. The kissing bug creeps up to your mouth as you sleep and bites you. It creates an open wound on your mouth and then defecates into it, and the parasite in its feces drops straight into your bloodstream.


More than 300,000 Americans have caught the parasite trypanosoma cruzi from the kissing bug, says the CDC. Some people may never know they have it as long as they live, but about a third of infections remain permanent, and a third of those develop serious chronic disease, Pritt says. It can cause intestinal tract problems, swelling of the esophagus, and swelling of the colon. “The cardiac manifestations are probably the worst,” she says. “Those can kill you.” When the parasites live in the heart walls, the tissue becomes so thin that the heart eventually can't function, and the person dies from cardiac arrest.

The last of the five neglected parasites is cysticercosis, also most often caught in Latin America, too. You catch it by swallowing microscopic parasitic eggs, which hatch in y our intestinal tract. The parasites swim into your bloodstream and then go throughout the body, creating cysts. The most dangerous place it can go is to the brain. Early cues that something is wrong is when a person begins having seizures, Pritt says, but by then the parasite has peppered the brain tissue with cysts. Cysticercosis that has invaded the brain can be deadly.

Cysticercosis is also called the pork tapeworm. In countries with a less-secure supply of pork, people eat the adult tapeworms in a meal. As it lives in them, they keep excreting the tapeworm's eggs in their stool like a walking parasite factory, and then the eggs make it back into peoples' food supply and drinking water. The Food and Drug Administration ensures that the US pork supply is safe, Pritt says, and we don't have to worry about it here. The CDC doesn't know how many people in the US have cysticercosis, Parise says, but at least 1,000 people are hospitalized each year in the US because it's spread into their brains.

People may breathe a sigh of relief that Chagas disease and cysticercosis are mainly caught outside the US, but Pritt says it doesn't much matter, because the people carrying them are in the US now. “Even though they're catching them elsewhere, these are people now living in the United States,” she says. “They're working here. They may be US citizens now, and so… our physicians need to be aware of these diseases and be able to treat them.”

She says the CDC's efforts are working, and that just the simple fact of identifying these five as neglected parasitic infections has meant more education material is becoming available to healthcare staff. “People are more likely to talk about them at meetings and bring them up,” she says, and adds that now they have manuscripts published regularly on the infections with updates from the CDC. The CDC is also working to distribute otherwise unavailable drugs to physicians fighting the infections, Parise says, and they're getting better at diagnostic testing to detect infections earlier.

It's a start, and yet it's confounding that more than 100 million people had to come down with the infections, some of them dying, for a public awareness campaign to take hold. Even then, the five—with the arguable exception of toxoplasmosis—are still barely known outside medical circles.

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