5 of the Worst Parasites You Can Get

No matter who you are, or where you live, you have, at some point in your life, served as the host to a parasite—“any organism that has a relationship with another organism where the first one gets a benefit and the other pays a cost,” Dan Riskin, biologist and expert on the Animal Planet series Monsters Inside Me, tells Mental Floss. “These relationships can be very short—like a mosquito you and then taking off—or they can last for decades, or your whole life.”

Ahead of the new season of Monsters Inside Me, which premieres on October 15, we asked Riskin to come up with a list of five parasites you really don’t want to host. When making his picks, Riskin went for the most common and the least common, then filled in with different parasites in between. He wasn’t lacking for creatures to choose from; more than half of the animal kingdom, he says, consists of parasites, which have been “hugely influential in the history of humankind.” Read on to be terrified—and amazed.

1. NEW WORLD SCREWWORM (COCHLIOMYIA HOMINIVORAX)

This nasty little parasite (above) made news in the U.S. last year when it infected a herd of rare Key Deer in the Florida Keys. (That outbreak, thankfully, is over.) These creepy crawlies can infect humans, too—and one such case is featured on the new season of Monsters Inside Me. “This family went to Colombia to volunteer at an orphanage,” Riskin explains. Near the end of the trip, the daughter had gone to a water park and came back with a sore on the side of her head.

“The next day, when they were supposed to fly home, the feces hit the fan,” Riskin says. The sore became incredibly painful and started pussing, and the family had to decide: Should they stay in Colombia or go home? Ultimately, they got on the plane. Once back in the States, they took their daughter to the ER, where she was given a haircut that allowed doctors to see the “mobile larvae,” a.k.a. maggots, in her head.

A female Cochliomyia hominivorax fly had landed on the girl’s head and deposited its eggs in a lesion on her scalp; the eggs soon hatched into nearly 2-centimeter-long maggots that began chomping away. Unlike other parasitic maggots—like botflies, for example—the New World Screwworm does not stay put. These maggots were burrowing into the girl’s skull. “If they hadn’t gotten to it, these things would’ve migrated right down sort of towards her face and out her eyeballs or who knows what,” Riskin says. Thankfully, doctors were able to remove the maggots using a combination of petroleum jelly and bacon therapy (basically, luring the creepy critters out with the smell of bacon and the threat of suffocation).

How to Avoid It: Wearing long pants and long-sleeve shirts, applying DEET-based repellents, and sleeping under a mosquito net are your best methods of defense. If you suspect you might be hosting some screwworms, “seek medical attention immediately,” Riskin advises.

2. MALARIA

According to the CDC, there were 212 million cases of malaria in 2015; 429,000 deaths were caused by the parasite. “You can get it almost anywhere tropical,” Riskin says. “All it takes is a mosquito bite.”

Mosquitos don’t hatch carrying malaria; the bugs pick it up from an infected person. Once inside the mosquito, the malaria parasite takes up residence in the insect’s salivary glands. “When the mosquito bites somebody else, it squirts a little bit of spit into them to help keep the blood flowing—that’s what causes an itching mosquito bite; it’s a reaction to that spit—but they can also be spitting these parasites into the next person,” Riskin says.

In humans, the parasites hang out in cells in the liver, then make their way to red blood cells, which they make explode, spreading more parasites that invade more blood cells. Anyone can get malaria, and people who contract it will experience fever and chills. “It just wreaks havoc on your body,” Riskin says.

The disease is potentially fatal; those who contract malaria and survive might relapse because some species of the parasite can lie dormant in the liver.

How to Avoid It: If you’re going to a country with malaria, consider taking preventative drugs. Otherwise, wear long pants and long sleeves, use DEET-based insect repellents, and sleep under a mosquito net when traveling in the tropics.

3. BRAIN-EATING AMOEBA (NAEGLERIA FOWLERI)

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Technically, this amoeba—which can be found in warm, untreated fresh water—isn’t a parasite. “Normally, it’s totally harmless, doing its own thing in the mud, eating whatever it finds there, going about its business, not bugging anybody,” Riskin says. That all changes when a person goes water skiing and gets water harboring N. fowleri violently shoved up their nose.

Now in a new environment, the amoeba resumes eating whatever it can find in the nose. “It makes its way up that olfactory nerve, reproducing and eating, until it hits the brain,” Riskin says. “And once it’s in the brain, it’s game over for the kid that had it shoved up his nose.”

Typically, a victim will begin to show symptoms—which include fever, headache, and vomiting—around five days after infection (although symptoms can appear as early as one day after infection or as late as nine), and will usually die about five days after that. The death rate associated with infection by N. fowleri in the U.S. is 97.2 percent.

That said, getting a brain-eating amoeba is very, very rare: There were 40 infections reported in the U.S. from 2007 to 2016, a rate of four cases per year. (The CDC reports that “36 people were infected by recreational water, 3 people were infected after performing nasal irrigation using contaminated tap water, and 1 person was infected by contaminated tap water used on a backyard slip-n-slide.”) That gives you odds of about 1 in 70 million of contracting a brain-eating amoeba.

How to Avoid It: Basically, just keep water from slamming up your nose. If you plan on engaging in an activity like water skiing during the summer months, “you could wear a nose plug,” Riskin says. “But this is so rare, that might be overkill.”

4. LUNG FLUKES A.K.A. PARAGONIMUS

Humans may find themselves hosting these coffee bean-sized parasites if they’ve eaten raw or undercooked shellfish. As the crab gets digested, the lung fluke larvae are released, and they “move through your body and go all over the place,” Riskin says. Typically, they tear through the abdominal wall and diaphragm to get into the lungs.

People infected with lung flukes may experience abdominal pain and a fever; eventually, they’ll begin to cough up blood laden with eggs. The blood is either spat out or swallowed, allowing eggs to pass in the stool (and, if a host is in water while defecating, the life cycle will continue).

“Our bodies are built to protect us from the environment,” Riskin says, but eating makes us vulnerable. “It’s like if you had a fortress with big stone walls all around it—you still have to get food to the people who live in the fortress. And so, every once in a while, you have to open the gate and let all these ox carts come in and then close the gate. And then, you’ve got to hope that there’s no Trojan horse.”

Untreated, lung flukes can live in the body for 20 years, according to the CDC. Thankfully, once an infection is identified, medication can wipe the infection out.

How to Avoid It: This one’s easy—just don’t eat raw or undercooked shellfish.

5. ELEPHANTIASIS A.K.A. LYMPHATIC FILARIASIS

This condition is caused by several species of roundworms; like malaria, these roundworms thrive in the tropics and are transmitted by mosquito. Once in your body, the roundworms set up shop in your lymphatic vessels—small, one-way tubes in your body that drain liquid away from tissues—and can live there for decades. “They’re cloaked like a Klingon bird of prey,” Riskin says. “The immune system doesn’t even know they’re there.” At least, not until the worms die.

“Once they’re dead, the cloaking mechanism doesn’t work anymore,” Riskin explains. Then your body’s like, ‘Whoa, we’ve got an invader.’” Your body sends white blood cells to the site where the bodies of the roundworms have piled up. The problem is, their bodies have clogged the very vessels meant to drain the liquid away, causing limbs to swell. “It gets more swollen, and the body sends more fluid, and that area gets puffier,” Riskin says. “This just keeps happening, and there’s no way to drain. The draining system is busted.”

The parasite infected 120 million people in 2000, according to the World Health Organization; 40 million people were disfigured and incapacitated by it. While medications have little effect on adult roundworms, there are drugs to help and prevent transmissions to others: In 2015, the scientists who developed a treatment that could prevent infections for around a year won the Nobel Prize for Medicine (alongside another researcher who developed a novel malaria therapy).

How to Avoid It: This mainly affects people in Africa and Asia, so if you’re traveling there, wear long-sleeve shirts and pants, liberally apply insect repellent, and sleep under a mosquito net.

A new season of Monsters Inside Me begins October 15 at 9 p.m. EST on Animal Planet.