Placeholder Image

Subtitles section Play video

  • [intro]

  • What do you think of when you hear the word parasite?

  • Maybe blood-sucking leeches

  • or swarms of mosquitoes spring to mind --

  • or maybe your dog had a tapeworm one time.

  • A parasite is any living thing that lives and gets its food at the expense of another,

  • generally on or in its host.

  • And some of them cause some nasty diseases.

  • But some parasites take that in thing to extremes.

  • They don't just live attached to another organism, or hide out in its gut.

  • No, they live inside their host's very cells.

  • Some of them are huge problems for humans,

  • while others are mere curiosities.

  • And while some are bacteria, or other tiny organisms,

  • others aren't what you'd expect at all.

  • Here are 6 such tiny parasites, and how they do it.

  • First up on our list is a group of microbes belonging to the genus Plasmodium,

  • parasites that cause malaria in humans.

  • As you may have heard,

  • this parasite kills a lot of people.

  • Plasmodium has a complicated life cycle that takes it from a mosquito's gut to its saliva

  • to a human liver to the bloodstream.

  • And that last part is what causes malaria symptoms.

  • After multiplying in the liver for a while,

  • the parasite bursts out and attacks red blood cells.

  • Incidentally, a single liver cell can produce thousands of Plasmodium cells --

  • and a red blood cell can breed a few dozen.

  • Once it's inside a red blood cell --

  • and this is what makes malaria so tricky to treat --

  • Plasmodium starts rearranging the furniture.

  • This includes sticking new proteins on the surface of the cell,

  • and creating tiny knobs that cause infected red blood cells to stick to blood vessel walls.

  • That prevents them from flowing through the bloodstream

  • and getting thrown out by the spleen, which normally identifies defective blood cells.

  • The infected blood cell can also clump together with uninfected red blood cells,

  • disguising itself among the healthy cells in something called a rosette.

  • And those clumps can block traffic.

  • If these sticky, nonfunctioning red blood cells bunch up in the wrong spot in a blood

  • vessel,

  • they can deprive that area of oxygen.

  • And you definitely don't want that to happen in the brain or other vital organs.

  • But the ability to make those little proteins that stick out of the cell

  • is also what makes malaria so hard to detect by our immune systems.

  • Plasmodium has about 60 genes that can code for a variety of these proteins.

  • And research published in 2017 reported that different patterns of those genes show up

  • in different patients --

  • and that likely hinders our ability to develop immunity.

  • There is a ton of research ongoing,

  • but malaria still kills hundreds of thousands of people every year, mainly in Africa.

  • So there's still work to do.

  • Number two on our list is Rickettsia,

  • a group of tiny bacterial parasites transmitted to humans by creepy crawlies like fleas and

  • ticks.

  • And you've likely heard about the diseases these parasites cause

  • sicknesses like Rocky Mountain Spotted Fever or typhus.

  • These are some of the nastier,

  • more deadly parasitic infections out there.

  • The U.S. Centers for Disease Control have even listed them as potential tools for bioterrorism.

  • Now, if a tick with Rickettsia bites you,

  • the bacteria get into your bloodstream.

  • But unlike Plasmodium, they don't target red blood cells.

  • Rather, Rickettsia tends to live in the cells lining your blood vessels.

  • The infection causes various problems with the circulatory system and widespread inflammation.

  • And if it gets severe enough,

  • the patient can face pneumonia, swelling in the lungs,

  • or kidney failure.

  • Life-threatening stuff.

  • Researchers are currently trying to understand how these bacteria can get into our cells

  • so easily

  • and of course, how to prevent them from doing so.

  • A 2015 paper found that proteins on the surface of Rickettsia cells

  • can actually induce host cells to grab them and pull them in.

  • Such entry mechanisms aren't exclusive to Rickettsia,

  • but we're interested in learning about them so we can use that in preventing these dangerous

  • diseases.

  • We can treat Rickettsia infections with antibiotics.

  • But prevention is always better.

  • So be careful out there if you're hiking through tick country.

  • Another one of those human-infecting parasites is Legionella pneumophila,

  • the bacterium behind Legionnaires' disease and Pontiac fever.

  • As the name pneumophila implies,

  • this type of bacteria tends to attack the lungs.

  • And it can cause pneumonia,

  • though this ranges from the fatal kind to much milder,

  • so-calledwalkingpneumonia.

  • Luckily, our immune systems can detect Legionella

  • and send white blood cells to the infected lung.

  • Plus, most patients respond well to antibiotics.

  • Legionella doesn't rely on bugs to spread it,

  • like our first two parasites.

  • Instead, it lives in water --

  • places like stagnant freshwater ponds, plumbing systems, or stale hot tubs.

  • Legionella has a particularly nasty feature where it's resistant to chlorine and heat,

  • our usual methods of sterilizing water.

  • It does need a host to reproduce,

  • but that can come in the form of amoebas sharing the water with it.

  • And it spreads when people breathe in tiny water droplets that harbor the bacteria.

  • That could mean anything from an outdoor mister to an indoor humidifier.

  • The important thing is that you inhale this water, not drink it.

  • Once in your system,

  • Legionella sticks to its host cell and triggers the cells to take it up.

  • But it's less picky than our first two examples about what kind of cells it infects.

  • And that means it probably has multiple ways of getting in.

  • Lung cells, in particular, engulf the bacterium in a compartment where it would normally be

  • destroyed.

  • But somehow, the bacterium keeps the destruction part from ever happening,

  • and lives there quite happily instead.

  • Meaning until we work out better strategies for keeping Legionella out of our water,

  • it's going to keep finding ways to call our lung cells home.

  • Number four on our list is one of the least dangerous but most common sexually transmitted

  • infections,

  • Chlamydia trachomatis, or affectionately, just chlamydia.

  • This parasite spreads very efficiently.

  • It's the most commonly reported bacterial STI in the U.S.,

  • though many people who have it don't show symptoms.

  • Chlamydia is a bit of an oddball in that it's a lot less independent than some other parasites.

  • They can't swim or move on their own,

  • and they lack the ability to produce their own energy,

  • so they depend on their host cell for just about every vital function.

  • So the chlamydia bacterium floats around in an inert form until it attaches to a host

  • cell.

  • Then the host cell wraps up the invader in a little package called an inclusion,

  • similar to if the cell were trying to destroy it.

  • Cells have these bodies called lysosomes that are full of destructive enzymes.

  • Now, usually, we'd expect the little intracellular inclusion bundle to fuse with a lysosome

  • and let those enzymes disintegrate the invader.

  • And if the host detects something really bad happening,

  • it can even destroy itself along with the parasite in the process.

  • Chlamydia not only avoids destruction,

  • but manipulates its host cell to hijack its energy sources.

  • They do that by using a variety of proteins to communicate with the host from inside their

  • cosy little inclusions.

  • After the bacterium establishes itself in the inclusion,

  • it starts replicating and matures, eventually bursting out from the host cell

  • to start the process over again.

  • From there, chlamydia spreads to new people through a few different methods,

  • including sexual activity or childbirth.

  • Luckily, chlamydia is pretty easy to cure with a round of antibiotics.

  • Either way, if you're sexually active, make sure you're routinely getting tested.

  • Next up is microsporidians, an entire phylum of single-celled organisms.

  • And they're exactly what the name implies

  • parasites with tiny spores that they use to invade host cells.

  • But these aren't bacteria -- current research suggests they're either a type of fungus,

  • or something closely related to fungus.

  • We're not sure!

  • Now, most humans that contract a microsporidian don't show any symptoms --

  • though not in all cases.

  • And they tend to show up more frequently in patients with compromised immune systems,

  • like folks with HIV or who are undergoing radiation therapy.

  • And because this is a big and diverse group of parasites,

  • an infection can cause different symptoms depending on the type of microsporidian

  • and what body part it infects.

  • Which includes the gut, lungs, and eyes, to name a few.

  • Lovely.

  • And they have a really fun way of getting into cells.

  • When a spore encounters a favorable environment, it starts to germinate.

  • Then it grows a tiny harpoon called a polar tubule,

  • which it uses to spear the unfortunate host cell.

  • It uses the polar tubule to inject reproductive material,

  • which then goes on to eventually develop into a whole bunch of mature spores --

  • which burst out of the host cell and start the process over again.

  • Microsporidians don't limit themselves to humans.

  • They can live in all sorts of cell types in a bunch of different kinds of animals,

  • as well as other organisms.

  • Which means contact with other animals is one way these things might spread --

  • but so might person-to-person contact, inhalation, ingestion, and direct contact with your eyes.

  • We actually don't understand how it's transmitted all that well.

  • Basically, these things are everywhere.

  • Plus, you might have one right now and not know it.