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Profile of a Parasite: Giardia

  • Writer: BJW
    BJW
  • Aug 26, 2019
  • 6 min read

Updated: Jun 17, 2021

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Giardia (Giardia lamblia), literally, and figuratively, sucks. A protozoan parasite, Giardia is the pathogenic agent behind the disease Giardiasis. Environmentally-resistant cysts pooped out by previous hosts sit around in water for about three months before becoming eventually inert [1]. If one of the many suitable hosts, say, Stephen who thinks that stream water is fine to drink (“Guys, it’s totally fine, it’s clear!), comes along and ingests as few as ten active cysts, Giardia is able to colonize his upper small intestine (although boiling the water would have rendered the cysts unviable) [2]. Great work Stephen, you now have giardiasis (also known as beaver fever)!


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Stephen you fool!

Let’s follow along and see what happens in Stephen’s small intestines. Stephen has now joined the not-so-exclusive club of Giardia-affected individuals, one that gains an estimated 1.2 million members each year in America, and 280 million globally. [2,3]


Stephen, hikes on, oblivious to the new arrivals. The cysts meanwhile, passively make their way to his stomach, then subsequently, the duodenum of his small intestine. These cysts are stimulated by exposure to the gastric acids of Stephen’s stomach and a dousing in the bile and digestive enzymes of the small intestine [4]. Thusly stimulated, each cyst releases its contents, something called an excyzoite, which divides twice to yield four mature and free-swimming trophozoites, the other form of Giardia. These trophozoites exhibit what is called “falling-leaf motility” a quite apt name, as you can see in this gif below [1].


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Click for source video

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Giardia cysts (left), trophozoites (right)

These trophozoites quickly make themselves busy, multiplying by asexual binary fission (just like bacteria), doubling in number every 9 to 12 hours [3]. Some drift down to the lower parts of Stephen’s intestines, reverting into the infectious and environmentally resistant cyst form of Giardia. They also attach themselves to the mucosal lining of Stephen’s upper small intestine. They accomplish this by primarily using a curious and quite unique structure: the ventral disc. Positioned on the “belly” of a trophozoite, it looks and functions pretty much like a microscopic suction cup. A structure called the ventrolateral flange, also positioned on the belly of a trophozoite, helps attachment as well, although it is less well understood how the latter accomplishes this [5]. You can see both of these structures in the electron micrograph below, labeled as “vd” for ventral disc and “vlf” for ventrolateral flange.


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Micrograph pulled from Dawson 2010 [6]


Another curious feature of Giardia worth mentioning: In a single protozoan, you’ll find not one, but two nuclei. You can actually see them in the picture of the trophozoites above. What looks like eyes are actually the nuclei. Both of these nuclei are transcriptionally active. Initial investigations seemed to show that these nuclei were exactly identical, but over time, differences became apparent. For example, sometimes, individual Giardia specimens would be found with differing numbers of chromosomes between each nucleus [2]. I remain to discover definitively what benefit possessing two nuclei, and with these characteristics, might grant Giardia.


Giardia also lack mitochondria, instead possessing simpler (non-cellular-respirating) mitosomes [2].


But back to Stephen. Now a week after gaining his new intestinal tenants, he begins to feel their presence. Much like a ticking time bomb, Stephen’s stomach has run out of time. A panoply of tummy perturbations and privations ensues. Stephen screams. He runs to the stream, splashing into the ankle-deep bubbling currents. With no time for sanitary concern or modesty, he rips off his poorly-fitting hiking shorts and lets loose. The results, much like said time bomb, are explosive.


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We told you Stephen

Diarrhea, alongside abdominal pain, nausea, and bloating, are among the most prominent symptoms of Giardiasis. It’s highly unlikely that Stephen’s condition will be fatal, but he’ll have to endure this gastrointestinal torture for around 2 to 4 weeks if he seeks no treatment [3]. Press f to pay respects.


How does Giardia cause diarrhea?

In the calm before this violent, diarrheic storm, the giardia trophozoites have been working to produce this outcome. Originally, it was thought that Giardia caused diarrhea by physically covering enough of the small intestine in order to sufficiently affect absorption, but this theory has fallen out of favor.


Instead, damaging of the villi and microvilli of the small intestine (the little finger-like projections) is now thought to be the main way that Giardia causes diarrheic symptoms. However, the exact way Giardia damages the small intestine is yet to be fully verified [7]. The suspected methods range from physical trauma by inflicted by the attachment of Giardia trophozoites to unintentional damage resulting from host immune response [2].


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Left, section of healthy gerbil villi. Right, section of infected gerbil villi. Note the “blunted” nature of the villi on the right. Pecková et al. 2018 [8]


As the primary function of these projections is to aid the absorption of fluid and nutrients, the decreasing of their surface area, and thus their effectiveness, has understandably disastrous results for gastrointestinal and overall health. The decreased function of the small intestine contributes to the increase in poop water content, of course, but there is another factor at play.


Remember osmosis?

If you’ve taken high school biology, you probably know how it works. You can go ahead and skip this section if so.


For those that haven’t taken biology yet, with osmosis, you’ve got three things to consider: a liquid (called a solvent), a “semi-permeable membrane,” and stuff dissolved in the liquid (called the solute). This membrane allows the liquid to pass, but not stuff dissolved in it. The deal with osmosis is that this liquid (usually water in biology) will move so that the “concentration” (or you could think of it as the ratio of water:solute) of water is the same on both sides. Note that this results in sometimes uneven final volumes as the amounts of solute on boths sides may differ.

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This equalizing action may sound like magic, but it’s really not. Things work out this way because when there are more water molecules on one side (left side of the container in the diagram), that means that more will randomly cross to the other side. Water molecules from the right side of the container in the diagram/lesser side will cross as well, but since there are less water molecules there, there are less crossings to the other side compared to the opposite. This results in a net gain of water from left to right.


Osmosis is a really relevant principle in biology, given that, well, our cells’ exteriors are semi-permeable membranes and it just so happens that we’ve got a lot of water in our bodies.


But how does this relate to Giardiasis?

When the villi of the small intestine gets damaged, another result is that the activity of sugar-digesting enzymes, disaccharidases, gets lowered [7]. In other words, they no longer get as much done as before. Sugars such as maltose, sucrose, and lactose need to be broken down by these disaccharides before they can be absorbed by the small intestine.


As a result of the reduction of disaccharidase activity, these undigested sugars accumulate interior the small intestine. This is where osmosis comes in. These sugars now form a high concentration, relative to the exterior of the small intestine. They serve as the solutes. The small intestine (the cells that is) serves as the semi-permeable membrane. Thanks to osmosis, water then has a net increase in the amount present inside the small intestine, contributing the severity of Stephen’s diarrhea.


There you have it. The causative mechanisms of diarrhea in a case of Giardiasis.


Thanks for reading!

- BJW











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