"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 14, 2010

February 14 - Giardia lamblia

Giardia is a very common parasite found all over the world and one that is capable of infecting humans as well as other animals. It is frequently found in water sources particularly those where human and animal contamination is likely. It's frequency of infecting hikers and campers who drink untreated water has led to its common name, "Beaver Fever." Humans or animals become infected when they ingest cysts. Trophozoites, the feeding stages emerge from these cysts in the digestive tract. These stages can asexually divide (as shown in the photo). The symptoms of a Giardia infection are a suite of gastrointestinal unpleasantries, some of which have been described as "explosive" and "violent." Giardia can be difficult to confirm medically but the good news is that it usually resolves itself in healthy people. This organism was discovered by Anton von Leeuwenhoek, who developed one of the very first microscopes, when he opted to examine his own feces using his new invention.

Giardia is a unicellular eukaryotic organism that possesses flagella. The placement of Giardia in the "tree of life" has been rather controversial. Initially, it was contended that these organisms represented the deepest branch of all eukaryotes, because they lacked mitochondria (like Trichomonas and Entoamoeba, two parasites we will see here in the near future). The belief was that these organisms, collectively termed the "Archezoa" diverged from other eukaryotes before the acquisition of mitochondria. However, it was later discovered that not only did Giardia have genes much like those of the alpha-proteobacterial ancestor of mitochondria within its nuclear genome, but that it also had structures, termed "mitosomes", that were thought to be remnants of previous true mitochondria in these cells. This is still something of an open question, partly because Giardia and some of these other amitochondriate organisms do not have many close relatives and so are subject to phylogenetic vagaries in the construction of the tree. Hopefully more genome sequences of protozoa and better methods to understand horizontal gene transfer processes can help to solve this riddle. In the meantime, filter your water when you're camping - and happy Valentine's Day.


  1. cool, they make a heart shape! but wait...this is a repost! u owe us another day!!!!

  2. Good eye - or memory - Alex (although this was hardly a repost given that the first one was one of the original ten that each consisted of about three lines). Today's has to stay, but the other has been swapped to a close cousin that looks just like it. I want everyone to get their full dose of 365 parasites - electronically, of course.

  3. I generally find microorganisms that have a very distinct shape, appealing and easy to remember. I’m intrigued by the fact that there are some protozoans like Giardia that lack mitochondria and yet are capable of carrying out cellular respiration and able to infect the intestines in anerobic conditions. It is also interesting to note that Giardia contains a very high number of flagella that are positioned at unique positions possibly enabling efficient locomotion. I am surprised to learn that Giardia was one of the first to be viewed under the microscope and I cannot contain my laughter when I found out that the great Anton Von Leeuwenhoek examined his own feces.

  4. Giardia are always a practical organism to be conscious of when around untreated water, and what is parasitology without practicality? Beyond that, I find the taxonomy and genetic uncertainty on their subject fascinating. As the above post discussed, there are, or rather were, uncertainties about the group’s exact taxonomic placement. In the 11 years since the post was made, genetic sequencing has come a long way, but it is still interesting to see an older perspective.

  5. The unique and distinct shape of Giardia lamblia is quite interesting and very appealing to my eyes, which attracts me to learn more about this parasite. It was very interesting to know that this particular parasite does not have true mitochondria but is still able to perform respiration and infect its host. Another very interesting fact from the text is that Giardia lamblia was discovered by the person who invented one of the first microscopes, Anton von Leeuwenhoek. The process by which Giardia lamblia enters a human host and thrive in the small/large intestine causing gastrointestinal unpleasantries is quite interesting to know as well as their morphology is also very fascinating.