"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.