|Okay, so this person has green gloves, but the purple |
are much more memorable to me!
|A pinned dissection of a female Ascaris.|
This morning I was thinking about what to post for this week, and I realized that I had never posted anything about this creature, which in all fairness was the first creature to spark my interest in parasites. So let's talk about it!
Not long ago, I learned that this worm has a common name...which I had never heard before, surprisingly. It is known in other places, especially in Europe, as "the maw-worm". The word "maw" has deep roots dating back to both the Old English "maga"and the Middle English "mawe" both referring to the stomach or the belly. Today, the word is more often used to describe the upper digestive tract, especially the mouth of a particularly ravenous animal. As we discuss the life cycle of this animal, keep both definitions in mind. But first things first.
Ascaris lumbricoides belongs to kingdom Animalia and to phylum Nematoda (roundworms). As roundworms, they lack circulatory and respiratory systems and have a cuticle that is shed during successive growth stages. This last characteristic is why these animals are thought to be more closely related to arthropods than to flatworms or segmented worms. This species is further classified under the class Rhabditea along with other parasitic nematodes such as threadworms and pinworms. They exist in the order Ascaridida on the basis of having three prominent lips, lots of cuticular sensory structures called caudal papillae, and lateral external labial papillae. These worms are placed in the family Ascarididae, which it shares with the famous dog parasite, Toxocara canis.
|Posterior end of a male A. lumbricoides.|
The eggs of these animals are particularly fascinating. They can be oval or round and appear to have a bumpy outer shell. This bumpy layer is comprised of proteins and is referred to as being "mammillated", which is really just a fancy word for bumpy. The lipids in the layer beneath the proteins in the egg shell are largely ascarosides. Ascarosides are glycosides (sugars + alcohol + glycosidic bond) which give the shell its incredible powers of impenetrability. The only things that can pass through such layers are gases and lipid solvents. This gives them a resistance to a variety of chemicals that one parasite text book referred to as "almost legendary". These eggs have been known to remain viable in formalin, potassium dichromate, hydrochloric acid, nitric acid, acetic acid, and even sulfuric acid. (*Note* The acids were at 50% solutions, not at full strength, but that's still pretty amazing!) There have been documented cases of these eggs remaining viable after 10-12 years in the environment. The eggs are sometimes carried accidentally by cockroaches and other household pests that may dwell in contaminated soils. (Another reason I hate cockroaches.)
The life cycle of these worms starts with their impervious eggs being shed in the feces of a human (or pig) host into soil. While in the environment, juvenile worms will begin to form inside of the protective egg shells of fertilized eggs. The juveniles will undergo a series of life stages, all inside the egg, before the egg becomes infective. At this point, a person (or pig) eats fruits or vegetables that are contaminated with egg-laden soil or drinks water with similar contamination. Sometimes the eggs can even become airborne under the right conditions and infect via the nasal passages of humans. The eggs travel down into the stomach, where they continue to be impervious to the acids of the stomach, and on into the small intestines. The eggs then hatch releasing the juvenile worms who then penetrate the intestinal walls to hitch a ride in the hepatic portal system. The now fourth-stage worms reach the lungs and penetrate through the alveoli. They move up through the bronchiole tubes and into the trachea before being coughed up and re-swallowed back down into the esophagus. Once back in the small intestine, the worms mature, mate, and start pumping out eggs to start the cycle over.
The question I have, which is a question other more experienced biologists still trying to answer, is why the long journey out of the small intestine only to wind back up in the same place? Why not just stay there and mature in a safer environment? Some think this odd behavior is something that is a relic of some evolutionary adaptation with origins much further back in time. It makes me think of a sort of parasite walk-about. Where juveniles must embark on a perilous journey through our bodies in order to prove they have the ability to survive our dangerous immune system. Upon returning home, they are given the privilege of mating and passing on their superior genes to the next generation of survivalists. (I feel like I could make a cartoon out of this...)
|A view of Ascaris within a small intestine.|
|A removed section of intestine that was |
cut open to reveal blockage by these worms.
Currently, there's no good way to identify juvenile stages of this parasite within a patient's body. Juveniles can be identified from sputum, but only by technicians who recognize them as being such. Fecal examinations can reveal eggs after worms have reached maturity and begun laying eggs. The symptoms described above can lead to a suspicion of ascariasis, but aren't enough for a full diagnosis alone.
The Moral of the Story
Wash your hands...especially if you've been playing in nightsoil (soil mixed with human feces). Also, don't use nightsoil as a fertilizer for your crops (don't laugh, many parts of the world do this)...and for God's sake, wash your fruits and vegetables well! This isn't something we worry about too much here in the states because ascariasis is found more often in tropical regions, but don't think we are immune here. Just a few weeks ago someone brought into a local parasite lab a worm that their son had passed right here in Nebraska. The worm was a nice-sized female, and upon dissection she was found to have fertilized eggs, meaning the boy also had at least one male still inside of him. (Don't worry, I'm sure the good parasitologists recommended the parent get ahold of some Mebendazole.) The theory is that he got this from working on a pig farm. Pigs carry these worms in the same way that humans do...in fact, the species known to infect pigs (Ascaris suum) is so similar to A. lumbricoides that some people argue these are actually the same species! So again...keep those hands clean!
The Other Moral of the Story
It's funny how some things stick in your mind. As I advanced through school after the day I opened up that worm I fell more and more in love with biology. I went from wanting to be a psychologist, wanting to do genetic engineering, to wanting to be a herpetologist, and finally to wanting a career as a parasitologist. The progression was slow and meandering, but I can still look back on my freshman year in Zoology Lab...to that exact day learning about nematodes....and it becomes clear that I was meant to be a parasitologist all along. I guess it just took meeting other parasitophiliacs to convince my brain of what my heart must have already known. <3