Saturday, April 6, 2013

A Love Letter About My First Parasite Encounter: Meet Ascaris lumbricoides

Okay, so this person has green gloves, but the purple
are much more memorable to me!
The name Ascaris lumbricoides holds special significance for me. This enormous nematode was the first parasite I ever learned about in any great detail.  I can remember sitting there in the Zoology lab on the second floor of Bolin Science Hall and staring at this odd preserved creature resting harmlessly in the dissection tray before me.  We had all been warned that the eggs from these animals could sometimes survive being fixed in formaldehyde, so our desks were free from the clutter of our books and drawings binders and we were all wearing those now all-too-familiar purple gloves. My labmates were not excited about this lab...they were rather repulsed in fact, so I had been elected to do the "cutting" for this one. I remember thinking I was a pretty big nerd to not feel the same repulsion. In fact, I was actually intrigued by this miniscule monster. I had learned only minutes before that the females of this species can lay up to 200,000 eggs in a single day...a fun fact that has stuck with me even to this day. They weren't gross, they were fascinating. As I carefully drug the dissection pin down the length of the specimen's delicate tegument, I recall being curious about how such biological simplicity could lead such a complex life, and how something so...well...dissectionally "boring"...could be such a huge problem for so many people all over the world. Their anatomy was simple, but profoundly efficient. Their life cycle was strange and led to many questions that I would ponder over the next few years as I learned more about these parasites.

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.
I won't got into too much detail here, but I will mention that these animals are quite large compared to other nematodes. Males typically range from 15cm-31cm in length and possess a distinctly curved posterior end. They are about 2mm-4mm in girth and have a few little spicules on their distinctive hook that are used for copulation.  The females are much larger, measuring between 20cm-49cm in length and 3mm-6mm in girth. Her ovaries extend the length of her body and her uterus is split into two parallel elongated structures that may hold as many as 27 million eggs at a time.

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.)

Life Cycle
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.
The infection of a person with these worms is termed ascariasis.  Some of the bigger issues associated with infection occur when juveniles stray from their path to the lungs and die in other organs like the spleen or liver.  This can cause immune responses such as inflammation in these areas. Juveniles have also been known to migrate into developing fetuses through placental walls of expectant mothers.  The worms that manage to make it to the lungs cause hemorrhaging of lung capillaries, which becomes more severe in heavy infections.  With heavy parasite burdens, some people suffer from pulmonary edema and air-space clogging. Dead lung tissue and white blood cells may accumulate to create Loeffler's pneumonia (a.k.a. Ascaris pneumonitis).  If  unchecked, this can lead to diseased lungs that invite bacterial infections and could result in death.

A removed section of intestine that was
cut open to reveal blockage by these worms.
This worm might occasionally suck blood, but most of the time it just sticks to absorbing liquids from intestine. With enough worms, this can lead to malnutrition or underdevelopment.  It can also impair cognitive abilities in children. Most hosts experience abdominal pain, rashes, insomnia, asthma, and/or eye pain.  With massive infections, the intestines can become blocked by these worms, which is a fatal condition.  Adults will also sometimes wander out of the intestine to cause strange problems elsewhere in the body.

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 choice drug for treating ascariasis is Mebendazole.  This drug binds with tubulin (a structural protein) and essentially paralyzes the muscles and intestinal walls of the worms. Ivermectin (a de-worming agent) is also used in some cases, as is Nitazoxanide (a drug for treating cryptosporidia). In cases of malnutrition, supplements are often administered as well. I have heard of people undergoing surgery to remove partial intestinal blockages due to these parasites, but I haven't actually read anything on that particular subject, so I don't know how often that actually happens.

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 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 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

A throw-back to nematode-day of General Zoology Lab, which I took as a freshman, then taught as a grad-student.  The top left shows a male above a female to show their dimorphism. I'm not sure about the top right, so I won't speculate. The bottom left is a cross-section through a male showing the intestine surrounded by the testes and other reproductive organs.  The bottom right is a cross-section through a female showing the intestine (bacon-looking strip), uteri (the two big round structures), and ovaries (other round structures).  I remember we could always tell the male from the female cross-section by looking for the "eggs and bacon" or the "smiley face" in the female. :p


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