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Ascaris Worms: Larvae & Reproduction

Instructor: Sarah Phenix
In this lesson we explore the wild ride of Ascaris worm growth and maturation. We'll look at the reproduction cycle that leads them on a journey through their host's body as well as the outside world.

Not Your Common Earthworm

Ascaris is a genus of worms that belongs to the phylum Nematoda, or roundworms. Now, you may hear roundworms and think of your common earthworm (which may or may not make your cringe), but earthworms are not classified as nematodes. Nematodes have a very different anatomical makeup than that little earthworm. And as a result, the Ascaris genus of the Nematoda phylum lives in an extremely different environment than an earthworm, mainly the intestines of a host organism.

Ascaris lumbricoides: giant intestinal roundworms

That's right, Ascaris worms are helminths, or parasitic worms, meaning that they survive by living or feeding off a host organism. Now, in the case of Ascaris worms, they don't actually feed directly off of their host like a mosquito or tick does. They don't consume the blood or tissue of their host, but they do live in their host's gut and utilize nutrients from the food that the host eats. What's more is that depending on the particular species of Ascaris worm, that host may or may not be human. With that said, I bet those little earthworms are looking a bit less cringe-worthy.

Species of Ascaris

There are two of the most common 'garden' variety (although never to be found in an actual garden) Ascaris worms to be familiar with. First is Ascaris suum. These worms typically infect both wild and domesticated pigs, but can also take up residence in a human host. The second, Ascaris lumbricoides, is commonly known as the giant intestinal roundworm, mainly infects humans, and can reach lengths of up to 14 inches.

Ascaris lumbricoides, while being one of the most common parasitic worms to invade a human host (about 1-2 billion people worldwide), are generally relegated to less developed countries or areas where human feces may be used as a fertilizer. However, unwashed food grown and shipped from those countries may also harbor these little castaways. Now that you know what they are, we will explore the life cycle of these admittedly disgusting, and sometimes not so little, parasitic worms.

Infection by Ingestion

Ascaris life cycle
Ascaris Life Cycle

Infection of Ascaris worms occurs from consuming food or beverages contaminated with fertilized Ascaris eggs. This lends to their designation as soil-transmitted helminths, or soil-transmitted parasitic worms. In other words, any food items grown in contaminated soil can be a carrier of Ascaris eggs. What's more is that these eggs can survive for several weeks to months outside of a host, which means they aren't opposed to waiting around for a while.

During this 'waiting' time, a larva develops within the protective casing of the egg. However, Ascaris larvae can't actually hatch from their eggs on their own. They need the acidic environment of the host's intestine to break down their eggs' shells.

Hatching in the Gut: The Journey Begins

Once released from their eggs the larvae are known as second-stage larvae. These guys are only just beginning their journey at this stage. Once hatched, they burrow into the intestinal walls to access blood vessels that will carry them to the liver and on to the heart. Once there, the heart pumps those second-stage larvae to their target location, the lungs, where they'll spend anywhere from 10-14 days. During this journey, they leave the thin walled vessels of the host's blood stream by breaking into the alveoli of the lungs, which is the thin sac-like tissue responsible for absorbing oxygen into our blood stream.

Invading the Host's Lungs

Now, you might think that these larvae would be done with their world travels at this point, but you'd be wrong. During their stay in the alveoli they grow into third-stage larvae by molting (shedding) their old skin. As third-stage larvae, these little invaders make their way up into the trachea (also known as the wind pipe), where the host will cough them up and then swallow them, sending them back into the stomach and to their end goal, the small intestines.

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