Popularly known as ‘roundworm’, Ascaris lumbricoides is probably the largest nematode to infect humans as it can grow up to a meter with an average size of about 45 cm. It affects over 1 billion people worldwide and its prevalence is high in the tropical climates where warm and moist soil can be found.
The initiating point for ascaris lumbricoides life cycle would be its eggs, which are excreted with feces of infected humans. There can be thousands of eggs excreted via stools and if it receives a warm and a moist environment such as in the soil of tropical climates, it can remain active and infective to humans. However, even in instances where the climate remains hostile to the eggs, it can remain dormant for many months or even years until it receives a favorable environment.
While remaining in the soil, ascaris eggs can contaminate water sources, vegetables, fruits, as well as human hands handling such soil. Through any of these contaminated routes, the eggs will be able to reach the human mouth, which will be the portal of entry for ascaris eggs in to the human digestive tract.
In the digestive tract:
Following being placed in the mouth, eggs will be swallowed and will withstand the acidity of the stomach until it reaches the upper portion of the small intestine. While it is in the small intestine, it will hatch into its larval form. The roundworm larvae will penetrate the intestinal mucosa in order to enter the blood circulation where it will undergo further maturation.
In the blood circulation:
Once in the blood circulation, the larval form will be able to travel through the liver and the heart and would reach the tiny blood vessels on the surface of alveolar spaces or ‘lung air sacs’. From these blood vessels, it will be able to penetrate into the ‘air sacs’ where they will develop further towards a more mature form.
In the lungs:
After certain degree of maturation, the larval form would be coughed-up or will ascend in to the upper airways where it can be swallowed back in to the digestive system.
Back again in the digestive tract:
During the second round, ascaris larvae would remain in the small intestine for a longer duration and would reach its adult form and size. However, the female roundworm would grow larger than its male counterpart and would reach nearly 45cm or greater in length.
Production of eggs and re-starting the life cycle:
Once the female roundworm reaches sexual maturity, it can produce thousands of eggs per day, which will mix with intestinal content and would be excreted via the infected person’s feces to begin another roundworm life cycle.
The entire life cycle of ascaris lumbricoides would last for around 2 – 3 months whereas, the adult worms can live up to a year producing millions of eggs.
Principles and Practice of Clinical Parasitology : Stephen Gillespie, Richard D. Pearson
Center for Disease Control