Ascariasis

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This time at Infection Landscapes I cover the first in the series on helminth infections. This week we will discuss ascariasis, which is one of the single biggest contributors to overall global morbidity and disability-adjusted life years.

The Worm. Ascariasis is caused by Ascaris lumbricoides, also known as the giant intestinal roundworm, which is a nematode:

A. lumbricoides worms undergo several unique developmental stages within their human host. An infection begins when the Ascaris eggs are consumed in contaminated water or food. The larvae emerge from the infective eggs in the small intestine, where the larvae then target the mucous membrane of the epithelium. The larvae penetrate the epithelium and make their way to the lungs by way of the blood and lymphatic circulation. The larvae will spend approximately two weeks in the lungs, where they continue their growth and development. Following this period of maturation, the larvae penetrate the alveoli and migrate up the bronchii and trachea until they reach the throat. Here they will be swallowed again and, after reaching the small intestine for the second time in their life cycle, develop into adult worms. If they are not agitated, the adult worms then remain in the small intestine where they obtain their nutrients from the ingested meals of the host. Female worms produce tens to hundreds of thousands of eggs per day, which are passed in the stool of the infected host. If the eggs are fertilized by an adult male in the lumen of the gut, then they will embryonate and become infective after three or more weeks outside the host in the environment. Eggs that are not fertilized will not become infective. Infective eggs that are ingested by a new host will emerge as larvae in the small intestine and the process begins again. The process within the human host, from the infection with embryonated eggs to the emergence of adult worms, typically requires approximately two to three months to complete. The adult worms will typically live up to 2 years within the gut lumen of their host. Below is a nice graph by the Centers for Disease Control and Prevention that depicts the complex life cycle of Ascaris lumbricoides:

And here is another nice graph by MetaPathogen.com that nicely contextualizes the life cycle within transmission:

The Disease. Ascariasis is most often an asymptomatic infection. However, given the invasive nature of these infections, particularly during the stages of larval development, clinical disease can manifest in a variety of organ systems. Hepatosplenomegaly and peritonitis are typical symptoms when clinical disease is apparent. Given the extended developmental period in the lungs, pneumonia is another common complication associated with ascariasis. In very high volume infections with large numbers of adult worms, the parasites can actually block the intestine, inhibiting digestion, as well as directly competing with the infected host for nutrients from food. The picture below shows a high volume infection, with many worms taken from a dissected intestinal section:

Agitation of the Ascaris worms is another potential problem during active infections. The most common causes of agitation are chemotoxicity, most often due to the effects of drugs used to treat other infections (a common occurrence in ascariasis-endemic areas), and fever, also a common occurrence among individuals co-infected with other pathogens. When agitated, the worms attempt to exit the host either by burrowing through tissue or by migrating up and out the gastrointestinal tract:

The former can cause severe tissue damage to various organ systems, while the latter can cause suffocation if the infection consists of a large number of worms. Overall, these infections are important contributors to morbidity because high volume or repeated infections involve worms that are competing with the host for food nutrients. As such, ongoing infections with large numbers of worms can lead to chronic malnutrition, and this is most pronounced in children.

The Epidemiology and the Landscape. Approximately one and a quarter billion people are infected with Ascaris lumbricoides. In some highly endemic areas of the Americas and Africa, infection prevalence can approach 50% to 95%, respectively. The sheer volume of infection is a major factor in why this neglected tropical disease is such an important contributor to global morbidity. 

Transmission is primarily via ingestion of water or food contaminated with the embryonated, infective Ascaris eggs. However, this is does not directly follow the fecal-oral route because, as described above, the eggs require an extended period outside the host in the environment to become infectious. As such, direct person to person transmission does not occur. Nevertheless, the transmission is still indirectly fecal-oral as the eggs are passed in the feces of an infected host and will ultimately infect a new susceptible host following ingestion of these eggs in a contaminated common vehicle, such as water or food.

Control and Prevention. Control and prevention of ascariasis begins by following the usual guidelines: improving sanitation in resource poor areas and maintaining vigilance in personal hygiene. In most settings in the world where ascariasis is a significant contributor to morbidity, improved infrastructure that can maintain adequate water resources is a first priority in its prevention.

Secondarily, personal hygiene at the individual level, especially in the context of food preparation, can also be very important in preventing ascariasis: consistent hand washing, boiling water, and thoroughly cooking food are all important in stopping the chain of transmission. The implementation of these strategies would go far toward eliminating the morbidity attributable to malnutrition in children in much of the world.