Ascariasis lumbricoides commonly known as the large intestinal roundworm, roundworm of man causing ascariasis or roundworm infection is the world’s most common intestinal helminth infection, affecting approximately 1 billion people. It is the second most common infection in the United States, after pinworm infection. Warm climates and areas of poor sanitation, particularly where human feces is used as fertilizer and children defecate directly on the ground, are the regions of the world and the United States most vulnerable to Ascaris. The Appalachian Mountains and the surrounding areas east, west, and south of them have the highest prevalence of Ascaris lumbricoides in the United States.
Children who put their contaminated hands in their mouths are the most vulnerable to Ascaris lumbricoides infection. Contamination sources range from children’s toys to the soil itself. In areas where vegetables are grown with contaminated human feces as fertilizer, people of all ages may become infected. Although water has been identified as a possible source of A. lumbricoides infection, this occurs only infrequently.
A. lumbricoides, in addition to pinworm, is suspected of being responsible for the transmission of Dientamoeba fragilis, though this theory has yet to be proven.
Morphology of Ascaris lumbricoides
Unfertilized Eggs
The average oblong, unfertilized Ascaris lumbricoides egg measures 85 to 95 m by 38 to 45 m. The inner amorphous mass of protoplasm is shielded by a thin shell. Normally, the egg is corticated (i.e., the egg possesses an outer mammillated, albuminous coating). Shape, size, and cortication variations are also possible.
Fertilized Eggs
The fertilized A. lumbricoides egg is more rounded than the unfertilized egg, usually measuring 40 to 75 µm by 30 to 50 µm. Fertilization of the egg transforms the amorphous mass of protoplasm into an undeveloped unicellular embryo. A thick nitrogen-containing polysaccharide coating called a chitin, also known as a shell, is sandwiched in between the embryo and mammillated albuminous material (corticated). Both layers protect the embryo from the outside environment. Eggs lacking an outer mammillated, albuminous coating (refered to as decorticated) fertilized eggs may also be present. The chitin shell is less evident in corticated eggs than in those that have lost their outer albuminous coating. Corticated eggs have a less visible chitin shell than those that have lost their outer albuminous coating.
Adults Ascaris lumbricoides
Adult A. lumbricoides worms are typically creamy white with a pink tint. Fine striations can be seen on the cuticle (a surface covering present on adult nematodes). Adult Ascaris worms are the largest intestinal nematodes known. The average adult male (is short, rarely exceeding 30 cm in length. The male is typically slender and has a prominent incurved tail. The adult female is 22 to 35 cm long and the thickness of a pencil lead.
Life Cycle of Ascaris lumbricoides
In comparison to the parasites presented thus far, A. lumbricoides’ life cycle is relatively complex. Infection begins with the ingestion of infected eggs containing viable larvae. The larvae hatch from the eggs once they reach the small intestine. The larvae then complete a liver-lung migration by first entering the bloodstream via intestinal wall penetration. The liver is the first stop on this journey. The larvae then travel through the bloodstream to the second stop, the lung. Once inside the lung, the larvae burrow their way into the alveoli by burrowing through the capillaries. The cells then migrate into the bronchioles.
The larvae are transferred from here to the pharynx via coughing, where they are swallowed and returned to the intestine.
Maturation of the larvae occurs, resulting in adult worms that live in the small intestine. The adults multiply, and a large number of undeveloped eggs (up to 250,000 per day) are passed in the feces. The outside environment, specifically soil, provides the necessary conditions for the embryonation of the eggs. Infectious eggs can live for years in soil, feces, sewage, or water. It is worth noting that these eggs may survive in the 10% formalin fixative used in stool processing. The longevity of these eggs is due in part to the fact that they are not easily destroyed by chemicals. When the resulting embryonated eggs are consumed by a human host, they initiate a new cycle.
Clinical Symptoms Ascariasis
Asymptomatic
Patients who are infected with a small number of worms (5 to 10) are often asymptomatic. These patients typically consume only a few eggs. They may only become aware of their infection if they find an adult worm in their recently passed feces or submit a stool for a routine parasite examination.
Ascariasis: Roundworm Infection
Patients with symptomatic ascariasis may only be infected with a single worm. As it moves through the host, such a worm may cause tissue damage. Following worm perforation out of the intestine, a secondary bacterial infection may occur.
Patients with multiple worm infections may experience vague abdominal pain, vomiting, fever, and distention. Mature worms can entangle themselves and form a mass that can obstruct the intestine, appendix, liver, or bile duct. Such intestinal complications can be fatal. Adult worms exiting the body through the anus, mouth, or nose may also cause discomfort. Children who are heavily infected and do not practice good eating habits may develop protein malnutrition.
Aside from symptoms associated with the intestinal phase of ascariasis, patients may also experience pulmonary symptoms as the worms migrate through the lungs. Patients may develop a low-grade fever, cough, eosinophilia, and/or pneumonia during this stage. An allergic asthmatic reaction to the presence of the worms is also possible.
Laboratory Diagnosis of Ascariasis
Stool is the preferred specimen for the recovery of A. lumbricoides eggs. Adult worms can be recovered from a variety of specimens, depending on the severity of the infection, including the small intestine, gallbladder, liver, and appendix. Adult worms may also be found in the stool, vomited up, or removed from the external nares, where they may be attempting to escape. There is also an enzyme-linked immunosorbent assay (ELISA) available.
Treatment of Ascariasis
Medication designed to rid the body of parasitic worms is recommended for A. lumbricoides infections that require treatment. Albendazole and mebenazole are two examples.
Prevention and Control
Avoiding the use of human feces as fertilizer, as well as practicing proper sanitation and personal hygiene practices, are critical steps in breaking the A. lumbricoides life cycle.
Reference
Clinical Parasitology: A PRACTICAL APPROACH. Elizabeth A. Gockel-Blessing (formerly Zeibig) P.212-216
