Ascaris lumbricoides, commonly known as “roundworm” is the largest nematode parasitizing man. The adult worms are cylindrical, with a tapering anterior end. When freshly passed from the intestine, Ascaris is light brown or pink in color, but gradually changes to white. Ascaris infection (ascariasis) is the most common human worm infection with nearly 1 billion cases every year.
Infection in humans is acquired through ingestion of the embryonated eggs from contaminated soil.
Ascaris lumbricoides is one of the soil-transmitted helminths, other most commons soil-transmitted helminths are whipworm (Trichuris trichiura), and hookworm (Ancylostoma duodenale and Necator americanus.
On ingestion, the embryonated eggs hatch in the stomach and duodenum, where the larvae actively penetrate the intestinal wall. They are then carried to the right heart via the hepatic portal circulation. Larvae within the eggs undergo one or possibly two molts prior to penetration of the intestinal wall.
From the right heart, they are carried into the pulmonary circulation, where they are filtered out by the capillaries. After approximately 10 days in the lung, the larvae break into the alveoli, migrate via the bronchi until they reach the trachea and pharynx, and then are swallowed.
The worms then mature and mate in the intestine, with the eventual production of eggs, which are passed in the stool. The stool may contain both fertilized and unfertilized eggs. After 2 weeks, fertilized eggs become infective if they are in warm, moist soil. Fertilized eggs may remain viable for months or even years.
This entire cycle of egg ingestion to egg passage takes about 8-12 weeks.
People get an infection with Ascaris by swallowing embryonated eggs of Ascaris with raw vegetables cultivated on soil fertilized by human excreta or by drinking water contaminated with mature eggs of Ascaris.
The major burden of this parasitic disease lies in under-developed or developing countries of tropical and sub-tropical regions where sanitation is not good. Children and early adolescents are mostly affected as they spend most of their time playing in the contaminated fields (in many such countries people defecate in open areas and maybe using the feces of an infected person as fertilizer).
Ascariasis is the disease caused by Ascaris lumbricoides. Pathogenesis caused by Ascaris infections is attributed to
- The host immune response,
- Effects of larval migration,
- Mechanical effects of the adult worms, and
- Nutritional deficiencies due to the presence of adult worms.
Host Immune Response
Various allergic manifestations such as fever, urticaria, angioneurotic edema, wheezing, and conjunctivitis are seen when the host immune cells react with the toxic body fluid (ascaron) of the adult worms.
Effects of Larval Migration
The worms are restless wanderers. The migrating larvae may cause inflammatory and hypersensitivity reactions in the lungs. Allergic inflammatory reactions to migrating larvae may involve other organs such as liver and kidneys. Loeffler’s syndrome is caused by migrating larvae.
Reinfection and subsequent larval migration cause intense tissue reactions in some individuals.
There may be pronounced tissue reaction around the larvae in the liver and lungs, with infiltration of eosinophils, macrophages, and epitheloid cells. This condition, also known as Ascaris pneumonitis is accompanied by an allergic reaction consisting of dyspnea, a dry or productive cough, wheezing or coarse rales, fever, transient eosinophilia, and a chest X-ray suggestive of viral pneumonia. Examinations of sputum or gastric washings may reveal larvae.
Mechanical Effect of Adult Worms
The presence of adult worms in the intestine usually causes no difficulties (sometimes may cause colicky cramps and loss of appetite) unless the worm burden is very heavy. However, during migration and/or if heavy infections adult worms may result in intestinal blockage.
Nutritional Deficiencies due to the Presence of Adult Worms
Adult worms rob the host of its nutrition and may cause malnutrition and night blindness (due to vitamin A deficiency). In children, particularly those younger than 5 years, there may be severe nutritional impairment related to worm burden which may cause malnutrition, stunting, and impairment in cognitive ability among others.
Microscopy and Staining
In the larval migration phase of infection, diagnosis can be made by finding the larvae in sputum or gastric washings; however, this is not a common finding.
During the intestinal phase, the diagnosis is made by finding the eggs (unfertilized or fertilized) or adult worms in the stool. The eggs are most easily seen on a direct wet mount or a wet preparation of the concentration sediment. Zinc-sulfate flotation concentration method or formal-ether concentration method is commonly used to concentrate the stool sediment.
Morphology of Eggs
Human excreta may contain both fertilized and unfertilized eggs of Ascaris. If the person is harboring only females, unfertilized eggs are only seen in the stool.
- Fertilized Eggs: Fertilized eggs of Ascaris lumbricoides are broadly oval, with a thick, mammillated coat, usually bile stained a golden brown. These eggs measure up to 75 um long and 50 um wide.
- Unfertilized Eggs: Unfertilized eggs of Ascaris are usually more oval, measure up to 90 um long, and may have a pronounced mammillated layer. Unfertilized eggs do not float (the eggs are too heavy) with the use of the zinc sulfate flotation concentration method.
Antibodies against Ascaris can be detected by the indirect hemagglutination method or by the immunofluorescent antibody (IFA) test. These tests are useful for the diagnosis of extraintestinal ascariasis like Loeffler’s syndrome.
References and further readings
- Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
- Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.