Ascaris lumbricoides
Life Cycle and Morphology
The adult worms are cylindrical, with a tapering anterior end. They are the largest of the common nematode parasites of humans; females measure 20 to 35 cm long and the males are 15 to 31 cm long, with a curved posterior end. Also, the three welldeveloped lips are characteristic of this group. Infection in humans is acquired through ingestion of the embryonated eggs from contaminated soil. On ingestion, the 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. Then the larvae 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 are then swallowed.
The worms then mature and mate in the intestine, with the eventual production of eggs which are passed in the stool. The entire developmental process from egg ingestion to egg passage from the adult female takes from 8 to 12 weeks. During her life span, egg deposition may reach a total of 27,000,000 eggs. Both unfertilized and fertilized eggs are passed. Often only female worms are recovered from the intestine. Fertilized eggs will become infective within 2 weeks if they are in moist, warm soil where they may remain viable for months or even years. The fertilized egg is broadly oval, with a thick, mammilated coat, usually bile-stained a golden brown. These eggs measure up to 75 µm long and 50 µm wide. Unfertilized eggs are usually more oval, measure up to 90 µm long, and may have a pronounced mammilated coat or an extremely minimal mammilated layer. Often both types of eggs are found in the same stool specimen. The total absence of fertilized eggs means only female worms are present in the intestine.
Clinical Disease
Pathogenesis caused by Ascaris infections is attributed to (1) the host's immune response, (2) effects of larval migration, (3) mechanical effects of the adult worms, and (4) nutritional deficiencies due to the presence of the adult worms. Although the initial passage of larvae through the liver and lungs usually elicits no symptoms, there can be signs of pneumonitis if the number of larvae is quite large. When the larvae break out of the lung tissue and into the alveoli, there may be some bronchial epithelium damage. With reinfection and subsequent arval migrations, there may be intense tissue reactions, even with small numbers of larvae. There may be pronounced tissue reaction around the larvae in the liver and lung with infiltration of eosinophils, macrophages, and epithelioid cells. This has been called Ascaris pneumonitis and is accompanied by an allergic reaction consisting of dyspnea, a dry or productive cough, wheezing or coarse rales, fever (39.9-40.0°C), transient eosinophilia, and a chest xray suggestive of viral pneumonia. This picture of transient pulmonary infiltrates, clearing within a couple of weeks, and associated with peripheral eosinophilia is frequently called Loeffler's syndrome. In addition to eosinophils and CharcotLeyden crystals, the sputum may also contain larvae. Asthma and urticaria may continue during the intestinal phase of ascariasis.
The presence of the adult worms in the intestine usually causes no difficulties unless the form burden is very heavy; however, due to the tendency of the adult worms to migrate, even a ingle worm can cause serious squeal. Worm migration may occur as a result of stimuli such as fever (usually over 38.9 (°C), the use of general anesthesia, or other abnormal conditions. This migration may result in intestinal blockage; entry into the bile duct, pancreatic duct, or other small spaces; or entry into the liver or peritoneal cavity. They can also migrate out of the anus or come out the mouth or nose. Other body sites have been involved such as the kidney, appendix, or pleural cavity. In children, particularly those under the age of 5, there may be severe nutritional impairment related to the worm burden. Directly measurable effects would include increased fecal nitrogen and fecal fat, and impaired carbohydrate absorption, all of which would return to normal with elimination of the adult worms. Worms can also be spontaneously passed without any therapy.
Diagnosis
In the larval migration phase of the infection, diagnosis can be made by finding the larvae in sputum or in gastric washings. The typical Loeffler's syndrome is more likely to be seen in areas where transmission is highly seasonal. During the intestinal phase, the diagnosis can be made by finding the eggs (unfertilized or fertilized) or adult worms in the stool. The eggs are most easily seen on a direct wet smear or a wet preparation of the concentration sediment. Unfertilized Ascaris eggs will not float using the zinc sulfate flotation concentration method (eggs too heavy). Also, if too much iodine is added to the wet preparations, the eggs may look like very dark debris. Eggs may be very difficult to identify on a permanent stained smear due to stain retention and asymmetrical shape. Intestinal disease can often be diagnosed from radiographic studies of the gastrointestinal tract where the worm intestinal tract may be visualized. This may be particularly obvious when two worms are lying parallel, like "trolly car lines". Other involved body sites would present specific symptoms indicative of bowel obstruction, biliary or pancreatic duct blockage, appendicitis, or peritonitis. Therapeutic measures would be related to specific symptoms and involved areas.
Ascaris lumbricoides —LABORATORY DIAGNOSIS
1. Both fertilized and unfertilized eggs can easily be recovered using the sedimentation concentration. (Unfertilized eggs will not float using zinc sulfate flotation concentration.)
2. Because of the potential problems caused by migration of the adult worms, patients who are undergoing elective surgery and general anesthetic should be checked for the presence of Ascaris if there is any possible exposure history (some anesthetics stimulate the worms to migrate). Usually a single stool examination will suffice to rule out the infection.
3. Larvae could be recovered from sputum (larval migration through the lungs); however, this is not a common finding.
4. Eggs may be very difficult to recognize on a permanent stained smear. They will usually be very darkly stained and may be mistaken for debris.
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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.
YanıtlaSilChildren 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. https://labweeks.com/ascaris-lumbricoides/