Ascaris lumbricoides
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p656-657
2025-10-21
138
GENERAL CHARACTERISTICS
Ascaris lumbricoides is the most common and the largest roundworm. The parasite has a worldwide distribution with higher prevalence in the tropical regions. Eggs are ingested and hatch in the duodenum, penetrate the intestinal wall, and migrate to the hepatic portal circulation. The adult worms live and reproduce in the lumen of the small intestine. The ovum is a thick, oval mammillated (outer protrusions) and embryonated egg. The eggs are passed in the feces and become infective 2 to 6 weeks following deposition, depending on the environment. The general life cycle is outlined in Figure 1. A. lumbricoides life cycle is classified as an indirect life cycle; transmission is not via a direct route from one host to the next.
Fig1. Life cycle of Ascaris lumbricoides (indirect life cycle).
EPIDEMIOLOGY
Geographic distribution is associated with climate and poor sanitation. The eggs of A. lumbricoides require a warm humid environment in order for the embryonated ovum to mature and become infective. Infection rates are elevated in poverty-stricken areas that have poor sanitation. Transmission is through the fecal-oral route, usually through the ingestion of eggs on contaminated material. Ascaris eggs are capable of survival within harsh environmental conditions, including dry or freezing temperatures.
PATHOGENESIS AND SPECTRUM OF DISEASE
Many A. lumbricoides infections are asymptomatic. The presentation of symptoms correlates with the length of infection, the number of worms present, and the overall health of the host. Intestinal symptoms range from mild to severe intestinal obstruction. Some patients will develop pulmonary symptoms and present with immune-mediated hypersensitivity pneumonia. The worms may cause an immune condition known as Löffler’s syndrome characterized by peripheral eosinophilia. See Table 1 for a summarized detail of associated diseases.
Table1. Pathogenesis and Spectrum of Associated Diseases
Table1. Pathogenesis and Spectrum of Disease—cont’d
LABORATORY DIAGNOSIS
Female worms have an extremely high daily output of eggs, making diagnosis relatively easy through the identification of eggs in feces. The large, broadly oval mammillated ova are typically stained brown from bile (Figure 2). Some eggs will be decorticated, or lacking the mammillated outer cover. Infertile eggs may be oval or irregular shaped with a thin shell and containing internal granules. Adult worms may also be identified in feces. The male is smaller (15 to 31 cm) with a curved posterior end (Figure 3) and contains three well-characterized lips. Larvae may be found in sputum or gastric aspirates as a result of larval migration during development within the human host.
Fig2. Bile-stained mammillated A. lumbricoides ovum. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Fig3. A. lumbricoides adult male worm. Note the curved posterior end. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
THERAPY
Anthelmintic treatment is recommended for all infections. Preferred therapy includes oral albendazole, mebendazole, or pyrantel pamoate.
PREVENTION
Prevention is managed through proper sanitation and good hygiene.
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