Ascaris lumbricoides, an intestinal roundworm, is a soil-transmitted helminth and one of the most common parasites in the world. It causes a variety of clinical manifestations known as ascariasis. Although A lumbricoides infects nearly 1 billion people worldwide and causes significant morbidity, it remains a neglected tropical disease.
PATHOGENESIS AND EPIDEMIOLOGY
Ascaris is the largest intestinal roundworm that commonly infects humans: the female measures 20 to 40 cm long, and the male measures 15 to 30 cm long (Fig. 319-1). The female lays approximately 200,000 eggs daily; eggs are broadly ovoid and 45 to 75 μm by 35 to 50 μm. Fertilized eggs have a 3-layer coat with a bile-stained, mamillated outer shell. Unfertilized eggs are broader and longer (ie, approximately 90 μm by 45 μm) and usually lack the mamillated outer coat (Fig. 319-2).
Life cycle of Ascaris lumbricoides.
Ascaris lumbricoides worm.
Transmission of A lumbricoides is through the fecal-oral route. There is no human-to-human transmission. Infected individuals excrete eggs in their stool. Infection occurs through ingestion of A lumbricoides eggs through contaminated food or soil. When eggs are ingested and stimulated by enzymes in the duodenum, the larvae emerge, traverse the intestinal mucosa, and enter the mesenteric lymphatics and venules (Fig. 319-3). They then enter the portal circulation and reach the pulmonary vascular bed, perforate the alveolar wall, ascend the tracheobronchial tree to the epiglottis, and are swallowed. The vast majority of ascarids finally settle in the jejunum, where mature worms mate and females begin laying eggs in 2 to 2.5 months. After a life span of 10 months to 2 years, they are passed in the stool. A video showing a live worm observed during colonoscopy is available at http://www.youtube.com/watch?v=HOaZCkA8Zvk (accessed July 1, 2016).
Ascaris lumbricoides eggs.
The World Health Organization estimates that over 1 billion people worldwide are infected with A lumbricoides, predominantly in South and East Asia. Tropical areas with warm, wet climates have year-round transmission of A lumbricoides and a high prevalence (approaching 95% in some regions). In the United States, the highest infection rates are among immigrants from developing countries (20–60% infected in some surveys). Young children are infected most frequently, with peak prevalence in school-age children living in the tropics. Intensity of infection, or worm burden, typically decreases significantly after the age of 15 years. Ascaris infections tend to cluster in families. Individuals may be asymptomatic and shed eggs for years, thus enhancing transmission. Behavioral and environmental factors are associated with intensity of infection, which is greatest in regions with poverty, malnutrition, and poor sanitation.